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PART II Disease Agents BACTERIA, FUNGI, AND VIRUSES Adenoviruses Agent. DNA virus. Two strains, MAd- 1 and MAd-2 (formerly called FL and K87, respectively), are recognized. Animals Affected. Mice and rats. Epizootiology. Prevalence is probably low. MAd- 1 is shed in urine, and MAd- 2 is shed in feces. Transmission is by the oral route. Clinical. Natural infection does not cause clinical disease. Pathology. There are no pathologic lesions associated with natural infections of MAd-l. Viral inclusions in intestinal mucosa are associated with MAd-2 infec- tions. Diagnosis. The preferred diagnostic procedures are the ELISA and the IFA test, which test sera for antigens of both MAd- 1 and MAd-2. Presumptive diagnosis of MAd-2 can be made by demonstration of characteristic intranuclear inclusions in histologic sections of intestinal epithelium. A fluorescent antibody method has been used for detecting MAd-2 antigen in the intestine. Definitive diagnosis requires virus isolation in tissue culture. Control. Cesarean derivation and barrier maintenance may be necessary for eliminating either virus strain. Interference with Research. MAd- 1 can produce extensive persistent lesions in the kidneys of adult mice and render them more susceptible to experimental Escherichia coli-induced pyelonephritis. MAd (strain not given) has been reported to accelerate experimental scrapie in mice. 9
10 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Suggested Reading Hamelin, C., C. Jacques, and G. Lussier. 1988. Genome typing of mouse adenoviruses. J. Clin. Microbiol. 26:31-33. Hasnelin, C., and G. Lussier. 1988. Genotypic differences between the mouse adenovirus strains FL and K87. Exper~entia 44:65-66. Luethans, T. N., and J. E. Wagner. 1983. A naturally occurring intestinal mouse adenov~rus infection associated with negative serologic findings. Lab. Anim. Sci. 33:270-272. Lussier G., A. L. Smith, D. Guenette, and J. P. Descoteaux. 1987. Serological relationship between mouse adenovirus strains FL and K87. Lab Anim. Sci. 37:55-57. Otten, J. A., and R. W. Tennant. 1982. Mouse adenovirus. Pp. 335-340 in The Mouse in BiomedicalResearch. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Smith, A. L., D. F. W~nograd, and T. G. Burrage.1986. Comparative biological characterization of mouse adenovirus strains FL and K87 and seroprevalence in laboratory rodents. Arch. Virol. 91:233-246. Bacillus piliformis Agent. Unclassified gram-negative bacterium, having vegetative and spore fonn ~ Animals Affected. Mice, rats, gerbils, hamsters, guineapigs, rabbits, cats, dogs, nonhuman primates, horses, and others. Epizootiology. Prevalence in the United States is unknown. Natural infection is thought to be caused by ingestion of spore-contaminated food or bedding. Clinical. Subclinical infection is probably more common than clinical disease (commonly called Tyzzer's disease). Contributors to clinical disease include poor sanitation, overcrowding, transportation stress, food deprivation, dietary modifi- cations, and altered host immune status. Clinical disease occurs most frequently in sucklings and weanlings, but animals of any age can be affected. Morbidity and mortality vary from low to high. Unexpected deaths, watery diarrhea, pasting of feces around the perineum, ruffled fur, and inactivity are the most common signs. Homozygous female and hemizygous male CBA/N-xid and C3.CBA/N-xid mice, which are deficient in a specific subpopulation of B cells, are highly susceptible, whereas T-cell-deficient athymic (nu/nu) mice are as resistant as immunocompe tent mice. Pathology. Primary infection occurs in the ileum and cecum, followed by ascension of the organisms by the portal vein to the liver and bacteremic spread to othertissues, most notably the myocardium. The organism preferentially replicates in the intestinal epithelium, smooth muscle, hepatocytes, and myocardium, but the degree of replication (and lesions) varies considerably between animals and between species. Intestinal lesions are usually more severe in rats than in mice. Myocardial lesions occur inconsistently. Gross lesions range from none to severe involvement of the intestine, liver, and/or heart. In mice, the most consistent finding is multiple pale to yellow foci in the liver. Infrequently, the ileum and cecum appear
DISEASE AGENTS 1 1 thickened, edematous, and hyperemic, and the myocardium contains circumscribed pale gray areas. Lesions in rats are similar, except that the ileum often appears dilated, atonic, and edematous (megaloileitis). The mesenteric lymph nodes are usually enlarged. In the ileum, in the cecum, and sometimes in the proximal colon, there is mild to severe loss of the mucosal epithelium, with blunting of villi in the ileum, thinning of the surface epithelium, and severe ulceration and hemorrhage. In more advanced stages, there is hyperplasia of crypt epithelium. Transmural acute to subacute inflammation can occur in areas of severe epithelial loss. In the liver there are multiple foci of coagulative necrosis that are rapidly converted to microabscesses. If the myocardium is affected, there is focal to diffuse myocardial necrosis with acute to subacute inflammation. In affected tissues, the characteristic large, filamentous bacilli are best demonstrated histologically in the cytoplasm of viable cells along the margin of necrotic tissues by the use of silver strains (Warthin- Starry or methenamine silver). Diagnosis. Diagnosis of clinical disease is based on finding typical gross and microscopic lesions and characteristic organisms in silver-stained histologic sec- tions. Both the IFA and the CF tests have been used for the diagnosis of subclinical infections, but neither test is available commercially in the United States. Alter- natively, weanling animals can be immunosuppressed by the administration of 100- 200 mg/kg cortisone acetate. Subclinical infection, if present, will become active disease, and characteristic lesions and organisms can be demonstrated histologically 7 days after cortisone administration. The presence of B. piliformis in tissues can be demonstrated by the finding of characteristic lesions and organisms histologi- cally 5-7 days following inoculation of the tissue into gerbils or into homozygous xid female or hemizygous xid male mice. Control. Cesarean-derivation and barrier-maintenance procedures, reduction of stress, and good sanitation procedures appear to minimize the occurrence of clinical disease. Good sanitation practices, avoidance of crowding, autoclaving of food and bedding, and the use of 0.3% sodium hypochlorite for disinfecting room surfaces are recommended for reducing spore contamination in conventional animal facilities. Oral administration of tetracycline can be helpful in controlling losses during outbreaks. Interference with Research. Tyzzer's disease can cause high mortality in breeding colonies of mice and in mice used in long-term carcinogenesis studies. Administration of cortisone or adrenocorticotropic hormone, whole-body x-irra- diation, transplantation of ascites tumors, and a high-protein diet can induce clinical disease. Tyzzer's disease alters the pharmacokinetics of warfarin and trimethoprim and the activity of hepatic transaminases. Suggested Reading Fries, A. S. 1980. Antibodies to Bacillus piliformis (Tyzzer's disease) in sera from man and
12 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS other species. Pp. 249-250 in Animal Quality and Models in Research, A. Spiegel, S. Enchsen, and H. A. Solleveld, eds. Stuttgart: Gustav Fischer Verlag. Fujiwara, K., M. Nakayama, and K. Takahashi. 1981. Serologic detection of inapparent Tyzzer's disease in rats. Jpn. J. Exp. Med. 51:197-200. Ganaway, J. R. 1980. Effect of heat and selected chemical disinfectants upon infectivity of spores of Bacillus piliformis (Tyzzer's disease). Lab. Anim. Sci. 30:192-196. Ganaway, J. R. 1982. Bacterial and mycotic diseases of the digestive system. Pp. 1-20 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Waggle, K. S., C. T. Hansen, J. R. Ganaway, and T. S. Spencer. 1981. A study of mouse strain susceptibility to Bacillus piliformis (Tyzzer's disease): The associahonofB-cellfunction and resistance. Lab. Anim. Sci. 31 :139-142. Weisbroth, S. H. 1979. Bacterial and mycotic diseases. Pp.191-241 in The Laboratory Rat. Vol.1: Biology and Diseases, H. J. Baker, J. R. Lindsey, and S. H. Weisbroth, eds. New York: Academic Press. Cilia-Associated Respiratory Bacillus Agent. Gram-negative bacterium. Animals Affected. Laboratory rats and mice, wild rats (Rattus norvegicus), African white-tailed rats (Mystromys albicaudatus), and rabbits. Epizootiology. Unknown. Clinical. Clinical manifestations in rats are similar to those of severe murine respiratory mycoplasmosis (MRM) and can include hunched posture, ruffled coat, inactivity, head tilt, and accumulation of prophyrin pigment around the eyes and external nares. No description of clinical disease in mice has been published. Pathology. In those instances in which cilia-associated respiratory (CAR) bacillus has been found in rats with natural disease, Mycoplasma pulmonis also was present. It is possible that M. pulmonis was the primary pathogen and the CAR bacillus increased disease severity. It is not known whether the CAR bacillus alone can cause natural clinical disease. The predominant lesions in rats are those of advanced MRM due to M. pulmonis with some additional distinctive features. Severe bronchiectasis andbronchiolectasis, pulmonary abscesses, and atelectasis are associated with the accumulation of purulent or mucopurulent exudate in airways. An abundance of mucus often is present in peribronchiolar alveoli. Multifocal necrosis and acute inflammation of bronchiolar and bronchial epithelia often progress to severe granulomatous in- flammation in airway walls and abscess formation in airway lumens. Disordered repair may result in distorted, scarred bronchioles and bronchiolitis obliterans. The ciliated border of the respiratory epithelium in affected airways often appears quite dense in hematoxylin- and eosin-stained sections because of the large numbers of CAR bacilli present between the cilia. The CAR bacillus can also be found on epithelial surfaces associated with MRM lesions in nasal passages, larynx, trachea, and middle ears. CAR bacillus-associated respiratory lesions similar to those seen in rats have been reported in C57BL/6J-ob/ob mice.
DISEASE AGENTS 13 Diagnosis. Recognition of the argyrophilic CAR bacillus in Warthin-Starry silver-stained histologic sections of affected lungs has been the main diagnostic method used. The organism also can be demonstrated by transmission electron microscopy. The ELISA and the IFA test for this infection are in use in some laboratories but have not been fully evaluated. Control. The infection probably can be eliminated by cesarean derivation, but definitive data are not available. Interference with Research. Uncertain. The organism might be an important contributor to the morbidity and mortality caused by MRM in rats. Suggested Reading Ganaway, J. R., T. H. Spencer, T. D. Moore, and A. M. Allen. 1985. Isolation, propagation, and characterization of a newly recognized pathogen, cilia-associated respiratory (CAR) bacillus of rats, an etiological agent of chronic respiratory disease. Infect. Immun. 47:472-479. Griffith, J. W., W. J. Whim, P. J. Danneman, and C. M. Lang. 1988. Cilia-associated respiratory bacillus infection of obese mice. Vet. Pathol. 25:72-76. Mackenzie, W. F., L. S. Magill, and M. Hulse. 1981. A filamentous bacterium associated with respiratory disease in wild rats. Vet. Pathol. 18:836-839. Matsushita, S. 1986. Spontaneous respiratory disease associated with cilia-associated respiratory (CARJ bacillus in a rat. Jpn. J. Vet. Sci. 48:437-440. Matsushita, S., M. Kashima, and H. Joshima. 1987. Serodiagnosis of cilia-associated respiratory bacillus infection by the indirect immunofluorescence assay technique. Lab. Anim. (London) 21:356-359. van Zwieten, M. J., H. A. Solleveld, J. R. Lindsey, F. G. deGroot, C. Zurcher, and C. F. Hollander. 1980. Respiratory disease in rats associated with a filamentous bacterium. Lab. Anim. Sci. 30:215-221. gen. Citrobacterfreundii Biotype 4280 Agent. Gram-negative bacterium. Usually considered an opportunistic patho Animals Affected. Mice. Epizootiology. Transmission is presumed to be by the fecal-oral route. The organism is rarely found in cesarean-derived, barrier-maintained mice. Clinical. Signs of disease are nonspecific and include ruffled fur, listlessness, weight loss, stunting, pasty feces around the anus and perineum, and rectal prolapse. Suckling mice are more susceptible to disease than are adults. Mortality can reach 60%, and the occurrence of rectal prolapse can reach 15 %. Mortality is significantly higher in C3H/HeJ than in DBA/2J, C57BL/6J, or N:NIHS (Swiss) mice. Pathology. Infection in mice lasts only about 4 weeks. Even if the infection is eliminated as early as 2 days postinfection by administration of neomycin sulfate or tetracycline hydrochloride, mucosal hyperplasia still occurs. Presence of the infection in the intestine for 10 days results in maximum hyperplasia. The descend
14 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS ing colon is most commonly affected, but the entire colon and cecum can be involved. Grossly, affected bowel is thickened and rigid in appearance. Micro- scopically, crypt height is increased threefold, mitotic activity is increased, goblet cells are decreased, and basophilia of the epithelium is increased. Crypt abscesses are common, and mucosal erosions and ulcers can occur. The occurrence of necrotizing and inflammatory lesions tends to parallel mortality. Variable numbers of neutrophils or mononuclear leukocytes can be present in the lamina propria, but there is often a paucity of inflammatory cells. Goblet-cell hyperplasia with muci- nous distension of crypts and streaming of mucin into the gut lumen can occur during regression of mucosal hyperplasia. Diagnosis. Diagnosis is by demonstration of characteristic lesions in the large intestine and isolation of organisms of the pathogenic biotype. Control. Definitive data are lacking. Control probably requires depopulation and restocking with cesarean-derived mice. Neomycin or tetracycline administered in drinking water reduces losses during outbreaks but probably does not completely eliminate infection. Interference with Research. The cytokinetics of the mucosal epithelium in the large intestine is profoundly altered in infected mice. Susceptibility to the carcinogen 1,2-dimethylhydrazine is increased, and the latent period for neoplasia induction is reduced. Suggested Reading Barthold, S. W. 1980. The microbiology of transmissible munne colonic hyperplasia. Lab. Anim. Sci. 30:167-173. Barthold, S. W., G. L. Coleman, P. N. Bhatt, G. W. Osbaldiston, and A. M. Jonas.1976. The etiology of transmissible murine colonic hypeIplasia. Lab. Anim. Sci. 26:889-894. Barthold, S. W., G. L. Coleman, R. O. Jacoby, E. M. Livstone, and A. M. Jonas. 1978. Transmissible murine colonic hype~plasia. Vet. Pathol. 15:223-236. Johnson, E., and S. W. Barthold. 1979. The ultrastructure of transmissible munne colonic hyperplasia. Am. J. Pathol. 97:291-313. Corynebacter~um kutscher~ Agent. Gram-positive bacterium. Animals Affected. Mice, rats, and rarely guinea pigs. Epizootiology. Persistent subclinical infections are thought to be common in conventionally reared stocks and rare in barrier-maintained stocks. The main sites of infection are probably the oropharynx, submaxillary lymph nodes, and large intestine, with transmission mainly by the fecal-oral route. Clinical Signs. Infection is usually inapparent. Disease occurs only after the immune system is compromised by experimental procedures, dietary deficiency, or concurrent infections with other agents. Signs of clinical disease in rats are usually those of a respiratory infection: dyspnea, rales, weight loss, humped posture, and
DISEASE AGENTS 15 anorexia. Signs in mice are usually those of severe septicemia: dead and moribund animals. Arthritis or abscesses can occur in either species. Pathology. Septicemia results in septic emboli in many organs. In mice, large bacterial emboli lodge in capillary beds, particularly in the kidney and liver. Em- bolic glomerulitis is characteristic. Abscess formation can occur at the focus of infection. In rats, bacterial emboli lodge in the capillaries of the lungs. Alveoli become packed with polymorphonuclear leukocytes and can form large necro- purulent centers. Fibrinous or fibrous pleuritis often develops. Occasionally, abscesses occur in the liver, kidneys, subcutis, peritoneal cavity, and other sites. Diagnosis. Detection of persistent subclinical infections is very difficult. Available methods are unsatisfactory for detecting persistent subclinical infections, although an ELISA has been developed that shows promise for detecting anti- bodies. There is also a new DNA probe method, but its usefulness has not been determined. Cortisone acetate can be given to provoke active disease, which is then diagnosed by culture of the organism, demonstration of characteristic lesions, and exclusion of other infectious agents and disease processes. Control. Cesarean derivation and barrier maintenance are effective means of control. Interference with Research. Infection with C. kutscheri can complicate ex- periments involving immunologically compromised mice or rats. Suggested Reading Ackerman, J. I., I. G. Fox, and J. C. Murphy. 1984. An enzyme linked immunosorbent assay for detection of antibodies to Corynebacterium kutscheri. Lab. Anim. Sci. 34:38-43. Barthold, S. W., and D. G. Brownstein. 1988. The effect of selected viruses on Coryne bacterium kutscheri infection in rats. Lab. Anim. Sci. 38:580-583. Brownstein, D. G., S. W. Barthold,R. L. Adams, G. A. Terwilliger, and J. G. Aftosmis. 1985. Experimental Corynebacterium kutscheri infection in rats: Bacteriology and serology. Lab. Anim. Sci. 36:135-138. Saltzgaber-Muller, J., and B. A. Stone. 1986. Detection of Corynebacterium kutscheri in animal tissues by DNA-DNA hybridization. J. Clin. Microbiol. 24:759-763. Suzuki, E., K. Mochida, and M. Nakagawa. 1988. Naturally occurring subclinical Corynebacterium kutscheri infection in laboratory rats: Strain and age related antibody response. Lab. Anim. Sci. 38:42-45. Weisbroth, S. H., and S. Scher. 1968. Corynebacterium kutscheri infection in the mouse. II. Diagnostic serology. Lab. Anim. Care 18:459-468. Cytomegalovirus, Mouse Agent. Double-stranded DNA virus, family Herpesviridae. Animals Affected. Wild mice and laboratory mice that have contracted the infection from wild mice. Epizootiology. Prevalence in laboratory mice is uncertain but is probably very low. Transmission occurs through the saliva, and infection persists throughout life.
16 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE kD RATS Vertical transmission also can occur but has not been fully explained. Direct passage of the virus across the placenta to the fetus and/or transmission via germ cells have been suggested. Clinical Signs. Natural infections are subclinical. Pathology. In natural infections, large acidophilic intranuclear inclusions are found in salivary gland acinar and duct cells. Affected cells typically are enlarged three to four times normal. The submaxillary glands are affected most, the sublingual glands less, and the parotid glands least. Diagnosis. The IFA and CF tests have been shown to be sensitive for acute experimental infections, and the ELISA has been shown to be sensitive for persistent experimental infections. These methods may prove useful for monitoring laboratory mice for cytomegalovirus infection in selected situations. Virus isola- tion can be accomplished using mouse embryo fibroblasts or other tissue culture systems. Control. Wild mice must be excluded from rodent facilities. Interference with Research. Natural infections have not been reported to Interfere with research. Suggested Reading Classen, D. C., J. M. Moringstar, and J. D. Shanley. 1987. Detection of antibody to murine cytomegalovirus by enzyme-linked immunosorbent and indirect immunofluorescence assays. J. Clin. Microbiol. 25:600-604. Lussier, G., D. Guenette, and J. P. Descoteaux. 1987. Comparison of serological tests for the detection of antibody to natural and experimental munne cytomegalovirus. Can. J. Vet. Res. 51:249-252. Mercer, J. A., C. A. Wiley, and D. H. Spector. 1988. Pathogenesis of murine cytomegalovrirus infection: Identification of infected cells in the spleen during acute and latent infections. J. Virol. 62:987-997. Mims, C. A., and J. Gould. 1979. Infection of salivary glands, kidneys, adrenals, ovaries, end epitheliabymur~necytomegalovirus. J.Med.Microbiol.12:113-122. Osborn, J. E. 1982. Cytomegalovirus and other he~pesviruses. Pp. 267-292 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Quinnan, G. V., and J. E. Manischewitz. 1987. Genetically detennined resistance to lethal mur~ne cytomegalovirus infection is mediated by interferon-dependent and -independent restriction of virus replication. J. Virol. 61: 1875-1881. Ectromelia Virus Agent. DNA virus, family Poxviridae. Animals Affected. Mice. Epizootiology. Ectromelia virus is possibly enzootic in some mouse colonies. Periodic epizootics have occurred in the United States since 1950, most commonly
DISEASE AGENTS 17 in research laboratories that exchange live mice and their tissues, sera, or transplant- able tumors. Natural transmission is dependent on direct contact and fomites. Skin abrasions are thought to be the main route of virus entry. Infected animals begin shedding virus about 10 days postinfection when characteristic skin lesions appear. Persistent infection (carrier state) was previously thought to be important in the epizootiology of mousepox. More recent data suggest that significant numbers of virus particles are shed from skin lesions for only about 3 weeks. Clinical. The severity of clinical disease (mousepox) varies greatly and depends on mouse strain, virus strain, length of time infection has been present in the colony, and husbandry practices. Inapparent infections occur mainly in resistant inbred mouse strains such as C57BL/6 or C57BL/10. Resistance in the these strains appears to be due to a single autosomal dominant trait. The more susceptible mouse strains include A, CBA, C3H, DBA/2, and BALB/c. Clinical manifestations include one or more of the following: ruffled hair; hunched posture; facial edema; conjunctivitis; swelling of the feet; cutaneous papules, erosions, or encrustations, mainly on the face, ears, feet, or tail; and necrotic amputation (ectromelia) of limbs or tails. Mortality varies from less than 1% to greater than 80%. Pathology. The incubation period is 7-10 days. Virus replicates in the skin and then in the regional lymph nodes, resulting in a mild primary viremia. Virus is taken up by splenic and hepatic macrophages, where there is extensive multiplication that results in a massive secondary viremia and sometimes in death due to diffuse splenic and hepatic necrosis. Virus from the secondary viremia localizes in a wide variety of tissues, especially the skin (basal cells), conjunctive, and lymphoid tissues. A primary lesion may appear at the site of skin inoculation about 4-7 days postinfection. Foot swelling and secondary generalized rash (pocks) may appear 7-10 days postinfection. Skin lesions heal within 2 weeks, leaving scars. In acute mousepox, there is severe necrosis of liver, spleen, lymph nodes, Peyer's patches, and thymus. Jejunal hemorrhage often results from mucosal erosions. Character- istic large eosinophilic cytoplasmic inclusions may be present in skin lesions. Diagnosis. The ELISA is sensitive and specific in unvaccinated mice; however, it may give false-positive results in NZW and NZB mice. The HAI is relatively insensitive but does not give positive reactions to sera from mice vaccinated with the IlID-T strain of vaccinia virus. Diagnosis of acute disease is based on sero- logic testing of survivors or on the demonstration, using transmission electron microscopy, of characteristic large virus particles in affected tissues. Differential diagnosis of skin lesions should exclude bite wounds and loss of limbs due to Streptobacillus moniliformis. Biologic materials such as cells and blood can be screened for ectromelia virus by injecting the tissue into known pathogen-free mice followed by serologic testing. Control. Quarantine and testing of incoming mice and mouse tissues from sources other than commercial barrier facilities are the best way to prevent the introduction of infection. In the past, the accepted practice for eradicating ectromelia virus was elimination of infected mouse colonies and all infected biologic materials,
18 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE ED RATS along with rigorous decontamination of rooms and equipment. Cesarean derivation of infected mouse stocks was not acceptable because intrauterine infection is known to occur in mice infected during pregnancy. More recently it has been suggested that quarantine and cessation of breeding might successfully eliminate the virus. Vaccination with a live-virus vaccine, the IHD-T strain of vaccinia virus adapted to growth in embryonated eggs, can be useful in eliminating disease from small closed colonies where all offspring can be vaccinated by 6 weeks of age. Vaccination can protect mice from fatal disease but does not prevent infection or virus transmission. Interference with Research. Up to 100% of the animals in an experiment can die in an explosive outbreak. Manipulations that exacerbate ectromelia virus infections or promote epizootics include experimental infection with tubercle bacilli, x-irradiation, administration of various toxic chemicals, shipping, tissue transplantation, castration, and tumors. Ectromelia virus infection can alter phagocytic response. Conversely, procedures that decrease phagocytosis can increase sus- ceptibility to ectromelia virus, e.g., large doses of endotoxin or splenectomy. Suggested Reading AALAS (American Association of Laboratory Animal Science).1981. Ec~omelia (mousepox) in the United States. Proceedings of a seminar presented at the 31st Annual Meeting of AALAS held in Indianapolis, Indiana, October 8,1980. Lab. Anim. Sci.31:549-631. Bhatt, P. N., and R. O. Jacoby. 1987. Mousepox in inbred mice innately resistant or susceptible to lethal infection with ectromelia virus. I. Clinical responses. Lab. Anim. Sci. 37:11-15. Bhatt, P. N., and R. O. Jacoby. 1987. Mousepox in inbred mice innately resistant or susceptible to lethal infection with ectromelia virus. III. Experimental transmission of infection and derivation of virus-free progeny from previously infected dams. Lab. Anim. Sci. 37:23-28. Bhatt, P. N., and R. O. Jacoby. 1987. Effect of vaccination on the clinical response, pathogenesis and transmission of mousepox. Lab. Anim. Sci. 37:610-614. Bhatt, P. N., R. O. Jacoby, and L. Gras. 1988. Mousepox in inbred mice innately resistant or susceptible to lethal infection with ectromelia virus. IV. Studies with the Moscow strain. Arch. Virol. 100:221-230. Penner, F. 1982. Mousepox. Pp. 209-230 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Encephalitozoon cuniculi Agent. A protozoan, order Microsporidia. Animals Affected. Mice, rats, rabbits, hamsters, guinea pigs, humans, end many other mammals. Epizootiology. Prevalence in mouse and rat stocks is not known but is thought to be very low in comparison with that in rabbits, in which the organism is considered ubiquitous. Rabbits undoubtedly provide the major source of infection
DISEASE AGENTS 19 for mice and rats in research facilities. Spores of E. cuniculi are shed in the urine and ingested by another host. Clinical. Natural infections usually are inapparent. Pathology. In cases of clinical disease, the classic lesion in rats and rabbits is meningoencephalitis with multifocal granulomatous inflmnmation. Activated macrophages form glial nodules in response to the organism. These nodules have necrotic centers or appear as solid sheets of cells. Varying numbers of lymphocytes and plasma cells are seen in the meninges and around vessels. The brain lesions in mice are similar, except for the lack ofthe granulomatous foci. The organism occurs intracellularly in the renal tubular epithelium with or without the presence of an inflammatory response. In chronic infections, focal destruction of tubules and replacement by fibrous connective tissue results in small pits on the cortical surface. Lesions in organs other than the kidney and brain are less consistent. Intrapentoneal inoculation of E. cuniculi, as when contaminated transplantable tumors are pas ~ . . . . saged, results In ascribes In mice. Diagnosis. Several serologic tests have been developed for diagnosis of the infection in rabbits; however, only the IFA and immunoperoxidase tests have been used in surveying mouse and rat colonies. Other methods used for diagnosis include detection of parasites in unne, demonstration of typical lesions and organisms in tissue sections, and an intradennal skin test. Control. Mice and rats should not be exposed to infected rabbits. Serologic testing of adult animals with selection of E. cuniculi-free breeding stocks has been used successfully for eradicating the infection in rabbits. Interference with Research. The histologic changes caused by E. cuniculi infection in the brain and kidneys can complicate the interpretation of lesions in studies requiring histopathology. E. cuniculi can contaminate transplantable tu- mors and alter host responses during tumor passage in mice. Mice experimentally infected with E. cuniculi have reduced humoral antibody titers to sheep erythro- cytes, reduced proliferative spleen cell responses to mitogens, and altered natural killer cell activity. Suggested Reading Beckwith, C., N. Peterson, J. J. Liu. and J. A. Shadduck. 1988. Dot enzyme-linked immunosorbent essay (dot ELISA) for antibodies toEncephalitozoon cuniculi. Lab. Anim. Sci. 38:573-576. Didier, E. S., and J. A. Shadduck. 1988. Modulated immune responsiveness associated with experimental Encephalitozoon cuniculi infection in BALB/c mice. Lab. Anim. Sci. 38:680-684. Cannon, J. 1980. The course of infection of Encephalitozoon cuniculi in immunodeficient and immunocompetent mice. Lab. Anim. (London) 14:189-192. Majeed, S. K., and A. J. Zubaidy. 1982. Histopathological lesions associated with Encephalitozoon cuniculi (nosematosis) infection in a colony of Wistar rats. Lab. Anim. (London) 16:244-247.
20 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Niederkorn, J. Y., J. A. Shadduck, and E. C. Schmidt. 1981. Susceptibility of selected inbred strains of mice to Encephali~ozoon cuniculi. J. Infect. Dis. 144:249-253. Shadduck, J. A., and S. P. Pakes. 1971. Encephalitozoonosis (nosematosis) and toxoplasmosis. Am. J. Pathol. 64:657-674. Hantaviruses Agent. Single-stranded RNA viruses, family Bunyaviridae, genus Hantavirus. Hantaan virus, the cause of Korean hemorrhagic fever in humans, is the prototype member of the genus and is a significant zoonotic pathogen of laboratory rodents. Animals Affected. Natural hosts of all hantaviruses appear to be small mam- mals, primarily rodents. Multiple species may serve as hosts in a given geographical area, and strain of virus and likelihood of causing disease in humans vary from region to region. Thus far, about five dominant associations between hantaviruses, rodent carriers, and human diseases (if present) have been described as follows: Hantaan virus, the field mouse Apodemus agrarius, Korean hemorrhagic fever (KHF) in Korea, and the severe fond of epidemic hemorrhagic fever in China; Puumala virus, the bank vole Clethrionomys glareolus, nephropathia epidemica (NE) in Eastern Europe and Scandinavia; urban and laboratory rat viruses, Rattus norvegicus, moderate disease found mostly in people in Asia but occasionally also in Europeans; Girard Point and other viruses from North and South America, Rattus norvegicus, no disease recognized in humans, although serologic evidence of infection has been found; and Prospect Hill virus, the meadow vole Microtus pennsylvanicus, no disease recognized in humans. Naturally infected laboratory rats have been the source of Hantavirus infections in research personnel in Japan, Belgium, the United Kingdom, and France. Epizootiology. Hantaan virus appears in the lungs of its reservoir host about 10 days postinfection and subsequently appears in the urine and saliva. Peak virus shedding occurs about 3 weeks after infection, but virus can be detected in the lungs for 6 months and occasionally for up to 2 years. Aerosols are the main mode of transmission. Other hantaviruses are assumed to have similar patterns of infec- tion in their reservoir hosts. Hantaviruses are transmitted to humans from persistently infected rodents and other small mammals. In laboratory settings, this is usually from laboratory rats or their tumors. The majormode oftransmission is by aerosols of urine, feces, or saliva containing infectious virus. Direct animal contact is not necessary. Animal bites can transmit the infection but appear to be of relatively minor importance. Clinical. Reservoir hosts have persistent subclinical infections. Human dis- ease has occurred only in Europe and Asia.* *Signs of KHF in humans vary from mild to severe and include (ever, headache, muscular pains, hemorrhages (cutaneous petechia or ecchymoses, hemoptysis, hematuria, hematemesis, melena), and . . protelnur~a.
DISEASE AGENTS 21 Pathology. Lesions have not been observed in reservoir hosts infected with hantaviruses. Lungs and other tissues contain large amounts of virus without morphologic changes. Diagnosis. Since Hantavirus infections in rodents are inapparent, diagnosis is most likely to be made through health surveillance. Recommended serologic tests are the IFA test, the ELISA, and the HAI test. Noninfectious antigen should be used, and work with animals and blood products should be done in a biological safety cabinet. P3 conditions are needed for working with unconcentrated virus in small amounts. The RAP test is recommended for testing transplantable tumors and other biologic materials for Hantavirus contamination. Control. Hantavirus infections can be prevented by obtaining animals, trans- plantable tumors, and other biologic materials that have been tested and found to be free of infection. Contamination of laboratory rodent stocks by wild rodents must be prevented. Laboratory rodent stocks found to be infected with a Hantavirus should be de- stroyed and replaced with pathogen-free stock. Although not proven to be effective, cesarean derivation has been recommended for eliminating the infection in valuable genetic stocks. Interference with Research. Some hantaviruses are important zoonoses. Suggested Reading Childs, J. E., G. E. Glass, G. W. Korch, J. W. LeDuc. 1987. Prospective seroepidemiology of hantaviruses and population dynamics of small mammal communities of Baltimore, Maryland. Am. J. Trop. Med. Hyg. 37:648-662. Hemorrhagic Fever with Renal Syndrome (HERS) Virus Guideline Committee. 1986. Guidelines for surveillance, prevention, and control of Hantaan virus infection in laboratory animal colonies. Pp. 209-216 in Viral and Mycoplasmal Infections of Laboratory Rodents: Effects on Biomedical Research, P. N. Bhatt, R. O. Jacoby, H. C. Morse III, and A. E. New, eds. Orlando, Fla.: Academic Press. Johnson, K. 1986. Hemorrhagic fever Hantaan virus. Pp. 193-207 in viral andMyco- plasmal Infections of Laboratory Rodents: Effects on Biomedical Research, P. N. Bhatt, R. O. Jacoby, H. C. Morse III, and A. E. New, eds. Orlando, Fla.: Academic Press. Kawamata, J., T. Yamanouchi, K. Dohmae, H. Miyamoto, M. Takahaski, K. Yamaniski, T. Kurata, and H. W. Lee. 1987. Control of laboratory acquired hemo~Thagic fever with renal syndrome (HFRS) in Japan. Lab. Anim. Sci. 37:431-436. Tsai, T. F. 1987. Hemorrhagic fever with renal syndrome: Mode of transmission to humans. Lab. Anim. Sci. 37:428-430. Tsai, T. F. 1987. Hemorrhagic fever with renal syndrome: Clinical aspects. Lab. Anim. Sci. 37:419-427. Hepatitis Virus, Mouse Agent. Single-stranded RNA virus, family Coronaviridae, genus Coronavirus. Animals Affected. Mice.
22 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Epizootiology. Mouse hepatitis virus (MHV) infection is extremely contagious; prevalence can exceed 80%. Pathogenesis is influenced by such factors as virus strain and mouse strain. Current evidence suggests that the infection runs its course in 2-3 weeks, and there is no carrier state. Transmission is by direct contact, fomites, and airborne particles. MHV is a frequent contaminant of transplantable tumors and cell lines. Clinical. In immunocompetent mice, MHV infections are usually subclinical. Infant mice of naive breeding populations can show diarrhea and high mortality when infected with the more virulent enterotropic MHV strains. Athymic (nu/nu) mice show progressive emaciation leading to debility and death. Pathology. Strains of MHV differ greatly in virulence and tissue tropism, and mouse strains differ greatly in susceptibility to MHV. These factors interact with host age and route and dose of virus inoculation to determine the outcome of infection. Mechanisms of host resistance to MHV infection are poorly understood. Mice are fully susceptible to the virus as neonates, but some strains acquire resistance at 2-3 weeks of age as lymphoreticular function matures. Cell-mediated immunity is important in the development of resistance; humoral immunity is considered relatively unimportant. Macrophages, interferon, and natural killer cells may also have important roles. There are two major disease patterns: the respiratory pattern and the enteric pattern. Lesions in immunocompetent mice are present for only about 7-10 days and are usually nonspecific and subtle, particularly those associated with the respiratory pattern. In the respiratory pattern, infection involves the nasal passages and lungs; intestinal involvement is minimal. Lesions include mild olfactory mucosal necro- sis, neuronal necrosis of olfactory bulbs and tracts, lymphoplasmacytic infiltrates and vacuolation in the brain, multifocal interstitial pneumonia with mild perivascular lymphoid infiltrates, and multifocal necrotizing hepatitis. In the enteric pattern, infection is primarily restricted to the bowel, with variable spread to other abdomi- nal organs such as the liver and abdominal lymph nodes. Lesions are most severe in neonatal mice because of their relatively slow kinetics of mucosal epithelium turnover. Varying degrees of epithelial lysis and blunting of villi occur in the small intestine. Numerous multinucleate syncytial giant cells (balloon cells) can occur on the villi, as well as in the crypts. In more severe cases, there can be ulceration of the mucosa. A similar lytic process occurs in the ascending colon and cecum. Occasionally, there is multifocal necrotizing hepatitis and/or encephalitis. MHV infection in athymic (nu/nu) or neonatally thymectomized mice becomes progressively more generalized, severe, and chronic, with involvement of many organs, including the liver, intestine, lungs, bone marrow, lymphoreticular organs, vascular endothelium, and brain. Multifocal necrosis with syncytial giant cells usually occurs in the liver. Splenomegaly may develop because of compensatory myelopoiesis, and large numbers of myelopoietic cells may appear in the liver. Diagnosis. The ELISA is the test of choice for serologic monitoring. An IFA test is also available and is about equal in sensitivity to the ELISA. CF and serum
DISEASE AGENTS 23 neu~alizai~on tests are less sensitive. Heterozygotes or sentinel mice should be used to test athymic stocks, because nu/nu homozygotes do not develop CF antibody to MHV and have a weak and variable antibody response to the ELISA and the serum neutralization test. The characteristic histologic lesions of MHV infections are useful in both health surveillance and necropsy diagnosis. Virus isolation can be difficult because not all strains grow equally well in all cell lines. Transplant- able tumors and other biologic materials can be screened by virus isolation or by the MAP test. Control. Strict adherence to barrier protocol, regular health surveillance, and testing of biologic materials from mice are necessary to prevent MHV infection. Cesarean derivation and battier maintenance traditionally have been recommend- ed for reservation of breeding stocks; however, these measures may be unneces- sary because recent evidence suggests that the virus is present for only 2-3 weeks. Another approach is to isolate individual breeding pairs from MHV-infected populations in separate containment devices, such as filter-top cage systems, and subsequently to select seronegative progeny as breeders. Interference with Research. MHV has been reported to after many exper~men- tal results. Examples are alterations in immune function and hepatic enzyme activities; inhibition of lymphocyte proliferative responses in mixed lymphocyte cultures and mitogen-stimulated cells; alteration of phagocytic and tumorcidal activity; increase of hepatic uptake of injected iron; increase of susceptibility to other indigenous pathogens; activation of natural killer cells and production of interferon; delay of the increase in plasma lactic dehydrogenase activity following infection with lactic dehydrogenase virus; and occurrence of anemia, leukopenia, and thrombocytopenia. In athymic (nu/nu) mice, the virus can also cause sponta- neous differentiation of lymphocytes bearing T-cell markers, alter IgM and IgG responses to sheep erythrocytes, enhance phagocytic activity of macrophages, cause rejection of xenograft tumors, impair liver regeneration after partial hepatectomy, and cause hepatosplenic myelopoiesis. Subclinical infections are exacerbated by thymectomy; whole-body irradiation; reticuloendothelial blockade by iron salts; and administration of cortisone, cyclophosphamide, antilymphocyte serum, chemotherapeutic agents, or halothane anesthesia. Suggested Reading Barthold, S. W. 1986. Research complications and state of knowledge of rodentcoronaviruses. Pp. 53-89 in Complications of Viral and Mycoplasma Infections in Rodents to Toxicology Research and Testing, T. E. Hamm, Jr., ed. Washington, D.C.: Hemisphere. Barthold, S. W. 1986. Mouse hepatitis virus biology and epizootiology. Pp. 571-601 in Viral and Mycoplasmal Infections of Laboratory Rodents: Effects on Biomedical Research, P. N. Bhatt, R. O. Jacoby, H. C. Morse III, and A. E. New, eds. Orlando, Fla.: Academic Press. Barthold, S. W. 1987. Host age and genotype effects on enterotropic mouse hepatitis virus infection. Lab. Anim. Sci. 37:36-40.
24 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Barthold, S. W., and A. L. Smith. 1984. Mouse hepatitis virus strain-related patterns of tissue tropism in suckling mice. Arch. Virol. 81:103-112. Boyle, J. F., D. G. Weismiller, and K. V. Holmes. 1987. Genetic resistance to mouse hepatitis virus correlates with absence of virus-binding activity on target tissues. I. Virol. 61 :185- 189. Holmes, K. V., J. F. Boyle, and M. F. Frana. 1986. Mouse hepatitis virus: Molecular biology and implications for pathogenesis. Pp. 603-624 in Viral and Mycoplasmal Infections of Laboratory Rodents: Effects on Biomedical Research, P. N. Bhatt, R. O. Jacoby, H. C. Morse III, and A. E. New, eds. Orlando, Fla.: Academic Press. H-1 Virus* Agent. Single-stranded DNA virus, family Parvovindae, genus Parvovirus. Animals Affected. Laboratory and wild rats (Rattus norvegicus). Epizootiology. Common infection of wild and laboratory rats; prevalence exceeds 50% in some populations. Epizootiological characteristics are generally assumed to be similar to those of Kilham rat virus infection. Transmission is primarily horizontal, and virus is shed in urine, feces, nasal secretions, and milk. Transplacental infection has not been demonstrated in natural infections. Clinical. Natural infections are inapparent. Pathology. There are no pathologic changes associated with natural infections. Diagnosis. The ELISA and IFA test are usually used for initial screening, followed by either the HAI, CF, or neutralization test for discriminating between H-1 virus and Kilham rat virus infections. Primary rat embryo, 324K, or BHK-21 cells can be used for virus isolation. Control. The same measures recommended for Kilham rat virus (see page 25) should be effective in controlling H-1 virus. Interference with Research. It might be possible for H- 1 virus to alter studies of fetal development or teratogenesis, but this has not been reported to occur as a result of natural infection. H-1 virus has been reported to cause hepatocellular necrosis when rats are subjected to liver injury by hepatotoxic chemicals, parasitism, or partial hepatectomy. H-1 virus has been reported to inhibit experimental tumor induction by adenovirus 12 and dimethylbenzanthracene in hamsters. Suggested Reading Jacoby, R. O., P. N. Bhatt, and A. M. Jonas. 1979. Viral diseases. Pp. 273-283 in The Laboratory Rat. Vol. I: Biology and Diseases, H. J. Baker, J. R. Lindsey, and S. H. Weisbroth, eds. New York: Academic Press. Kilham, L. 1966. Viruses of laboratory and wild rats. Pp. 1 17-140 in Viruses of Laboratory *This virus was designated "H-1" by Toolan in 1961, and this is its official taxonomic name. In the United States, the virus is commonly called Toolan H-1 virus.
DISEASE AGENTS 25 Rodents, R. Holdenried, ed. National CancerInstitute Monograph 20. Washington, D.C.: U.S. Department of Health, Education, and Welfare. Ruffolo, P. R., G. Margolis, and L. Kilham. 1966. The induction of hepatitis by prior partial hepatectomy in resistant adult rats infected with H-1 virus. Am. J. Pathol. 49:795-824. Tattersall, P., and S. F. Cotmore. 1986. The rodent parvoviruses. Pp. 305-348 in Viral and Mycoplasmal Infections of Laboratory Rodents: Effects on Biomedical Research, P. N. Bhatt, R. O. Jacoby, H. C. Morse III, and A. E. New, eds. Orlando, Fla.: Academic Press. Toolan, H. W., S. L. Rhode III, and J. F. Gierthy. 1982. Inhibition of 7,12- dimethylbenz~ajanthracene tumors in Syrian hamsters by prior infection with H-1 parvoviruses. Cancer Res. 42:2552-2555. Kilham Rat Virus Agents. Single-stranded DNA virus, family Parvoviridae, genus Parvovirus. Animals Affected. Laboratory and wild rats are the natural hosts. Epizootiology. Kilham rat virus (KRV) is a common infection in wild and laboratory rats; prevalence exceeds 50% in some populations. Transmission is primarily by the horizontal route, either through direct contact or fomites. Virus is shed in urine, feces, milk, and nasal secretions. Transplacental infection is not considered important. Persistent infection can last up to 14 weeks. KRV is a frequent contaminant of cultured cell lines and transplantable tumors. Clinical. KRV infections rarely cause clinical disease. Signs in the few instances of overt disease that have been reported include increased numbers of uterine resorption sites in pregnant dams and ranting, ataxia, cerebellar hypoplasia, and jaundice in their pups. Spontaneous deaths, cerebellar hypoplasia, scrotal cyanosis, jaundice, abdominal swelling, dehydration, and other signs of severe illness have occurred in juvenile and young adult rats. Pathology. Parvoviruses attack rapidly dividing cells. In newborn and young rats, KRV can cause jaundice, hemorrhagic infarction with thrombosis in multiple organs (including brain, spinal cord, testes, and epididymis), and cerebellar hyp- oplasia. Amphophilic intranuclear inclusions occur in the endothelium and other cells of affected organs. Focal necrosis, hypertrophy and vacuolar degeneration of hepatocytes, cholangitis, and biliary hyperplasia also occur. Hemorrhagic encephalopathy has been reported in naturally infected LEW rats given cyclo- phosphamide. Diagnosis. The ELISA and IFA test are the most sensitive tests but do not discriminate between different serotypes of parvoviruses. The HAI, CF, and neutralization tests are used for serotype discrimination. In clinical disease, typi- cal lesions should be demonstrated, and virus isolation should be carried out. The virus can be grown in primary rat embryo, 324K, and BHK-21 cells. Control. The most practical approach to controlling infection is to obtain animals demonstrated free of KRV by serologic monitoring. Biologic materials should be tested for KRV infection by the MAP test. Cesarean section should be
26 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS successful for rederiving valuable breeding stocks of rats because transplacental transmission is not considered important. Interference with Research. KRV can contaminate transplantable tumors and rat cell cultures, interfere with in vitro lymphocyte responses, suppress the devel- opment of Moloney virus-induced leukemia, and alter in vitro lymphocyte re- sponses and cytotoxic lymphocyte activity. KRV also has been reported to induce interferon production. Immunosuppression can cause clinical disease in inapparently infected rats. Suggested Reading Coleman, G. L., R. O. Jacoby, P. N. Bhatt, A. L. Smith, and A. M. Jonas. 1983. Naturally occurring lethal parvovirus infection in juvenile and young adult rats. Vet. Pathol.20:44- 56. Jacoby, R. O., P. N. Bhatt, and A. M. Jonas. 1979. Viral diseases. Pp. 273-283 in The Laboratory Rat. Vol. I: Biology and Diseases, H. J. Baker, J. R. Lindsey, and S. H. Weisbroth, eds. New York: Academic Press. Jacoby, R. O., P. N. Bhatt, D. J. Gaertner, A. L. Smith, and E. A. Johnson. 1987. The pathogenesis of rat virus infection in infant and juvenile rats after oronasal inoculation. Arch. Virol. 95:251-270. Jacoby, R. O., D. J. Gaertner, P. N. Bhatt, F. X. Paturzo, andA. L. Smith. 1988. Transmission of exper~mentally-induced rat virus infection. Lab. Anim. Sci. 38:11-14. Kilham, L., and G. Margolis. 1966. Spontaneous hepatitis and cerebellar hypoplasia in suckling rats due to congenital infection with rat virus. Am. J. Pathol. 49:457-475. Paturzo, F. X., R. O. Jacoby, P. N. Bhatt, A. L. Smith, D. J. Gaertner, and R. B. Ardito. 1987. Persistence of rat virus in seropositive rats as detected by explant culture. Arch. Virol. 95:137-142. Lactic Dehydrogenase-Elevating Virus Agent. RNA virus, family Togaviridae. Animals Affected. Mice. Epizootiology. Wild mice presumably serve as reservoir hosts. Transmission occurs primarily during passage of contaminated tumors, cells, or other biologic materials. Lactic dehydrogenase-elevating virus (LDV) is shed in feces, urine, saliva, and milk. After the first week of infection, the virus titer in these excretions declines sufficiently to make the risk of transmission to other mice relatively low. Transplacental transmission can occur. Bite wounds increase transmission between cagemates. Clinical. Natural infections are usually subclinical. There is lifelong viremia in which the virus is complexed to antiviral antibody and lifelong elevation in plasma lactic dehydrogenase (LDH) and other plasma enzymes. The virus causes overt disease in C58 and AKR mice that are naturally immunosuppressed because of a loss of Lyt-1,2 cells between 5 months and 1 year of age. These mice develop
DISEASE AGENTS 27 polioencephalomyelitis with flaccid hind limb paralysis. C58 are more susceptible than AKR mice. Inheritance of this susceptibility is thought to be polygenic, possibly involving the H-2 complex. Pathology. Virus titer in serum peaks 12-14 hours postinfection and gradually decreases until about 2 weeks postinfection, at which time the titer stabilizes for life. Serum LDH increases to 8- to 11-fold above normal by 72-96 hours postinfection because of the decreased clearance of LDH V, one of the five LDH isozymes. It gradually declines over the next 3 months, although it remains significantly elevated for life. SJL/J mice carry a recessive trait that causes a 15- to 20-fold increase in serum LDH. The activity of several other serum enzymes is also increased but to a lesser degree than LDH. LDV selectively replicates in a small subpopulation of macrophages, the specific identity of which is not known. As a result, cellular immunity is depressed during the first few weeks of infection and gradually returns to normal after weeks or months. There is an enhanced humoral response to antigenic challenge with T-cell-dependent antigens during the first 24 hours postinfection but a diminished humoral response to such challenge 3 weeks or longer after infection. There is a similar enhanced response to early postinfection challenge with T-cell-independent antigen but no diminution of response in chronic infection, which suggests a defect in T-cell function. Circulating antigen-antibody complexes, which partially neutralize LDV, are produced by 4 weeks postinfection. These complexes are deposited in glomeruli, but they produce only a mild membranous glomerulopathy. Protective antibody is not produced. After 5 months of age, infected C58 and AKR mice develop age-dependent encephalomyelitis characterized by neuronal destruction, mononuclear infiltration, and microglial proliferation in the gray matter of the central nervous system. Diagnosis. Diagnosis is usually based on the presence of increased plasma LDH activity. Transplantable tumors, virus inocula, and other biologic materials can be screened for LDV contamination by inoculating pathogen-free mice with the test material and performing a plasma or serum LDH assay 72-96 hours later. Virus isolation is not practical for most diagnostic purposes. Control. Mice from commercial barrier breeding facilities are not likely to be infected. LDV-free mice can be derived from contaminated stocks by selection of animals with normal plasma LDH concentration or by cesarean derivation. LDV can be eliminated from tumors by passage of tumor cells in a rodent species other than the mouse or by maintenance of tumor cells in tissue culture. Interference with Research. The effects of LDV on research results can be subtle and complex. Subclinical infection with LDV lasts throughout life, and the effects on many biologic endpoints can differ dramatically with time after infection. LDV-induced immunosuppression can cause an alteration in defense mechanisms against other infectious agents. It has been reported that tumor growth is enhanced early after LDV infection because of depressed cellular immunity and is influenced less during chronic infection. LDV infection also has been shown to alter the incidence and behavior of spontaneous virus-induced neoplasms, including the
28 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Bittner mammary tumor and murine sarcoma viruses; to suppress the development of urethan-induced pulmonary adenomas; and to suppress vinyl chloride-vinyl acetate-induced carcinogenesis. It has been reported to cause delayed allograft rejection, to prevent the development of experimental allergic encephalomyelitis, and to prevent autoimmunediseaseinNZB and (NZB xNZW)F~ hybrid mice. Serum gamma globulin levels and humoral antibody responses have been shown to increase during early infection, and humoral antibody responses have been shown to decrease during chronic infection. LDV has been found to be a polyclonal lymphocyte activator during the early stages of infection. Suggested Reading fronton, M. A. 1982. Lactate dehydrogenase-elevating virus. Pp. 193-208 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Buxton, I. K., S. P. K. Chan, and P. G. W. Plagemann. 1988. The Ia antigen is not the major receptor for lactate dehydrogenase-elevating virus on macrophages from CBA and BALB/e mice. Virus Res.9:205-219. Contag, C. H., S. P. K. Chan, S. W. Wietgrefe, and P. G. W. Plagemann. 1986. Correlation between presence of lactate dehydrogenase-elevating virus RNA and antigens in motor neurons and paralysis in infected C58 mice. Virus Res. 6:195-209. Crispens, C. G.1964. On the epizootiology of the lactic dehydrogenase agent. J. Natl. Cancer Inst. 35:975-979. Dillberger, J. E., P. Monroy, and N. H. Altman. 1987. Delayed increase in plasma lactic dehydrogenase activity in mouse hepatitis virus-infected mice subsequently infected with lactic dehydrogenase virus. Lab. Anim. Sci. 37:792-794. Rowson, K. E. K., and B. W. J. Mahy. 1985. Lactate dehydrogenase-elevating virus. J. Gen. Virol. 66:2297-2312. Leukemia Viruses, Murine Agents. RNA virus, family Retroviridae, type C oncovirus group. Animals Affected. Laboratory and wild mice. Epizootiology. Murine leukemia viruses (MuLVs) are integrated into the DNA of the host's sex cells and are transmitted vertically as Mendelian traits. All laboratory and wild Mus musculus are thought to harbor these viruses. Horizontal transmission is inefficient but can occur through infected saliva, sputum, urine, feces, or milk or by intrauterine infection. Clinical. Despite the fact that all mice have endogenous MuLVs, leukemias and related malignancies occur naturally in only 1-2% of most strains. Strain suscep- tibility is influenced by the number of MuLV gene copies in the genome and by other genes, including Fv-l, Fv-2, In, nu, hr. and Ir. The AKR strain, for example, has a high incidence of MuLV-related spontaneous thymic lymphoma, which reaches 90% by the time the mice are 9 months of age. The most common signs of thymic lymphoma are dyspnea, peripheral lymphadenopathy, and abdominal enlargement.
DISEASE AGENTS 29 Pathology. Mouse strains with a high incidence of leukemia (e.g., AKR, C58, C3H/Fa) spontaneously express high titers of ecotropic MuLV in all organs throughout life. Mouse strains with a low incidence of leukemia (e.g., BALB/c, A/J, C3H/He, CBA/J) express only low titers of virus. The mouse leukemias are, with few exceptions, actually lymphomas because they are predominantly solid tumors of lymphocytes or other hematopoietic cells. The majority of those occur- nng before 1 year of age ale thymic lymphomas, while those seen in older mice are predominantly histiocytic (reticulum cell) lymphomas. Other types include nonthymic lymphomas, lymphatic leukemias, granulocytic leukemias, erythroleukemia, plasma cell tumors, and mast cell tumors. Diagnosis. Diagnosis of neoplasia is based on morphologic features in histo- logic sections. Tumor cell types can also be determined by analysis of cell-surface antigens. Isolation and characterization of MuLVs require specialized techniques usually available only in research laboratories dedicated to viral oncology. MuLV type and group specificity can be determined by immunofluorescence of fixed cells using an appropriate panel of type- or group-specific antisera. Control. Control measures are not usually considered useful since all mice probably have endogenous MuLV proviruses as an integral part of their genomes. Horizontal transmission is considered inefficient. Interference with Research. The presence of MuLVs probably has little significance for most research purposes. MuLV expression and the associated occurrence of neoplasms, however, can present competing endpoints in some studies, e.g., studies of aging processes in venous organs. Thus, an awareness of the incidence of spontaneous tumors in different mouse strains or the ability of specific test chemicals to induce tumors can be useful in designing some expen- ments. Active MuLV infection can cause suppression of humoral and cellular immunity without causing clinical disease. Suggested Reading Bishop, J. M., and H. E. Varmus. 1975. The molecular biology of RNA viruses. Pp. 1-48 in Cancer: A Comprehensive Treatise, vol. 2., F. F. Becker, ed. New York: Plenum. Dunn, T. B.1954. Normal and pathologic anatomy of reticular tissue in laboratory mice with classification and discussion of neoplasms. J. Natl. Cancer Inst. 14:1281-1433. Furmanski, P., and M. A. Rich.1982. Neoplasms of the hematopoietic system. Pp.352-371 in The Mouse in Biomedical Research. Vol. IV: Experimental Biology and Oncology, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Lilly, F., and A. Mayer. 1980. Genetic aspects of murine type-C viruses and their hosts in oncogenesis. Pp. 89-108 in Viral Oncology, G. Klein, ed. New York: Raven Press. Risser, R., J. M. Horowitz, and J. McCubrey. 1983. Endogenous mouse leukemia viruses. Annul Rev. Genet. 17:85-121. Squire, R. A., D. G. Goodman, M. G. Valerio, T. Frederickson, J. D. Strandberg, M. H. Levitt, C. H. Lingeman, J. C. Harshbarger, and C. J. Dawe. 1978. Pp. 1051-1283 in Pathology of Laboratory Animals, vol. II, K. Benirschke, F. M. Garner, and T. C. Jones, eds. New York: Springer-Verlag.
30 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Lymphocytic Choriomeningitis Virus Agent. RNA virus, family Arenaviridae, genus Arenavirus. Animals Affected. Wild mice are the principal reservoir hosts, but laboratory mice and Syrian hamsters also serve as important natural hosts. Humans, monkeys, dogs, rabbits, guinea pigs, rats, and chickens are susceptible to the virus. Epizootiology. Natural lymphocytic choriomeningitis virus (LCMV) infec- tions of laboratory mice are rare. Only infected mice and hamsters are known to transmit the virus. Both species can have chronic infections, with high concentra- tions of virus shed in the urine, saliva, and milk. The portals of entry are probably mucus membranes and broken skin. Vertical (transovarian and/or transuterine) transmission occurs in mice and probably in hamsters. Once introduced into a population of mice, the infection can spread to all members of that population. Since 1960 three epidemics involving at least 236 human cases have occurred in the United States, and all have been associated with Syrian hamsters, either as laboratory animals bearing transplantable tumors or as pets.* Clinical. Clinical signs are highly variable depending on the virus strain, the mouse strain, and the age of the mouse at the time of infection. Persistent tolerant infection is acquired if infection occurs in utero or within a few days of birth. There is lifelong viremia and shedding of virus. Transient runting can occur during the first 3 weeks of life. Thereafter, the mice appear normal. At 7-10 months of age, immune complex glomerulonephritis occurs and is associated with emaciation, ruffled fur, hunched posture, ascites, and sometimes death. Nontolerant (acute) infection is acquired if infection occurs after the first week of life when the animals are immunocompetent. Viremia occurs, but there is no shedding of virus. The outcome is either death within a few days or weeks or recovery with elimination of the virus. Natural infections in adult mice range from inapparent infection to severe dis- ease with high mortality. Natural infections in hamsters are usually subclinical. Pathology. T-cell, but not B-cell, activity is suppressed in persistent tolerant infection. Infectious virus circulates bound to LCMV-specific IgG and complement. These complexes accumulate in the renal glomeruli; choroid plexus; and, to a lesser degree, in synovial membranes, in blood vessel walls, and beneath the epidermis of the skin to cause late-onset disease that becomes clinically apparent around 7-10 months of age. There is generalized lymphoid hyperplasia and perivascular accu- mulation of lymphocytes and plasma cells in all visceral organs. In nontolerant (acute) infection, there is multifocal hepatic necrosis and generalized necrosis of lymphoid tissues. Both the morphologic lesions and elimination of the virus are due to cell-mediated immune responses involving H-2-restricted, cytotoxic T lym- phocytes and possibly natural killer cells. LCMV-infected adult mice can be protected from disease by numerous immunosuppressive regimens. Athymic (nu/ *In humans the usual clinical manifestations are those of flu-like disease, with fever, headache, myalgia, nausea' vomiting, sore throat, and photophobia being the major symptoms. The following occur rarely: rash, alopecia, diarrhea, cough, arthritis, lymphadenopathy, orchitis, delirium, amnesia, meningitis.
DISEASE AGENTS 31 nu) mice inoculated with the virus at 3-6 weeks of age do not develop disease but become persistently viremic. Diagnosis. The serologic methods of choice are the IFA test, the micro plaque- reduction test for neutralizing antibody, and the ELISA. The IFA test is particularly useful for rapid diagnosis early in the course of infection, while the micro plaque- reduction test is considered better for chronic infection. The CF test is considered relatively insensitive and is not recommended. Drawing and processing of blood from an animal suspected of LCMV infection should be done with care because of the likelihood of viremia. The MAP test can be used in testing transplantable tumors and other biologic materials. The virus can also be identified by using the IFA test in tissues or cultures of isolated virus. Control. The most practical method of control is to obtain mice only from colonies known to be free of LCMV and to maintain them in a barrier facility that excludes wild rodents. Biologic materials such as transplantable tumors should be pretested and shown to be free of the virus before experimental use. If LCMV infection is diagnosed, the entire stock should be destroyed and incinerated. Ani- mal cages arid other equipment should be autoclaved. Animal rooms should be fumigated either with forrnalin (40% formaldehyde in water, sprayed on all room surfaces at 36 ml/m3) or parafo~maldehyde (11 g/m3 vaporized in a high- temperature silicone fluid at 96°C) and allowed to remain vacant for 7-10 days. Cesarean derivation of animal stocks is of no value because of transovar~an or . . . . . transuter~ne transmission of infection. Interference with Research. LCMV infection is an important zoonotic infec- tion that can cause serious disease and sometimes fatality in personnel. LCMV is a frequent contaminant of biologic materials, including transplantable tumors of mice, hamsters, and guinea pigs; tissue culture cell lines; virus stocks, including leukemia viruses, distemper virus, rabies virus, and mouse poliomyelitis virus; and Toxoplasma gondii sublines. LCMV infection has an inhibitory effect on tumor induction by polyomavirus, Rauscher virus, and mammary tumor virus. Acute infection has been reported to cause induction of natural killer cell activity, depression of both humoral and cellular immunity, delayed rejection of skin and tumor allografts, increased susceptibility to mousepox virus or Eperythrozoon coccoides infection, and increased susceptibility to bacterial endotoxin and x- irradiation. Abrogation of the naturally occurring insulin-dependent diabetes mellitus of BB strain rats has also been reported. Chronic infection has been shown to cause proliferation of virus-specific cytotoxic T lymphocytes. Suggested Reading Bhatt, P. N., R. O. Jacoby, and S. W. Barthold. 1986. Contamination of transplantable murine tumors with lymphocytic choriomeningitis virus. Lab. Anim. Sci. 36:136-139. Biggar, R. J., R. Deibel, and J. P. Woodall. 1976. Implications, monitoring, and control
32 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS of accidental transmission of lymphocytic chonomeningitis virus within hamster tumor cell lines. Cancer Res. 36:537-553. Lehmann-Grube, F. 1982. Lymphocytic chonomeningitis virus. Pp.231 -266 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, alla J. G. Fox, eds. New York: Academic Press. Lohler, J., and F. Lehmann-Grube. 1981. Immunopathologic alterations of lymphatic tissues of mice infected with lymphocytic chonomeningitis virus. I. Histopathologic findings. Lab. Invest. 44:193-204. Moskophidis, D., S. P. Coppold, H. Waldmann, and F. Lehmann-Grube. 1987. Mechanism of recovery from acute virus infection: Treatment of lymphocytic chonomeningitis virus- infected mice with monoclonal antibodies reveals that Lyt-2+ T lymphocytes mediate clearance of virus and regulate the antiviral antibody response. J. Virol. 61: 1867- 1874. van der Zeijst, B. A. M., B. E. Noyes, M.-E. Mirault, B. Parker, A. D. M. E. Osterhaus, E. A. Swyryd, N. Bleumink, M. C. Horzinek, and G. R. Stark. 1983. Persistent infec- tion of some standard cell lines by lymphocytic chonomeningitis virus: Transmission of infection by an intracellular agent. J. Virol. 48:249-261. Mammary Tumor Virus, Mouse Agent. RNA virus, family Retroviridae. Fourmajorvariants have been identified: MMTV-S (standard; the Bittner virus), which is transmitted through the milk to nursing young and is highly oncogenic; MMTV-L (low oncogenic), which is transmitted through germ cells and is weakly oncogenic; MMTV-P (pregnancy dependent), which is transmitted through both milk and germ cells and is highly oncogenic; MMTV-O (overlooked), which is considered an endogenous virus in the ~ . genome ot most mice. Animals Affected. Wild and laboratory mice. Epizootiology. When infected, strains such as C3H, DBA/2, and A readily express MMTV-S, and the virus can be demonstrated in a variety of locations throughout the body, especially in mammary tissue and milk. Transmission of MMTV-S is by ingestion of infected milk, resulting in a high incidence of mammary tumors early in life (6-12 months) when the associated genetic and hormonal factors are also present. Clinical. Tumors occur in any part of the body where mammary tissue is located. The lungs are the most common site for distant metastases. Pathology. It is thought that the virus initially induces hyperplastic alveolar nodules that progress to neoplasia. The average latency period from infection to tumor expression is 6-9 months. Susceptibility to MMTV is genetically determined, and tumor development is enhanced by administration of estrogen to both males and females, forced breeding, and administration of carcinogens. Mammary tumors are usually circumscribed, round to nodular, gray to white masses located in the subcutaneous tissue. Ulcerations and hemorrhages are common in large tumors. Histologically, most are types A or B adenocarcinomas. Type A tumors are characterized by uniform acini lined by a single layer of cuboidal cells. Type B
DISEASE AGENTS 33 tumors are variable in the extent of differentiation but usually consist of irregular cords and sheets of cells. Types C, Y. L, and P adenocarcinomas; carcinomas with squamous cell differentiation; and carcinosarcomas occur less frequently. Other histologic types are rare. Mice of many strains develop humoral and cellular immune responses to MMTV, indicating that mice infected early in life are not immunologically tolerant. Diagnosis. Pathologic diagnosis of mouse mammary tumors is based on histopathologic characteristics. Detection and characterization of the virus require test procedures normally available only in specialized viral oncology laboratories, including nucleic acid hybridization, immunologic assays for viral antigens, and bioassays for infectivity in different strains of mice. Control. The most practical method of control is by selection of mouse strains without MMTV. Foster nursing of young on mouse strains that are free of the virus has been used to eliminate MMTV-S. Interference with Research. MMTV infection can be a complicating factor in experimental carcinogenesis studies. Suggested Reading Bentvelzen, P., and J. Hilgers. 1980. Munne mammary tumor virus. Pp. 311-355 in Viral Oncology, G. Klein, ed. New York: Raven Press. Bittner, J. J.1936. Some possible effects of nursing on the mammary gland tumor incidence in mice. Science 84:162. Medina, D. 1982. Mammary tumors. Pp. 373-396 in The Mouse in Biomedical Research. Vol. IV: Expenmental Biology and Oncology, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Michalides, R., A. van Ooyen, and R. Nusse. 1983. Mammary tumor virus expression and mammary tumor development. Curr. Top. Microbiol. Immunol. 106:57-78. Ringhold, G. M. 1983. Regulation of mouse mammary tumor virus gene expression by glucocorticoid hormones. Curr. Top. Microbiol. Immunol. 106:79-103. Sass, B., and T. B. Dunn. 1979. Classification of mouse mammary tumors in Dunn's miscellaneous group including recently reported types. J. Natl. Cancer Inst.62: 1287-1293. Minute Virus of Mice Agent. DNA virus, family Parvoviridae, genus Parvovirus. Animals Affected. Wild and laboratory mice. Epizootiology. Wild mice serve as reservoir hosts. Enzootic infection is common in barrier-maintained and conventional breeding colonies of mice. Minute virus of mice (MVM) is highly contagious. In infected colonies, maternal antibodies are protective until the young are 6-8 weeks of age; however, most mice become infected and seroconvert by 3 months of age. Transmission occurs by direct contact and by urine and fecal contamination. Airborne and transplacental infection are not considered important.
34 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Clinical. Natural infections are inapparent. Pathology. There are no pathologic changes associated with natural infections. Diagnosis. The ELISA and the IFA test are the most sensitive serologic tests. The HAI, CF, and neutralization tests are used to discriminate infections caused by MVM from Hose caused by Kilham rat virus or H-1 virus. MRL/MpJ arid MR:L/ MpJ-lpr/lpr mice frequently show false-positive HAI test results for MVM. Virus isolation can be done by using rat embryo tissue culture. The MAP test is commonly used for detection of MVM in transplantable tumors and other biologic materials. Control. Infection can be eliminated from stocks of mice by cesarean denva- tion, but elimination of infected mice followed by replacement with MVM-free mice is often more practical. Strict adherence to barrier procedures is required to maintain the MVM-free state. Wild mice must be excluded. Transplantable tumors, virus stocks, and other biologic materials should be monitored before admission to a facility. Interference with Research. MVM is a frequent contaminant of mouse leuke- mia virus preparations, transplantable tumors, hybndomas, and cell lines. Evidence that MVM can interfere with research has come from studies of MVM(i), a single variant of the virus that may or may not occur as a natural infection in contemporary mice. MVM(i) grows lyrically in cytotoxic T-lymphocyte clones, abrogates cytotoxic T-lymphocyte responses, suppresses T-lymphocyte mitogenic responses, and suppresses T-helper-dependent B-lymphocyte responses in vitro. The intra- muscular inoculation of MVM(p) into mice suppresses the growth of Ehrlich ascites tumor cells given intrapentoneally. Suggested Reading Cross, S. S., and J. C. Parker. 1972. Some antigenic relationships of the murine parvovi- ruses: Minute virus of mice, rat virus, and H-1 virus. Proc. Soc. Exp. Biol. Med. 139:105-108. Engers, H. D., J. A. Louis, R. H. Zubler, and B. Hirt. 1981. Inhibition of T cell mediated functions by MVM(i), a parvovirus closely related to minute virus of mice. J. Immunol. 127:2280-2285. Parker, J. C., S. S. Cross, M. J. Collins, Jr., and W. P. Rowe. 1970. Minute virus of mice. I. Procedures for quantitation and detection. J. Natl. Cancer Inst. 45:297-303. Parker, J. C., M. J. Collins, Jr., S. S. Cross, and W. P. Rowe.1970. Minute virus of mice. II. Prevalence, epidemiology, and occurrence as a contaminant of transplanted tumors. J. Natl. Cancer Inst. 45: 305-310. Tattersall, P., and S. F. Cotmore. 1986. The rodent parvoviruses. Pp.305-348 in Viral and Mycoplasmal Infections of Laboratory Rodents: Effects on Biomedical Research, P. N. Bhatt, R. O. Jacoby, H. C. Morse III., and A. E. New, eds. Orlando, Fla.: Academic Press. Ward, D. C., and P. J. Tattersall. 1982. Minute virus of mice. Pp.313-334 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press.
DISEASE AGENTS 35 Mycoplasma arthritidis Agent. Gram-negative bacterium, family Mycoplasmataceae. Animals Affected. Rats and mice. Epizootiology. M. arthritidis occurs as a common subclinical infection in rats and mice, including ces~arean-denved, bamer-maintained stocks. Clinical. The infection is usually subclinical; disease caused by this agent is extremely rare. Pathology. There are no pathologic changes associated with subclinical ntectlons. Diagnosis. The ELISA should be used for screening; immunoblot on positive sera can be used to discriminate between species of Mycoplasma. Isolation of M. arthritidis from animals with subclinical infection requires culture of tissue homogenates from multiple organ sites, which is not practical in most instances. In the rare event that clinical arthritis occurs, the organism should be cultured from joint exudates. Control. Definitive information is not available. Interference with Research. Infection can cause spontaneous polyarthntis in rats. Subclinical infections can be activated to complicate experimentally induced arthritis in rats. M. arthritidis can contaminate transplantable tumors of rats, caus- ing arthritis and/or abscesses at the injection site in recipients. Expenmental infections of rodents with M. arthritidis can be complicated by preexisting latent infection with this organism. M. arthritidis is a frequent contaminant of rodent cell cultures. Suggested Reading Cassell, G. H., M. K. Davidson, J. K. Davis, and J. R. Lindsey. 1983. Recovery and identification of murine mycoplasmas. Pp.129- 142 in Methods in Mycoplasmology, vol. II, J. G. Tully and S. Razin, eds. New York: Academic Press. Davidson, M. K.; J. R. Lindsey, M. B. Brown, G. H. Cassell, and G. H. Boorman. 1983. Natural infection of Mycoplasma arthritidis in mice. Curr. Microbiol. 8:205-208. Lindsey, J. R., M. K. Davidson, T. R. Schoeb, and G. H. Cassell.1986. Murine mycoplasmal infections. Pp. 91-121 in Complications of Viral and Mycoplasma Infections in Ro- dents to Toxicology Research and Testing, T. E. Hamm, Jr., ed. Washington, D.C.: Hemisphere. Minion, F. C., M. B. Brown, and G. H. Cassell. 1984. Identification of cross-reactive anti- gens between Mycoplasma pulmonis and Mycoplasma arthritidis. Infect. Immun. 43:115-121. Razin, S., and E. A. Freundt. 1984. The mycoplasmas. Pp.740-793 in Bergey's Manual of Systematic Bacteriology, vol. I, N. R. Krieg and J. G. Hold, eds. Baltimore: Williams & WiLkins. Thirkill, C. E., and D. S. Gregerson. 1982. Mycoplasma arthritidis-induced ocular in flammatorydisease. Infect.Immun.36:775-781.
36 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Mycoplasma pulmonis Agent. Gram-negative bacterium, family Mycoplasmataceae. Animals Affected. Rats and mice. The organism is occasionally isolated from wild rats (Rattus norvegicus), cotton rats (Sigmodon hispidus hispidus), rabbits, Syrian hamsters, and guinea pigs. Epizootiology. Infection and disease are common in conventionally reared rats and mice. Subclinical infection occurs in some cesarean-derived, barrier-maintained stocks. Transmission is thought to be by the intrauterine route and by aerosol between cagemates, including from dam to offspring and between adjacent cages. Clinical. Infections are usually subclinical. Signs of murine respiratory mycoplasmosis (MRM), the disease caused by M. pulmonis, are nonspecific but can include rates, polypnea, weight loss, hunched posture, ruffled coat, inactivity, and "head tilt," in both rats and mice; "snuffling" and accumulation of porphyrin pigment around the eyes and external nares in rats; and "chattering" in mice. Athymic (nu/nu) mice are no more susceptible to MRM than are immunocom- petent mice. Pathology. M. pulmonis is an extracellular parasite that preferentially colonizes the luminal surface of respiratory epithelium. Organisms and lesions, if present, tend to decrease from proximal to distal airways. Usually, the organism is a commensal. Intracage ammonia concentrations of 19 ,ug/1 of air or greater appear to exacerbate MRM by increasing the growth of M. pulmonis in the respiratory tract. Other influencing factors include concurrent infection with Sendai virus, sialodacryoadenitis virus or cilia-associated respiratory bacillus; administration of hexamethylphosphoramide or cyclophosphamide; a deficiency of vitamins A or E; inhalation of tobacco smoke; genetic susceptibility of the host (e.g., LEW are more susceptible than F344 rats; C3H/HeN are more susceptible than C57BL/6N mice); and, possibly, virulence of the M. pulmonis strain. Characteristic changes at any level in the respiratory tract include neutrophils in the airways, hyperplasia of the mucosal epithelium, and a lymphoid response in the sub- mucosa. Lesions can be acute or chronic and include rhinitis, otitis media, laryn- gitis, tracheitis, bronchitis, bronchiectasis, pulmonary abscesses, and alveolitis. Pleuritis and emphysema are rare. Hyperplasia of bronchus-associated lymph- oid tissue is characteristic in rats. Syncytial epithelial giant cells can occur in nasal and bronchial mucosa in mice. LEW rats show genital disease, characterized by purulent endometritis, pyometra, salpingitis, and perioophoritis. In mice humoral antibody is protective and can be passively transferred. Cellular immunity appears to be more important in rats than in mice. Diagnosis. Cultural isolation can be achieved by using a medium that has been pretested and shown to support growth. The nasopharynx is probably the best single site from which to obtain a culture sample, but culturing samples from multiple sites increases the isolation rate. A battery of bacterial, viral, and histopathologic procedures should be used to identify the responsible agent~s) and to exclude other possible causes or contributors. Efforts should be made to identify factors that
DISEASE AGENTS 37 exacerbate MRM. The ELISA is the method of choice for rodent health surveil- lance; it is more sensitive and cost effective than is culturing the organism. The ELISAs currently in use are only genus specific; therefore, immunoblot is used to differentiate between species of Mycoplasma. Detection of subclinical infection is often a major problem; ELISA seropositivity might occur only sporadically, the number of ELISA-positive animals might be very small, and seropositive animals might become negative again. The best results are obtained when only adults are tested (weanlings with subclinical infection usually are ELISA negative), sample size is increased, and testing is done repeatedly. Control. Cesarean-denvation and barner-maintenance programs appear to have reduced the prevalence of disease but may not have been equally successful in reducing the prevalence of infection. The major emphasis should be on selecting mycoplasma-free breeding stocks. This can be achieved by housing small groups of young adult breeders in plastic film isolators and testing them monthly using the ELISA until they are 12 months of age. Young animals from stocks found to be consistently negative can then be used to establish breeder production populations under barrier programs. Definitive information on eliminating M. pulmonis from clinically or subclinically infected stocks is lacking. Darns should be several months old and have been found repeatedly to be ELISA negative. Administration of antimicrobial agents might help to control clinical signs; however, such agents are not curative and can introduce variables if used in animals on experimental protocols. Interference with Research. Morbidity and mortality caused by MRM can disrupt long-term studies. MRM alters ciliary function, cell kinetics, and immunity in the respiratory tract and changes the response to carcinogens. Infection in LEW rats delays the onset and reduces the seventy of adjuvant arthntis, reduces the incidence of experimental collagen-induced a~thntis, and reduces antibody re- sponse to collagen. Genital infection alters genital tract histology. In mice infection can activate natural killer cells, contaminate transplantable tumors, and cause arthritis in the recipients. Subclinical infection can be exacerbated by some experimental procedures (e.g., deficiencies of vitamin A or E, administration of hexarnethylphosphorarnide). M. pulmonis is a frequent contaminant of rodent cell cultures. Mycoplasmas produce lymphokine-like substances that are mitogenic for B and T lymphocytes in vitro. Suggested Reading Cassell, G. H., N. R. Cox, J. K. Davis, M. B Brown, F. C. Minion, and J. R. Lindsey. 1986. State-of-the-art detection methods for rodent mycoplasmas. Pp. 143-160 in Complica- tions of Viral and Mycoplasmal Infections in Rodents to Toxicology Research and Testing, T. E. Hamm, Jr., ed. Washington, D.C.: Hemisphere. Cassell, G. H., M. K. Davidson, J. K. Davis, and J. R. Lindsey. 1983. Recovery and identification of murine mycoplasmas. Pp. 129-142 in Methods in Mycoplasmology, vol. II, J. G. Tully and S. Razin, eds. New York: Academic Press.
38 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Lindsey, J. R., G. H. Cassell, and H. J. Baker. 1978. Mycoplasmatales and rickettsiales. Pp. 1481-1550 in Pathology of Laboratory Animals, vol. II, K. Benirschke, F. Garner, and C. Jones, eds. New York: Springer-Verlag. Lindsey, J. R., M. K. Davidson, T. R. Schoeb, and G. H. Cassell. 1986. Murine mycoplasmal infections. Pp. 91-121 in Complications of Viral and Mycoplasma Infections in Rodents to Toxicology Research and Testing, T. E. Hamm, Jr., ed. Washington, D.C.: Hemisphere. Minion, F. C., M. B. Brown, and G. H. Cassell. 1984. Identification of cross-reactive antigens between Mycoplasma pulmonis and Mycoplasma arthritidis. Infect. Immun. 43:1 15-121. Razin, S., and E. A. Freundt. 1984. The mycoplasmas. Pp. 740-793 in Bergey's Manual of Systematic Bacteriology, vol. I, N. R. Krieg and J. G. Holt, eds. Baltimore: Williams & Wilkins. Pasteurella pneumotropica Agent. Gram-negative coccobacillus, family Pasteurellaceae. Animals Affected. Mice, rats, hamsters, guinea pigs, and many others. Epizootiology. P. pneumotropica can be isolated from up to 95% of healthy animals in some colonies. It can be isolated from many organs, including the respiratory tract, oral cavity, intestine, uterus, urinary bladder, skin, and conjuncti~ra. Transmission is probably by contact and fomites. Clinical. Infections are usually subclinical. When clinical infections do occur, signs in mice include conjunctivitis, panophthalmitis, dacryoadenitis, subcutane- ous and cervical abscesses, bulbourethral gland infections, utenne infections, and otitis media, while signs in rats include ophthalmitis, conjunctivitis, subcuta- neous abscesses, and mastitis. Pathology. P. pneumotropica is an opportunist that most frequently causes lesions of the skin and adnexal structures. Lesions are usually character~zed by suppurative inflammation. Diagnosis. Diagnosis must discr~minate between P. pneumotropica infection and P. pneumotropica-induced disease and rule out other possible causative agents end disease processes. Pasteurella spp.,Actinobacillus spp.,Haemophilus spp., and Yersinia spp., which are commonly found in mice and rats, give similar reactions in many biochemical tests. Therefore, extensive biochemical testing is required to accurately identify these organisms. An ELISA for detection of serum antibody to P. pneumotropica has recently been developed. Control. Cesarean der~vation and maintenance in a gnotobiotic isolator may be necessary to exclude the organism completely. Antibiotic therapy has limited value. Interference with Research. There have been no reports of interference with research results. Suggested Reading Carter, G. R. 1984. Genus 1. Pasteurella. Pp. 552-558 in Bergey's Manual of Systematic Bacteriology, vol. I, N. R. Krieg and J. G. Holt, eds. Baltimore: Williams & Wilkins.
DISEASE AGENTS 39 Jawetz, E. 1950. A pneumotropic Pasteurella of laboratory animals. I. Bacteriological and serologic characteristics of the organism. J. Infect. Dis. 86:172-183. Jawetz, E., and W. H. Baker. 1950. A pneumotropic Pasteurella of laboratory animals. II. Pathological and immunological studies with the organism. J. Infect. Dis. 86:184-196. Kunstyr, I., and D. Hartman. 1983. Pasteurella pneumotropica and the prevalence of the AHP (Actinobacillus, Haemophilus, Pasteurella)-group in laboratory animals. Lab. Anim. (London' 17: 156-160. Moore, T. D., A. M. Allen, and J. R. Ganaway. 1973. Latent Pasteurella pneumotropica infection of the intestine of gnotobiotic and barrier-held rats. Lab. Anim. Sci.23 :657-661. Pneumocysfis cannii Agent. Classification uncertain. Considered a fungus or a protozoan. Animals Affected. Mice, rats, humans, and numerous other mammals. Epizootiology. P. carinii is a ubiquitous organism of low virulence. It causes active pulmonary disease only in immunocompromised hosts. It is extremely prevalent as a persistent subclinical infection in mice and rats. Transmission is thought to be by inhalation of infective cysts expelled during exhalation or coughing. Clinical Signs. Rats and mice show no clinical signs unless they are immuno- deficient or immunosuppressed. Clinical signs include weight loss, cyanosis, rough hair coat, and dyspnea. Pathology. In animals with active disease the lungs are enlarged, rubbery in consistency, plum colored, and heavierthan normal. Histologically, alveolar septae are variably thickened, and there is a meager inflammatory response of lymphoid cells. Many alveoli are distended by homogeneous, foamy, eosinophilic material characteristic of P. carinii pneumonia. Diagnosis. Definitive identification of P. carinii in active infection depends on the demonstration of cysts (characteristically containing eight sporozoites) and trophozoites. Cysts measure 5-7 ,um in diameter and have thick walls that stain with methenamine silver, cresyl violet, periodic acid-Schiff, or toluidine blue. Giemsa stain is preferred for the demonstration of trophozoites and sporozoites in lung imprints. An IFA method also has been used. Acridine orange has been proposed for rapid screening of imprints; trophozoites stain yellow to orange; cyst walls do not stain. Persistent infections can be diagnosed by immunosuppressing some animals and then demonstrating the organisms in diseased lung tissue. Control. Subclinical infection is probably very common in conventionally reared and pathogen-free colonies. Gnotobiotic methods are likely useful in excluding the infection but might not be completely effective because of possible . . vertical transm~ss~on. Interference with Research. P. carinii can complicate long-term studies in which severely immunosuppressed animals are used. Athymic (nu/nu) and severe combined immunodeficient (~scid/scid) mice can develop active infection.
40 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Suggested Reading Bartlett, M. S., M. M. Durkin, M. A. Jay, S. F. Queener, and J. W. Smith. 1987. Sources of rats free of latent Pneumocystis carinii. J. Clin. Microbiol. 25:1794-1795. Walzer, P. D., R. D. Powell, and K. Yoneda. 1979. Experimental Pneumocystis carinii pneumonia in different strains of cortisonized mice. Infect. Immun. 24:939-947. Walzer, P. D., R. Powell, Jr., K. Yoneda, M. E. Rutledge, and J. E. Milder. 1980. Growth characteristics and pathogenesis of experimental Pnemocystis carinii pneumonia. Infect. Immun. 27:928-937. Walzer, P. D., M. E. Rutledge, and K. Yoneda. 1983. Experimental Pneumocystis carinii pneumonia in C3H/HeJ and C3HeB/FeJ mice. J. Reticuloendothel. Soc. 33:1-9. Walzer, P. D., C. K. Kim, J. Linke, C. L. Pogue, M. J. Huerkamp, C. E. Chrisp, A. V. Lerro, S. K. Wixson, E. Hail, and L. D. Shultz. 1989. Outbreaks of Pneumocystis carinii pneumonia in colonies of immunodeficient mice. Infect. Immun. 57:62-70. Weir, E. C., D. G. Brownstein, and S. W. Barthold. 1986. Spontaneous wasting disease in nude mice associated with Pneumocystis carinii infection. Lab. Anim. Sci.36: 140-144. Pneumonia Virus of Mice Agent. RNA virus, family Paramyxoviridae, genus Pneumovirus. Animals Affected. Mice, rats, and hamsters. Epizootiology. Prevalence rates are approximately 50% in rat and hamster colonies and 20% in mouse colonies. Active infection in mice (and presumably in rats and hamsters) lasts about 9 days. Chronic or latent infections do not occur. Transmission is exclusively horizontal via the respiratory tract, mainly by direct contact and aerosols. Fomites are probably not important in transmission. Clinical Signs. Natural infections are subclinical, except in immunocompro- mised hosts. Chronic illness, emaciation, and death have been reported in infected athymic (nu/nu) mice. Pathology. No pathologic lesions have been associated with natural infections in immunocompetent hosts. Chronic pneumonia has been reported to occur in naturally infected athymic mice. Diagnosis. The ELISA is the most sensitive method for routine monitoring. The HAI test is also reliable, although occasionally it gives false-positive results. The CF test is useful for detecting recent PVM infections. The MAP test is useful for detecting the virus in biologic specimens. The virus can be isolated by using primary hamster kidney, BHK-21, Vero, or hamster embryo cells. Control. Cesarean derivation and barrier maintenance are effective methods of controlling infection. Interference with Research. There have been no reports of interference with research results. Suggested Reading Carrano, V. A., S. W. Barthold, D. S. Beck, and A. L. Smith. 1984. Alteration of viral
DISEASE AGENTS 41 respiratory infections of mice by prior infection with mouse hepatitis virus. Lab. Anim. Sci. 34:573-576. Jacoby, R. O., P. N. Bhatt, and A. M. Jonas. 1979. Viral diseases. Pp. 271-306 in The Laboratory Rat. Vol. I: Biology and Diseases, H. J. Baker, J. R. Lindsey, and S. H. Weisbroth, eds. New York: Academic Press. Parker, J. C., and C. B. Richter. 1982. Viral diseases of the respiratory system. Pp. 107-152 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Richter, C. B., J. E. Thigpen, C. S. Richter, and J. M. MacKenzie, Jr. 1988. Fatal pneumonia win terminal emaciation in nude mice caused by pneumonia virus of mice. Lab. Anim. Sci. 38:255-261. Smith, A. L., V. A. Carrano, and D. G. Brownstein. 1984. Response of weanling random- bred mice to infection with pneumonia virus of mice (PVM). Lab. Anim. Sci. 34:35-37. Weir, E. C., D. G. Brownstein, A. L. Smith, and E. A. Johnson. 1988. Respiratory disease and wasting in athymic mice infected with pneumonia virus of mice. Lab. Anim. Sci. 38:133-137. Polyomavirus Agent. DNA virus, family Papovaviridae, genus Polyomavirus. Animals Affected. Wild and laboratory mice. Epizootiology. Polyomavirus is very rare in mice from commercial barrier breeding facilities in the United States. The virus is highly contagious and is shed in large quantities in saliva, urine, and feces of infected mice. In persistently infected dams, the titer of virus in the kidney increases during late pregnancy. The virus has a propensity for airborne dissemination and intranasal infection. Contaminated feed and bedding can also be important sources of infection. Clinical. Natural infections are usually inapparent. Athymic (nu/nu) mice given heterotransplants of human tumors contaminated with the virus have been reported to develop a syndrome characterized by wasting and paralysis of the rear legs and tail. Pathology. Morphologic lesions, including polyomavirus-induced tumors, are usually not found in naturally infected mice. Diagnosis. The ELISA and HAI test are commonly used in routine health monitoring. The MAP test can be used for screening tumor lines and other biologic materials. Virus isolation using mouse embryo tissue culture can be useful in selected situations. Control. Cesarean derivation and barrier maintenance are usually effective in eliminating the virus because transplacental transmission does not occur. Strict isolation from polyomavirus-infected stocks and exclusion of wild mice from animal facilities are essential for preventing infection. Laminar-flow units and filter-top cages are helpful in reducing the spread of infection in laboratories in which the agent is used experimentally. Interference with Research. Polyomavirus can complicate research by con
42 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS laminating tumor lines, stocks of other viruses, and other biologic materials that are passaged in mice. Polyomavirus-contaminated tumors can cause paralysis and wasting in athymic (nu/nu) mice. Suggested Reading Eddy, B. E. 1982. Polyomavirus. Pp. 293-311 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Harper, J. S., III, C. J. Dawe, B. D. Trapp, P. E. McKeever, M. Collins, J. L. Woyclechowska, D. L. Madden, and J. L. Sever. 1983. Paralysis in nude mice caused by polomavirus- induced vertebral tumors. Pp. 359-367 in Polyomaviruses and Human Neurological Diseases. New York: Alan R. Liss. McCance, D. J., A. Sebesteny, B. E. Griffin, F. BaLLwill, R. Tilly, and N. A. Gregson.1983. A paralytic disease in nude mice associated with polyoma virus infection. J. Gen. Virol. 64:57-67. McGarrity, G. J., and A. S. Dion. 1978. Detection of airborne polyoma virus. J. Hyg. 81 :9 13. McGamty, G. J., L. L. Coriell, and V. Ammen. 1976. Airborne transmission of poly- omavirus. J. Natl. Cancer Inst. 56: 159- 162. Shah, K. V., and C. Christian. 1986. Mouse polyoma and otherpapovaviruses. Pp.505-527 in Viral and Mycoplasmal Infections of Laboratory Rodents: Effects on Biomedical Research, P. N. Bhatt, R. O. Jacoby, H. C. Morse III, and A. E. New, eds. Orlando, Fla.: Academic Press. Pseudomonas aeruginosa Agent. Gram-negative bacterium, family Pseudomonadaceae. Animals Affected. Mice, rats, humans, and numerous other species. Epizootiology. P. aeruginosa is ubiquitous, occurring widely in soil, water, sewage, and air. It is widely distributed in conventional stocks of rodents and is transmitted by fomites (e.g., contaminated food, bedding, or water) or by contact with infected humans or rodents. Clinical. The organism is sometimes part of the normal flora in the digestive tract, and clinical signs are not present. Fulminating septicemia, resulting in death with few clinical signs, can occur in immunosuppressed animals. There are a few reports of "circling" or"rolling" in mice associated with otitis media and intema caused by this organism. Pathology. Gross and histopathologic lesions are nonspecific. Occasionally there is suppurative otitis media with extension into the inner ears and to the adjacent meninges or brain. Animals subjected to severe immunosuppression (e.g., whole- body irradiation or cyclophosphamide administration) can develop fulminant septicemia with hemorrhage and multifocal necrosis in multiple organs. Diagnosis. Diagnosis of P. aeruginosa infection is made by isolation and iden- tification of the organism and exclusion of other possible causes of disease. In
DISEASE AGENTS 43 animals that have been immunosuppressed, septicemia should be demonstrated by culture of the organism. Control. Control is only necessary for immunosuppressed rodents. Cesarean derivation followed by maintenance under gnotobiotic conditions completely eliminates the organism. P. aeruginosa can be eliminated from animal facilities by rigorous sanitation measures coupled with acidification and/or hyperchlorination of the water. Interference with Research. Indigenous infections are of little importance except when the research involves immunosuppressed animals. Mice and rats naturally infected with P. aeruginosa typically die earlier than do noninfected controls when exposed to lethal doses of whole-body irradiation, cyclophosphamide, cortisone, or other immunosuppressants. Suggested Reading Brownstein, D. G. 1978. Pathogenesis of Pseudomonas aeruginosa bacteremia in cyclo- phosphamide treated mice and potentiation of endogenous streptococcal virulence. J. Infect. Dis. 137:795-801. Hammond, C. W. 1963. Pseudomonas aeruginosa infection and its effects on radiobio- logic research. Lab. Anim. Care 13(1, Part 21:6-11. Kohn, D. F., and W. F. MacKenzie. 1980. Inner ear disease characterized by rolling in C3H mice. J. Am. Vet. Med. Assoc. 177:815-817. Matsumoto, T. 1982. Influence of Escherichia colt, Klebsiella pneumonias and Proteus vulgaris on the mortality pattern of mice after lethal irradiation with y rays. Lab. Anim. (London) 16:36-39. McDougall, P. T., N. S. Wolf, W. A. Steinbach, and J. J. Trentin. 1967. Control of Pseudomonas aeruginosa in an experimental mouse colony. Lab. Anim. Care 17:204- 215. McPherson, C. W. 1963. Reduction of Pseudomonas aeruginosa and coliform bacteria in mouse drinking water following treatment with hydrochloric acid or chlorine. Lab. Anim. Care 13:737-744. Reovirus-3 Agent. RNA virus. family Reoviridae, genus Reovirus. Animals Affected. Mice, rats, hamsters, guinea pigs, and others. Epizootiology. Reovirus-3 is prevalent in contemporary rodents. Transmission is by the fecal-oral route and probably by aerosol. Infected fomites may be important because reoviruses are relatively resistant to environmental conditions. Clinical. Natural infections are subclinical. Pathology. There are no pathologic changes associated with natural infections. Diagnosis. The ELISA is the most sensitive method. The OF test is not reovirus- type specific, and the HAI test is prone to give false-positive results for reovirus-3. Virus isolations can be performed using L cells or embryonic kidney cells.
44 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Transplantable tumors and cell lines can be screened for reoviruses by using tissue- culture methods or the MAP test. Control. Cesarean derivation and barrier maintenance appear to be effective methods of control. Interference with Research. Reovirus-3 is an occasional contaminant of and may interfere with research involving transplantable tumors and cell lines. Suggested Reading Hartley, J. W., W. P. Rowe, and R. J. Huebner. 1961. Recovery of reoviruses from wild and laboratory mice. Proc. Soc. Exp. Biol. Med. 108:390-395. Nelson, J. B. 1964. Response of mice to reovirus type 3 in presence and absence of ascites tumour cells. Proc. Soc. Exp. Biol. Med. 1 16:1086-1089. Rotavirus, Mouse Agent. RNA virus, family Reoviridae, genus Rotavirus, group A. Animals Affected. Mice. Epizootiology. Mouse rotavirus is thought to be widely prevalent, but this has not been documented for contemporary mouse stocks. Transmission is by airborne infection in which contaminated dust and bedding from adjacent cages probably play key roles. Mice are most susceptible to infection from birth to about 17 days of age. Infected neonates shed high concentrations of virus in the feces from about 2 to 8-10 days postinfection. Transient viremia and viruria can occur. Mice infected after about 17 days of age shed lower concentrations of virus in the feces for 2-4 days. It is not known whether there is persistent infection or whether very low concentrations of virus are shed in the feces beyond these time points. There is no evidence for transplacental transmission. Clinical. Susceptibility to disease is age dependent, being greatest from 4 to 17 days of age. Infection in adults is subclinical. Diarrhea during the first 2 weeks of life is the only consistent sign of disease. Watery, yellow stools usually begin around 48 hours postinfection and persist for about 1 week. Varying amounts of stool accumulate around the anus and base of the tail, soiling the coats of neonates and their dams. Affected neonates may appear lethargic and have distended abdomens. Usually there is no mortality. Pathology. The infection progresses from the proximal to distal end of the intestine, involving the duodenum, jejunum, ileum, and colon. The virus selectively infects epithelial cytoplasm and destroys cells near the tips of the villi in the small intestine, resulting in mild villous atrophy. Cytoplasmic vacuoles of varying size occur in the mucosal epithelium. Diagnosis. Diagnosis is made by serologic testing using the CF test, radioim- munoassay, IFA test, ELISA, or other methods; by demonstration of rotaviral antigen lesions in the small intestine; or by isolation of the virus using trypsinized primary monkey kidney cells.
DISEASE AGENTS 45 Control. Cesarean derivation and exclusion by barrier maintenance are the traditional methods of control. However, it is more practical to use such methods as isolation and quarantine of individual breeding pairs with subsequent selection of seronegative progeny for breeding. The use of filter-top cage systems can be beneficial in controlling the transmission of infection among subpopulations housed in the same room. Interference with Research. Infection can alterresearch results in studies using infant mice. Protein-calorie deprivation of nursing dams increases the severity of diarrhea, increases mortality, and reduces the weight gains of MRV-infected infant pups. Folic acid deficiency also increases the severity of diarrhea! disease. Rotavirus-infected infant mice show increased mortality when challenged with enterotoxigenic Escherichia coli. Suggested Reading Eiden, J., H. M. Ledennan, S. Vonderfecht, and R. Yolken. 1986. T-cell-deficient mice display normal recovery from experimental rotavirus infection. J. Virol. 57:706-708. Eydelloth, R. S., S. L. Vonderfecht, J. F. Sheridan, L. D. Enders, and R. H. Yolken. 1984. Kinetics of viral replication and local and systematic immune responses in experimental rotavirus infection. J. Virol. 50:947-950. Ferner, W. T., R. L. Miskuff, R. H. Yolken, and S. L. Vonderfecht. 1987. Comparison of methods for detection of serum antibody to munne rotavirus. J. Clin. Microbiol.25: 1364- 1369. Kraft, L. M. 1982. Viral diseases of the digestive system. Pp. 159-191 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Little, L. M., and J. A. Shadduck. 1982. Pathogenesis of rotavirus infection in mice. Infect. Immun. 38:755-763. Riepenhoff-Talty, M., T. Dharakul, E. Kowalski, D. Stennan, and P. L. Orga. 1987. Rotavirus infection in mice: Pathogenesis and immunity. Adv. Exp. Med. Biol. 216B:1015-1023. Salmonella ententidis Agent. Gram-negative bacterium, family Enterobacteriaceae, tribe Salmonellae. There are more than 1,500 serotypes of Salmonella enteritidis. Animals Affected. Mice, rats, humans, and many others. Epizootiology. Prevalence in laboratory rodents is unknown, but there is evidence that subclinical infections caused by weakly virulent strains are common in the United States. Infections are acquired primarily by ingestion, usually from contaminated food, water, or bedding. After initial colony infection, about 5% of the animals become asymptomatic, chronic carriers and shed organisms in the feces for many months. Clinical. Most infections in mice are subclinical. In clinical outbreaks, nonspecific signs, including ruffled fur, hunched posture, reduced activity, weight
46 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS loss, conjunctivitis, and variable mortality, follow an incubation period of 2-6 days. Diarrhea occurs in only about 20% of animals. Enzootically infected breeding colonies can have alternating periods of inapparent infection and overt clinical disease. Litter sizes and birth weights can be reduced. Clinical salmonellosis in rats is extremely rare. Virulence and dose of the organism and the host's age, strain, in- testinal microflora, nutritional state, immune status, and intercurrent infections affect the expression of disease. C3H/HeN, C3H/St, C3H/Bi, CBA/Ca, BRVR, A/J, A/HeN, SWR/J, and DBA/2 mice are relatively resistant to S. enteritidis-induced disease, while BSVS, DBA/1, BALB/c, C57BL/6, C3H/HeJ, and CBA/N mice are relatively susceptible. Three distinct genetic loci affect susceptibility: Ity (immunity to S. typhimurium), Lps (lipopolysaccharide response), and xid (X-linked immune deficiency). Weanling mice are more susceptible than adults. Susceptibility to clin- ical disease is increased by food and water deprivation, nutritional iron deficiency in rats, nutritional iron overload in mice, pretreatment with sodium bicarbonate by Savage, and administration of morphine sulfate to slow gastrointestinal motility. Pathology. Gross lesions are extremely variable, depending on the stage of disease. Animals that die of acute infection may have only hyperemia and congestion of visceral organs. Animals that survive a week or longer often appear emaciated, have hyperemia and thickening of the ileal and cecal walls, an empty or fluid-filled large intestine, multiple white or yellow foci in the liver, splenomegaly, enlarged mesenteric lymph nodes, and fibrinous exudate in the peritoneal cavity. Chronic carriers usually do not have gross lesions. The predominant microscopic lesions of salmonellosis are ileocecitis, mesenteric lymphadenitis, and multifocal inflammation in the liver and spleen, each varying in character depending on the stage of disease. There is multifocal to diffuse destruction of villous epithelium in the ileum, with blunting of villi and hyperplasia of crypt epithelium and purulent to pyogranulomatous inflammation in the lamina propria. Similar changes occur in the cecum, which is more severely affected in rats than in mice. Ulcerative cecitis, accompanied by severe pyogranulomatous inflammation in the lamina propria and purulent to chronic inflammation leading to atrophy and cyst formation in the paracecal lymph nodes, is characteristic of salmonellosis in rats. Multifocal purulent, pyogranulomatous, or granulomatous inflammation occurs in the mesenteric lymph nodes, liver, and spleen. So-called cell-fragment thrombi commonly occur in liver and spleen in which necrotic foci break into venous channels. Peritonitis is commonly caused by extension of the infection through the capsule of the liver, lymph nodes, or spleen. Cholangitis and cholecystitis are seen infrequently. Pyo- granulomatous inflammation occasionally occurs in other organs such as the lungs. Diagnosis. S. enteritidis is diagnosed by cultural isolation and serologic typing of isolates. It is usually best to culture the liver, spleen, intestine, feces, and blood. Characteristic histopathologic lesions help to rule out diseases with similar gross lesions (e.g., mousepox, Tyzzer's disease, and streptobacillosis in mice). Carriers must be detected by cultural isolation of the organism because there is no satisfactory serologic method for testing suspected carriers.
DISEASE AGENTS 47 Control. Affected animals pose a zoonotic risk to personnel, are a source of infection for other animals, and are unsuitable for research. The usual method of control is to destroy the entire population and replace it with animals from a pathogen-free source. For extremely valuable stocks, cesarean derivations can be attempted. Prevention is by barrier maintenance, avoidance of Salmonella-con- taminated food and water, and exclusion of infected animals and wild rodents from the animal facility. Regular monitoring is necessary. Interference with Research. Salmonella-infected mice can have a nonspecific resistance to challenge with other intracellular parasites such as Listeria monocytogenes. Prior immunization of mice with viable S. enteritidis has resulted in the suppression of growth of transplantable tumors. Concurrent infections of S. enteritidis and Plasmodium berghei in mice have resulted in higher mortality than infection by either agent alone. S. enteritidis infection has caused reduced blood glucose and hepatic enzyme levels. Mice orally infected with S. enteritidis have reduced intestinal enzyme activities. Susceptibility to infection is increased by some experimental procedures. Suggested Reading Collins, F. M., and P. B. Carter. 1978. Growth of salmonellae in orally infected germfree mice. Infect. Immun. 21:41-47. Ganaway, J. R. 1982. Bactenal and mycotic diseases of the digestive system. Pp. 1-20 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Casebolt, D. B., and T. R. Schoeb. 1988. An outbreak in mice of salmonellosis caused by Salmonella enteritidis serotype enteritidis. Lab. Anim. Sci. 38:190-192. Farmer, J. J., III, B. R. Davis, F. W. Hickman-Brenner, A. McWhorter, G. P. Huntley-Carter, M. A. Asbury, C. Riddle, H. G. Wathen-Grady, C. Elias, G. R. Fanning, A. G. Steigerwalt, C. M. O'Hara, G. K. Morris, P. B. Smith, and D. J. Brenner. 1985. Biochemical identification of new species and biogroups of Enterobacteriaceae isolated from clinical specimens. J. Clin. Microbiol. 21:46-76. Hsu, H. -S. 1989. Pathogenesis and immunity in munne salmonellosis. Microbiol. Rev. 53:390-409. Kirchner, B. K., L. W. Dixon, R. H. Lentsch, and J. E. Wagner. 1982. Recovery and pathogenicity of several salmonella species isolated from mice. Lab. Anim. Sci.32:506- 507. Sendai Virus Agent. RNA virus, family Paramyxoviridae, genus Paramyxovirus. Animals Affected. Mice, rats, hamsters, and possibly guinea pigs. Epizootiology. Sendai virus (SV) is extremely contagious. Prevalence is considered high in colonies of laboratory mice and rats worldwide. Natural infection occurs via the respiratory tract. Transmission is by direct contact and fomites and is highly efficient. Viral replication is thought to be limited to the respiratory tract and occurs for only about 1 week postinfection.
48 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Clinical. Natural SV infection in rats is usually subclinical. In pregnant rats, the infection can cause fetal resorptions, retarded embryonic development, and mortal- ity of neonates. Susceptibility to SV-induced disease in mice is dependent on the strain or stock. Those that are the most susceptible include 129/ReJ, 129/J, DBA/1J, DBA/2J, and S-nu/nu (Swiss carrying the mutation nude). Strains of intermediate susceptibility include A/HeJ, A/J, SWR/J, C57BL/lOSn, and BALB/c. The most resistant strains and stocks include SJL/J, RF/J, C57BL/6J, and S. The mode of inheritance and mechanisms of host resistance are poorly understood. Clinical disease caused by natural SV infection in mice falls into one of two patterns. Enzootic (subclinical) infection commonly occurs in breeding populations. Adults have active immunity due to prior infection, and newborn mice are passively protected by maternal antibody until around 4-8 weeks of age, when they become infected. Recovery is prompt end usually without morbidity or mortality. Epizootic (clinically apparent) infection occurs when a mouse population is first infected. Infection quickly spreads through the entire population. Signs are variable but may include chattering; mild respiratory distress; and prolonged gestation in adults, deaths in neonates and sucklings, and poor growth in weanling and young adults. Breeding colonies return to normal productivity in 2 months and thereafter main- tain the enzootic pattern of infection. Epizootics of disease that exceed these gen- eral patterns in clinical severity should arouse suspicion of complication by other agents, particularly Mycoplasma pulmonis and cilia-associated respiratory bacillus. Pathology. There are few gross lesions in uncomplicated SV infections. The lungs can appear focally reddened or atelectatic, and serous fluid can be visible in the pleural and pericardial cavities. The most severe lesions are seen in mice that are infected as sucklings or weanlings and in mice of the more susceptible strains. Severe necrotizing bronchitis and bronchiolitis often cause intense inflammatory injury to terminal bronchioles, resulting in scarring with severe distortion of the smaller airways and formation of polypoid outgrowths into bronchiole lumens. There is also pronounced hyperplasia of airway epithelium resulting in peribron- chiolar adenomatous hyperplasia that can persist throughout life. In aged mice the air spaces in these lesions are often filled with mucus, large macrophages, and cellular debris. There can be large eosinophilic crystals in the air spaces, cyto- plasm of the macrophages, and cells forming the adenomatoid structures. The terminal bronchioles of rats may be scarred and distorted but do not show the hyperplastic peribronchiolar changes seen in mice. Athymic (nu/nu) mice develop chronic pneumonia similar to that in immuno- competent mice but have abundant intranuclear and intracytoplasmic inclusions in laryngeal, tracheal, bronchial, and bronchiolar epithelia, as well as in type I and II pneumocytes and alveolar macrophages. The virus persists for 10 weeks or longer. Athymic (rnulrnu) rats also have increased susceptibility to SV and de- velop a similar chronic lung disease. Diagnosis. The ELISA is the test of choice for routine serologic monitoring. The
DISEASE AGENTS 49 ELISA also successfully detects anti-SV antibody in infected athymic (nu/nu) mice. An avidin-biotin-peroxidase-complex method has been used successfully for demonstrating SV antigen in histologic sections. Isolation of SV can be achieved by using BHK-21 or primary monkey kidney cell cultures or by inoculating the amniotic or allantoic sacs of 8- to 10-day-old embryonated hen's eggs. The MAP test can be used for determining contamination of transplantable tumors and other biologic materials. Control. To prevent the introduction of infection, only animals known to be free of SV should be obtained, and the animals should be maintained under strict barrier conditions. In addition, all biologic matenals, such as transplantable tumors, should be pretested and shown to be free of the virus. If SV infection is detected, prompt elimination of infected sub populations is essential to prevent spread of the infection to other rodents. A less effective alternative is to place the infected ani- mals under strict quarantine, remove all young and pregnant females, suspend all breeding, and prevent the addition of other susceptible animals for a period of 6-8 weeks until the infection has run its course and the virus has been eliminated naturally. Cesarean derivation is effective but usually is not warranted. Vaccina- tion might prove useful in some situations. Interference with Research. Experimental SV infection alters the phagocytic function of pulmonary macrophages. Concurrent SV and M. pulmonis infections are synergistic in mice, causing disease of far greater severity than that caused by either agent alone. It has been reported (but not confirmed) that infected mice have deficiencies in T- and B-cell function that persist throughout life, but most of the evidence indicates that such deficiencies are transient, lasting only a few weeks. SV infection inhibits in vitro mitogenesis of lymphocytes, increases natural killer cell- mediated cytotoxicity, and increases cytotoxic lymphocyte responses after in vivo stimulation with SV-coated syngeneic cells. Isograft rejection is altered and the neoplastic response to respiratory carcinogens can be increased or decreased. Wound healing is delayed. Cyclophospharnide increases the clinical end pathologic severity of SV infection. SV infection in rats alters the mitogenic responses of T cells, reduces the severity of adjuvant arthntis, and decreases antibody response to sheep erythrocytes. SV infection alters host responses to transplantable tumors. Suggested Reading Brownstein, D. G. 1986. Sendai virus. Pp. 37-61 in Viral and Mycoplasmal Infections of Laboratory Rodents: Effects on Biomedical Research, P. N. Bhatt, R. O. Jacoby, H. S. Morse III, and A. E. New, eds. Orlando, Fla.: Academic Press. Castleman, W. L., L. J. Brundage-Anguish, L. Kreitzer, and S. B. Neuenschwander. 1987. Pathogenesis of bronchiolitis and pneumonia induced in neonatal and weanling rats by parainfluenza (Sendai) virus. Am. J. Pathol. 129:277-286. Hall, W. C., R. A. Lubet, C. J. Henry, and M. J. Collins, Jr. 1985. Sendai virus Disease processes and research complications. Pp. 25-52 in Complications of Viral and My
50 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS coplasmal Infections in Rodents to Toxicology Research and Testing, T. E. Hamm, Jr., ed. Washington, D.C.: Hemisphere. Jakab, G. J. 1981. Interactions between Sendai virus and bacterial pathogens in the mur~ne lung: A review. Lab. Anim. Sci. 31:170-177. Parker, J. C., and C. B. Richter. 1982. Viral diseases of the respiratory system. Pp. 107-152 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Schoeb, T. R., K. C. Kervin, and J. R. Lindsey. 1985. Exacerbation of muting respiratory mycoplasmosis in gnotobiotic F344/N rats by Sendai virus infection. Vet. Pathol. 22:272-282. Sialodacryoadenitis Virus Agent. RNA virus, family Coronaviridae, genus Coronavirus. Animals Affected. Rats. Mice are susceptible to experimental infection, but natural infection has not been reported for this species. Epizootiology. Sialodacryoadenitis virus (SDAV) is one of the most common viruses in laboratory rats. It is highly contagious and transmitted by contact and aerosol. It is not transmitted vertically. LEW, WAG/Rij, and SHR rats are more susceptible than WI (Wistar), SD (Sprague-Dawley(~)), LE (Long Evans), and F344. Clinical Signs. Enzootic disease: Adults are immune because of previous infection. Suckling rats have a mild, transient (1 week or less) conjunctivitis accompanied by blinking. Occasionally, exudate causes the eyelids to adhere together. Clinical signs usually disappear by weaning. Epizootic disease: Overt disease occurs in naive rat populations. There is a sudden high incidence of overt disease. Signs include cervical edema; sneezing; photophobia; serous to seropuru- lent, often porphyrin-stained nasal and ocular discharge; corneal ulceration; and keratoconus. There is high morbidity but no mortality. Most clinical signs dis- appear in a week, but the eyes might be more prominent than normal for 1-2 weeks because of inflammation of retroorbital tissues. Pathology. SDAV has a positive tropism for serous or mixed serous-mucous tubuloalveolar glands. The submaxillary and parotid salivary, exorbital lacrimal, Harderian, and intraorbital lacrimal glands are the major target organs. There are also mild changes in the cervical lymph nodes, thymus, and respiratory tract. Characteristically, by 5 days postinfection, there is diffuse necrosis of alveolar and ductal epithelium in the salivary and lacrimal glands, and polymorphonuclear leukocytes quickly infiltrate the necrotic debris and interstitium accompanied by interstitial edema. The ductal epithelium is rapidly repaired, becoming hyperplastic and squamous in appearance by 10 days postinfection. Intranuclear inclusions are occasionally observed. Complete restoration of normal glandular architecture requires about 30 days. Eye lesions include interstitial keratitis, corneal ulceration, keratoconus, synechia, hypopyon, hyphema, and conjunctivitis. Sequelae of infection can include megaloglobus with lenticular and retinal degeneration. Thymic lesions are limited to focal necrosis of the cortex and medulla with some
DISEASE AGENTS 51 widening of the interlobular septae. Focal necrosis and lymphoid hyperplasia occur in cervical lymph nodes. Diagnosis. The ELISA and the IFA test are more sensitive than the CF test. Presumptive diagnosis often can be based on characteristic histologic changes in Harderian, submaxillary, and parotid glands. Lesions can be bilateral or unilateral and are frequently found in animals with negative serologic tests for coronavirus antibody. The virus can be isolated by culture methods using primary rat kidney cells or by intracerebral inoculation of neonatal mice. The virus canbe demonstrated in affected tissues by immunofluorescence for only about 7 days postinoculation. Control. Control requires very strict adherence to preventative measures, including procurement only of rats known to be free of SDAV and adherence to strict barrier housing procedures. Prompt elimination of infected subpopulationks) is essential to prevent spread of infection to other rodents. A less effective al- ternative is to place infected animals under strict quarantine, remove all young and pregnant females, suspend all breeding, and discontinue adding other suscep- tible animals for a period of 6-8 weeks until the infection has run its course and the virus has been eliminated naturally. Interference with Research. The virus can seriously complicate studies in- volving the eyes, salivary glands, lacrimal glands, or respiratory tract. It is reported to reduce reproductive rate in breeding populations and slow growth rate of young rats. It inhibits phagocytosis and interleukin-1 production by pulmonary macro- phages. SDAV infection exacerbates concurrent Mycoplasma pulmonis infection. Suggested Reading Bhatt, P. N., D. H. Percy, and A. M. Jonas. 1972. Characterization of the virus of sialodacryoadenitis of rats: A member of the coronavirus group. J. Infect. Dis.126: 123- 130. Boschert, K. R., T. R. Schoeb, D. B. Chandler, and D. L. Dillehay. 1988. Inhibition of phagocytosis and interleukin-1 production in pulmonary macrophages from rats with sialodacryoadenitis virus infection. J. Leukocyte Biol. 44:87-92. Jacoby, R. O., P. N. Bhatt, and A. M. Jonas. 1975. Pathogenesis of sialodacryoadenitis in gnotobiotic rats. Vet. Pathol. 12: 196-209. Jacoby, R. O. 1986. Rat coronavirus. Pp.625-638 in Viral and Mycoplasmal Infections of Laboratory Rodents: Effects on Biomedical Research, P. N. Bhatt, R. O. Jacoby, H. C. Morse III, and A. E. New, eds. Orlando, Fla.: Academic Press. Schoeb, T. R., and J. R. Lindsey. 1987. Exacerbation of murine respiratory mycoplasmosis by sialodacryoadenitis virus infection in gnotobiotic F344 rats. Vet. Pathol.24:392-399. Wojcinski, Z. W., and D. H. Percy. 1986. Sialodacryoadenitis virus-associated lesions in the lower respiratory tract of rats. Vet. Pathol. 23:278-286. Staphylococcus aureus Agent. Gram-positive bacterium, family Micrococcaceae. Animals Affected. Mice, rats, humans, and many others.
52 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Epizootiology. S. aureus commonly colonizes the nasopharynx, lower diges- tive tract, fur, and skin. It is readily cultured from cages, room surfaces, and per- sonnel. The epizootiology of pathogenic types in animals is poorly understood. Human carriers might be an important source of infection for rodent colonies and vice versa. Many rodent colonies have infection without overt disease. Pathogen- esis is probably dependent on many factors, including phage types, traumatic injures of skin or mucosal surfaces, host factors, and sanitation. Clinical. Rats: Ulcerative dermatitis with intensely pruritic, moist eczematous lesions (usually 1-2 cm in diameter) occurs on the lateral surfaces of the shoulders and neck. Lesions appear to be initiated or aggravated by scratching. Mice: Ulcerative dermatitis with moist eczematous lesions occurs on the face, neck, ears, and forelegs. Multiple abscesses and botryomycotic granulomas can develop in deeper tissues of the face, including the orbital tissues, facial muscles, pendontium, and mandibles. Purulent lesions of varying size occur commonly around the eyes and on the face of athymic (nu/nu) mice. Abscesses also occur in preputial glands, which become film and enlarged to a few millimeters in diameter. The highest incidence of preputial gland abscesses is in strain C3H/HeN. In strain C57BL/6N, the organism has been associated with a syndrome involving self-mutilation of the penis. Pathology. Suppurative inflmnmation is a hallmark of tissue invasion by S. aureus. In ulcerative dermatitis, there is destruction of the epidermis, and the underlying dermis contains pustules; abscesses; and, eventually, chronic or granulomatous inflmnmation. Large numbers of organisms are usually present and can be demonstrated readily in Gram-stained sections or imprints. Pre- putial abscesses in C3H/HeN mice apparently are caused by ascending infection from the ducts of the preputial glands. Diagnosis. Diagnosis depends on the isolation and identification of S. aureus and exclusion of other agents (e.g., dermatophytes and mites) as possible causes of the lesions. Control. The best methods of control are improved sanitation, frequent sterilization of cages and other equipment, elimination of equipment that could cause skin injury, and reduction in the number of animals per cage. Interference with Research. S. aureus alters host immune responses (e.g., activates suppressor B cells). Rats maintained on prolonged immunosuppression with corticosteroids can develop S. aureus-induced renal abscesses. Suggested Reading Clarke, M. C, R. J. Taylor, G. A. Hall, and P. W. Jones. 1978. The occurrence of facial and mandibular abscesses associated with Staphylococcus aureus. Lab. Anim. (London) 12:121-123. Easmon, C. S. F., and A. A. Glynn. 1979. The cellular control of delayed hypersensitivity to Staphylococcus aureus in mice. Immunology 38:103-108. Hong, C. C., and R. D. Ediger. 1978. Self-mutilation of the penis in C57BL/6N mice. Lab. Anim. (London) 12:55-57.
DISEASE AGENTS 5 3 Hong, C. C., end R. D. Edger. 1978. Preputial gland abscess ~ ce. Lab. Anim. Sci. 28:153-156. Fox, J. G., S. M. Niemi, J. C. Murphy, and F. W. Quimby. 1977. Ulcerative dermatitis in the rat. Lab. Anim. Sci. 27:671-678. Wagner, J. E., D. R. Owens, M. C. LaRegina, and G. A. Vogler. 1977. Self-aauma and Staphylo- coccus aureus In ulcerative dennahus In rats. J. Arn. Vet. Med. Assoc. 171:839-841. Streptobacillus moniliformis Agent. Gram-negative bacterium; taxonomy uncertain. Animals Affected Rats. Mice, guinea pigs, humans, and others can contract the infection from rats. Epizootiology. S. moniliformis is a commensal of the nasopharynx in wild and conventionally reared laboratory rats. It has not been reported in cesarean-derived, barrier-maintained rats or mice. Epizootic disease is most likely to occur in mice housed near infected rats. Transmission is by rat bites, aerosols, and fomites. Clinical. In rats the infection is subclinical. The organism can be isolated from the nasopharynx, middle ear, respiratory tract, and subcutaneous abscesses. In mice, early signs include conjunctivitis, photophobia, cyanosis, diarrhea, anemia, hemoglobinuria, emaciation, and high mortality. Septicemia clears in survivors in a few weeks, but infection persists around the joints for about 6 months. During this chronic phase of infection, there can be diffuse swelling and reddening of limbs or tail, with the development of chronic arthritis, deformity and ankylosis, or ampu- tation (ectromelia). Spinal lesions, accompanied by posterior paralysis, kyphosis, and priapism, can occur. Pregnant females can abort or produce stillborn young. Pathology. In mice, early lesions are associated with septicemia, including focal necrosis of the spleen and liver, splenomegaly, and lymphadenopathy. Subsequent lesions are primarily those of chronic arthritis in various stages of development and severity. Diagnosis. Diagnosis depends on cultural isolation and identification of the organism. Differential diagnosis should distinguish between S. moniliformis-in- duced disease and mousepox or bacterial septicemias caused by other organisms (e.g., Corynebacterium kutscheri, Salmonella enteritidis). Control. Cesarean derivation, barrier maintenance, and regular monitoring for rodent pathogens by a comprehensive health surveillance program are the best methods of control. Mice should not be housed in the same room as rats that have not been monitored for S. moniliformis infection. Interference with Research. There have been no reports of interference with the results of research in which rats were used. S. moniliformis can cause high mor- tality in mice and is a serious zoonotic infection in humans.* * In humans the incubation period is usually 3-10 days, followed by the abrupt onset of fever, chills, vomiting, headache, and myalgia. There is a maculopapular rash that is most pronounced on the extremities. Arthritis occurs in about two-thirds of cases, and other complications such as endocarditis and focal abscesses occur in some untreated cases.
54 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Suggested Reading Anderson, L. C., S. L. Leary, and P. J. Manning. 1983. Rat-bite fever in animal research laboratory personnel. Lab. Anim. Sci. 33:292-294. Lerner, E. M., II, and L. Sokoloff. 1959. The pathogenesis of bone and joint infections produced by Streptobacillus moniliformis. Arch. Pathol. 67:364-372. Savage, N. 1984. Genus Streptobacillus. Pp. 598-600 in Bergey's Manual of Systematic Bacteriology, vol. I, N. R. Kreig and J. G. Holt, eds. Baltimore: Williams & Wilkins. Savage, N. L., G. N. Joiner, and D. W. Florey. 1981. Clincial, microbiological, and histological manifestations of Streptobacillus moniliformis-induced arthritis in mice. Infect. Immun. 34:605-609. Weisbroth, S. H.1979. Bacterial and mycotic diseases. Pp. 191-241 in The Laboratory Rat. Vol. I: Biology and Diseases, H. J. Baker, J. R. Lindsey, and S. H. Weisbroth, eds. New York: Academic Press. Streptococcus pneumonias Agent. Gram-positive bacterium, family Streptoccaceae. Animals Affected. Only subclinical infection has been reported in mice. Disease has been reported occasionally in rats, guinea pigs, and monkeys. Humans are the main natural hosts. Epizootiology. Transmission is mainly by aerosol. Clinical Signs. Signs include dyspnea, weight loss, hunched posture, snuffling respiratory sounds, and abdominal breathing. Clinical onset can be sudden, and young rats are affected most often. Pathology. Predominant lesions in rats are suppurative rhinitis and otitis media. The disease often extends into distal airways, causing acute tracheitis and fibrinous lobar pneumonia, and into organs adjacent to the lungs, causing fibrinous pleuritis or empyema, fibrinous pericarditis, and/or acute mediastinitis. Lesions associated with severe bacteremia include suppurative arthritis, meningitis, hepatitis, splenitis, peritonitis, and orchitis. Splenic and testicular infarcts can occur. Abdominal lesions are frequently the primary cause of death. Diagnosis. Diagnosis is made by isolating the organism from sites with characteristic lesions and excluding other possible causes and contributors to the disease. Control. Cesarean derivation and barrier maintenance are extremely effective methods of control. It might be helpful for personnel to wear masks because of the high prevalence of the infection in humans. Interference with Research. S. pneumoniae-induced septicemia alters hepatic metabolism, serum biochemistries, and thyroid function. Studies involving the rat respiratory tract can be jeopardized. Suggested Reading Fallon, M. T., M. K. Reinhard, B. M. Gray, T. W. Davis, and J. R. Lindsey. 1988. Inapparent
DISEASE AGENTS 55 Streptococcus pneumonias type 35 infections in commercial rats and mice. Lab Anim. Sci. 38:129-132. Harding, B., F. Kenny, F. Given, B. Murphy, and S. Lavelle. 1987. Autotransplantation of splenic tissue after splenectomy in rats offers partial protection against intravenous pneumococcal challenge. Eur. Surg. Res. 19: 135 - 139. Johanson, W. G., Jr., S. J. Jay, and A. K. Pierce. 1974. Bacterial growth in viva. An important determinant of the pulmonary clearance of Diplococcus pneumonias in rats. J. Clin. Invest. 53:1320- 1325. Quie, P. G., G. S. Giebink, and J. A. Winkelstein, eds. 1981. Thepneumococcus. Rev. Infect. Dis. 3:183-395. Weisbroth, S. H. 1979. Bacterial and mycotic diseases. Pp.193-241 in The Laboratory Rat. Vol. I: Biology and Diseases, H. J. Baker, J. R. Lindsey, and S. H. Weisbroth, eds. New York: Academic Press. Wood, W. B., Jr., and M. R. Smith. 1950. Host-parasite relationships in experimental pneumonia due to pneumococcus type III. J. Exp. Med. 92:85-100. Theiler's Murine Encephalomyelitis Virus Agent. RNA virus, family Picornaviridae, genus Enterovirus. Animals Affected. Laboratory mice and rats. The infection has not been reported in wild mice or rats. Epizootiology. The prevalence of Theiler's murine encephalomyelitis virus (TMEV) in mice is generally thought to be very low in barrier breeding colonies in the United States. Prevalence in laboratory rats is unknown. In naturally infected mice, there is a low titer of virus in intestinal mucosa; intestinal contents; feces; and, less frequently, mesenteric lymph nodes. Virus shedding in the feces has been documented to occur as long as 154 days postinfection. Transmission is by the fecal-oral route. Transplacental infection does not occur. Clinical. Natural infections in mice are usually inapparent and, presumably, are caused by less virulent, wild-type strains of TMEV resembling Theiler's original (TO) strain of virus. Clinical disease appears at a rate of only 1 in 4,000-10,000 infected animals. Affected mice may have flaccid paralysis of one or both rear legs. There is little or no mortality. Signs of infection with a highly virulent strain of TMEV in one natural outbreak included circling, rolling, hyperexcitability, convul- sions, tremors, weakness or flaccid paralysis of the hind legs, and high mortality. Cinical signs of infection with the MHG strain of TMEV in rats included circling, incoordination, tremors, and torticollis. Pathology. Natural disease in mice results from the rare occurrence of viremia, i.e., the dissemination of virus from the intestine to the spinal cord and brain. This occurs most frequently around 6-10 weeks of age. The predominant lesion is poliomyelitis, with necrosis and neuronophagia of anterior horn cells and nonsuppurative inflammation composed primarily of lymphocytes. Little if any secondary demyelination is seen in the natural disease. TMEV can be isolated from the lesions for at least 1 year.
56 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Diagnosis. The ELISA is the method of choice for serologic screening. If the HAI test with GDVII antigen and human type O erythrocytes is used, it is essential to perform the test at 4°C to avoid false-positive results. The CF and serum neutral- ization tests might also be useful for some purposes, such comparison of the cross-reactivity of TMEV strains. The MAP test can be used for screening bio- logic materials. Definitive diagnosis usually is made by isolating the virus from the spinal cords or brains of mice with clinical disease, but it is also possible to isolate the virus from the intestinal contents of mice with asymptomatic infections. Control. The most practical method of control is to obtain mice from breeding populations that have been shown serologically to be free of the infection, followed by barrier maintenance and regular testing to reconfirm TMEV-free status. TMEV infection has been eliminated from valuable mouse stocks by foster nursing infant mice on TMEV-free mice or rats. Cesarean derivation is also effective but usually is not warranted. Interference with Research. Indigenous TMEV infections occasionally in- terfere with studies of other unrelated viruses in mice. Suggested Reading Brownstein, D., P. Bhatt, R. Ardito, F. Paturzo, and E. Johnson. 1989. Duration and patterns of transmission of Theiler's mouse encephalomyelitis virus infection. Lab. Anim. Sci. 39:299-301. Downs, W. G. 1982. Mouse encephalomyelitis virus. Pp. 341-352 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. McConnell, S. J., D. L. Huxsoll, F. M. Garner, R. O. Spertzel, A. R. Warner, Jr., and R. H. Yager. 1964. Isolation and characterization of a neurotropic agent (MHG virus) from adult rats. Proc. Soc. Exp. Biol. Med. 115:362-367. Theiler, M., and S. Gard. 1940. Encephalomyelitis of mice. III. Epidemiology. J. Exp. Med. 72:79-90. Thompson, R., V. M. Harnson, and F. P. Myers. 1951. A spontaneous epizootic of mouse encephalomyelitis. Proc. Soc. Exp. Biol. Med. 77:262-266. von Magnus, H., and P. von Magnus. 1948. Breeding of a colony of white mice free of encephalomyelitis virus. Acta Pathol. Microbiol. Scand. 26:175-177. Thymic Virus, Mouse Agent. Considered a herpesvirus because of its ultrastructural features and properties of heat and ether lability. Animals Affected. Wild and laboratory mice. Epizootiology. Wild mice apparently serve as reservoir hosts. Prevalence in laboratory mice is unknown; however, limited data suggest natural infections might be common. Mouse thymic virus (MTV) apparently occurs as a persistent subclinical infection in salivary glands, with virus shed in saliva. Horizontal transmission is most important, but vertical transmission cannot be ruled out.
DISEASE AGENTS 57 Clinical Signs. Natural infections are subclinical. Pathology. There are no pathologic changes associated with natural infections. Diagnosis. The ELISA is the most sensitive serologic test, although the IFA and CF tests are also in use. However, these tests are only useful for screening adult mice; neonatally infected mice do not produce serum antibody. To detect MTV infection in neonatal mice, it is necessary to inoculate pathogen-free neonatal mice with salivary gland homogenate, saliva, or other material from the neonatal mice to be tested. If the test mice are infected with MTV, histologic examination of the thymuses, lymph nodes, and spleens of the pathogen-free mice 10-14 days postinoculation will disclose lymphoid necrosis and intranuclear inclusions. Virus isolation is impossible because no cell culture system is known to support its growth. Control. No data are available. Interference with Research. MTV infection might complicate experiments involving the passage of tissues in neonatal mice. Suggested Reading Cross, S. S. 1973. Development of Bioassays and Studies on the Biology of Mouse Thymic Virus. Ph.D.Dissertation. Washington,D.C.: George Washington University. Cross, S. S., J. C. Parker, W. P. Rowe, and M. L. Robbins. 1979. Biology of mouse thymic virus, a herpesvirus of mice, and the antigenic relationship to mouse cytomegalovirus. Infect. Immun. 26:1186-1195. Lussier, G., D. Guenette, W. R. Shek, and J. P. Descoteaux. 1988. Detection of antibodies to mouse thymic virus by enzyme-linked immunosorbent assay. Can. J. Vet. Res.52:236- 238. Morse, S. S. 1987. Mouse thymic necrosis virus: A novel murine lymphotropic agent. Lab. Anim. Sci. 37:717-725. Morse, S. S., and J. E. Valinsky. 1989. Mouse thymic virus (MTLV). A mammalian herpesvirus cytolytic for CD4+(L3T4+) T lymphocytes. J. Exp. Med. 169:591-596. Rowe, W. P., and W. I. Capps. 1961. A new mouse virus causing necrosis of the thymus in newborn mice. J.Exp.Med.113:831-844. DERMATOPHYTES Tr~chophyton spp. and Microsporum spp. Agents. Fungi. Animals Affected. Mice, rats, humans, and numerous other animals. Epizootiology. Dermatophytes have not been reported in cesarean-derived, barrier-maintained rodent stocks. Other animals are probably the major reservoirs of infection for mice and rats. The organisms are parasites of keratin, i.e., hair and superficial layers of skin. Clinical. Infections are rare, and when they do occur, they are usually sub- clinical. Clinical disease has been seen more frequently in mice than in rats.
58 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Lesions consist of irregularly defined areas of alopecia with a scaly to crusty appearance and occasional pustules at the edges. Lesions most commonly occur on the head near the mouth and eyes, but they can be found anywhere on the body. Pathology. Uncomplicated lesions are very subtle; microscopic examination reveals only thickening of the stratum corneum in sections stained with hematoxylin and eosin. Special stains such as periodic acid-Schiff or Gridley's fungus stain are valuable in demonstrating the organisms. Diagnosis. If infection is suspected in asymptomatic animals, several animals should be held over opened plates of culture medium while the fur is brushed. The plates should then be cultured for dermatophytes. In clinical cases, the hair should be plucked or skin scrapings should be taken from the periphery of the lesions and mounted onto slides in 10% potassium hydroxide for visualization of hyphae and endospores. Definitive diagnosis is dependent on culture and identification of the organisms by using Sabouraud's or other dermatophyte mediums. Control. Where feasible, infected stocks should be destroyed and replaced by dermatophyte-free stock after thorough sterilization and disinfection of the facili- ties and equipment. Treatment of affected animals is not recommended. For prevention of infection, barrier maintenance appears to be effective. Rodents should be housed well away from laboratory animal species known to be more frequently infected (e.g., cats and dogs). Dermatophyte infections of mice and rats are rare in contemporary stocks and, therefore, do not represent important zoonoses. Suggested Reading Balsari, A., C. Bianchi, A. Cocilova, I. Dragoni, G. Poll, and W. Ponti. 1981. Dennatophytes in clinically healthy laboratory animals. Lab. Anim. 15:75-77. Fox, J. G., and J. B. Brayton. 1982. Mycoses. Pp. 411-413 in The Mouse in Biomedical Reseach. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Kunstyr, I. 1980. Laboratory animals as apossible source of dermatophytic infections inman. Zbl. Bakt. Med. Mycol. 8:361-367. Weisbroth, S. H. 1979. Dermatomycosis (Trichophyton mentagrophytes). Pp. 228-229 in The Laboratory Rat. Vol. I: Biology and Diseases, H. J. Baker, J. R. Lindsey, and S. H. Weisbroth, eds. New York: Academic Press. Williford, C. B., and J. E. Wagner. 1982. Mycotic diseases. Pp. 65-68 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. COMMON ECTOPARASITES Myobia musculi Agent. Fur mite, order Acarina. Life Cycle. Myobia musculi has egg, larval, nymphal, and adult stages. Eggs are oval, about 200 ~m long, and usually found either attached to the base of hairs or
DISEASE AGENTS 59 inside mature females. Eggs hatch in about 7 days, and completion of the entire life cycle requires about 23 days. Animals Affected. Mice and rarely rats and other laboratory rodents. Epizootiology. Mites can be seen anywhere on the body but are most numerous alongside the hair bases in the more densely furred parts of the body (i.e., over the head and back). Transmission is by direct contact. The dynamics of mite popula- tions on a host are very complex and are influenced by factors that include grooming, strain susceptibility, and host immune responses. Athymic (nu/nu) and other furless mice are not susceptible to infestation. Clinical. The general appearance of infested mice is not directly related to the size of the mite population. Infestations are commonly subclinical. Clinical signs include scruffiness, pruritis, patchy alopecia, self-trauma, ulceration of the skin, and pyoderma. Close inspection often reveals bran-like hyperkeratotic debris and mites on the skin around the base of the hairs. Pathology. Mice of the C57BL strains and their congeneic sublines are particularly susceptible to severe M. musculi-related skin disease. Lesions vary from mild to severe. Initially there is mild hyperkeratosis, but this often progresses to severe hyperkeratosis with fine bran-like material on the skin over virtually all of the body but particularly abundant over the dorsum, head, and shoulders. In more advanced cases, there is patchy alopecia and chronic ulcerative dermatitis most frequently distributed asymmetrically in the shoulder and neck regions. Second- ary bacterial infection commonly leads to suppurative and granulomatous in- flammation. Hyperplasia of regional lymph nodes, splenic lymphoid hypeIplasia, and increased serum immunoglobulins are common. Diagnosis. Diagnosis requires demonstration and identification of mites, while excluding other causes of dermatitis such as fungi (ringworm) or Staphylococcus aureus. Mites can be demonstrated by using a stereoscopic microscope or hand lens to examine the pelage, particularly over the back and head. Alternatively, mice can be killed and placed either on black paper and left at room temperature or in tape- sealed Petri dishes and refrigerated for an hour. As the body cools, the mites leave it and can be collected from the paper or Petri dish. The mites are mounted under a coverslip on glass slides with immersion oil and identified microscopically on the basis of anatomic features. Control. Cesarean derivation and barrier maintenance are the most effective methods for eradication of mite infestations. Insecticides can be used, but they may alter experimental results. Interference with Research. Behavioral patterns are likely to be altered by hypersensitivity to these mites. Secondary amyloidosis caused by chronic infes- tation can interfere with research results. Suggested Reading Flynn, R. J. 1973. Parasites of Laboratory Animals. Ames, Iowa: Iowa State University Press. 884 pp.
60 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Friedman, S., and S. H. Weisbroth. 1977. The parasitic ecology of the rodent mite, Myobia musculi. IV. Life cycle. Lab. Anim. Sci. 27:34-37. Gallon, M. 1963. Myobic mange in the mouse leading to skin ulceration and amyloidosis. Am. J. Pathol. 43:855-865. Weisbroth, S. H.1982. Arthropods. Pp.385-402 in The Mouse in BiomedicalResearch. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Weisbroth, S. H., S. Friedman, and S. Scher.1976. The parasitic ecology of the rodent mite Myobia musculi. III. Lesions in certain host strains. Lab. Anim. Sci. 26:725-735. Myocoptes masculines and Radfordia affinis Agent. Mites, order Acanna. Pathology. M. musculinus causes lesions similar to, but usually milder than, those caused by Myobia musculi. R. affinis is not a significant pathogen. Interference with Research. Mite infestations due to M. musculinus have been reported to reduce the contact sensitivity of mice to oxazolone. Suggested Reading Flynn, R. J. 1973. Parasites of Laboratory Animals. Ames, Iowa: Iowa State University Press. 884 pp. Laltoo, H., and L. S. Kind. 1979. Reduction in contact sensitivity reactions to oxazolone in mite-infested mice. Infect. Immun. 26:30-35. Weisbroth, S. H. 1982. Arthropods. Pp. 385-402 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Other Ectoparasites For more in-depth coverage or information on less common ectoparasites of mice and rats, consult comprehensive reference works on the subject. Suggested Reading Flynn, R. J. 1973. Parasites of Laboratory Animals. Ames, Iowa: Iowa State University Press. 884 pp. Hsu, C.-K.1979. Parasitic diseases. Pp.307-331 in The Laboratory Rat. Vol. I: Biology and Diseases, H. J. Baker, J. R. Lindsey, and S. H. Weisbroth, eds. New York: Academic Press. Owen, D. 1972. Common Parasites of Laboratory Rodents and Lagomorphs. MRC Labo- ratory Animals Centre Handbook No. 1. London: Medical Research Council. 64 pp. Pratt, H. D., and J. S. Wiseman.1962. Fleas of Public Health Importance and Their Control. Insect Control Series: Part VIII. PHS Publ. No.772. Washington, D.C.: U.S. Department of Health, Education, and Welfare. Weisbroth, S. H.1982. Arthropods. Pp.385-402 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press.
DISEASE AGENTS 61 ENDOPARASITES Aspicularzs tetraptera (Mouse Pinworm) Agent. Roundworm, order Ascarida, suborder Oxyurina. Life Cycle. Direct, requires 23-25 days. The adults reside in the colon. Females lay their eggs in the colon, and the eggs subsequently leave the host on fecal pellets. The eggs become infective after 6-7 days et room temperature. Transmission occurs when the infective eggs are ingested by another host. The eggs hatch in the colon, where the larvae develop to maturity, and the cycle begins again. Animals Affected. Mice, rats (rarely), and wild rodents. Epizootiology. A. tetraptera inhabits and lays eggs in the colon. The eggs . ,% . . . ~ . survive for weeks in animal room environments. Clinical. Infections are subclinical. Pathology. A. tetraptera is not considered pathogenic. Diagnosis. Diagnosis is made by demonstration of distinctive eggs by fecal flotation (the cellophane tape method is of no value) and by demonstration and identification of the adult worms in the colon at necropsy. Control. Cesarean derivation and barrier maintenance are effective. Infection can be controlled to some extent by using hygienic methods, such as frequent cage and room sanitization. Cage-to-cage transmission can be prevented by using filter- top cages. Several anthelminthics are effective in eliminating a high percentage of adult worms, but many are inefficient in clearing immature worms or eggs. Interference with Research. See Syphacia obvelata (p. 661. Suggested Reading Flynn, R. J. 1973. Nematodes. Pp. 203-320 in Parasites of Laboratory Animals. Ames, Iowa: Iowa State University Press. Hsu, C.-K. 1979. Parasitic diseases. Pp. 307-331 in The Laboratory Rat. Vol. I: Biology and Diseases, H. J. Baker, J. R. Lindsey, and S. H. Weisbroth, eds. New York: Aca demic Press. Wescott, R. B. 1982. Helminths. Pp. 374-384 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Entamoeba muris Agent. Protozoan, order Amoebida, family Entamoebidae. Life Cycle. Direct. Trophozoites, which inhabit the cecum and colon, form cysts that are passed in the feces. Transmission is by ingestion of cysts. Animals Affected. Mice, rats, hamsters, and other rodent species. Epizootiology. Trophozoites are most commonly found at the interface between the fecal stream and the intestinal epithelium in the cecum and colon. Cysts are resistant to environmental conditions.
62 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Clinical. Infection is subclinical. Pathology. The organism is nonpathogenic. Diagnosis. Diagnosis is made by demonstrating cysts in the feces or trophozoi- tes in wet mounts of intestinal contents from the cecum or colon. In sections stained by hematoxylin and eosin, the trophozoites usually have a distinct magenta-stained nucleus and violet-stained cytoplasm that can appear vacuolated. The outer cell membrane of the trophozoites is usually distinctly visible. Control. Entamoeba muris can be eliminated by cesarean derivation and barrier maintenance; however, infection with this agent is generally considered inconse- quential, and control measures are usually not necessary. Interference with Research. There have been no reports of interference with research results. Suggested Reading Levine, N. D. 1961. Protozoan Parasites of Domestic Animals and of Man. Minneapolis, Minn.: Burgess. 412 pp. Levine, N. D. 1974. Diseases of laboratory animals-parasitic. Pp. 209-327 in CRC Handbook of Laboratory Animal Science, vol. II, E. C. Melby and N. H. Altman, eds. Cleveland: CRC Press. Giardia mutts Agent. Flagellated protozoan, order Diplomonadida, family Hexamitidae, subfamily Giardinae. Life Cycle. Direct. Trophozoites reproduce by longitudinal fission and form cysts that are passed in the feces. Transmission is by ingestion of cysts. The minimal infectious dose for a mouse is approximately 10 cysts. Animals Affected. Mice, rats, hamsters, humans, and many other species. Epizootiology. Trophozoites colonize the proximal one-fourth of the small intestine, where they are found mainly adhering to columnar cells of the villi and *lee in the adjacent mucous layer. The number of trophozoites in the small intes- tine correlates directly with the number of cysts in the large intestine and feces. Cysts are resistant to most environmental conditions but are inactivated by treat- ment with a 2.5% phenol solution and by temperatures above 50°C. Clinical. Infections in mice and rats are usually subclinical but can cause reduced weight gain, rough hair coats, and enlarged abdomens. Infection may be associated with morbidity and mortality in athymic (nu/nu) and other immunocompromised mice. Pathology. Pathogenesis has been studied most extensively in mice. The acute phase of infection involves the proliferation of trophozoites in the small intestine, with the peak period of cyst release occurring during the second week of infection. In the elimination phase, cysts released in the feces are reduced to undetectable
DISEASE AGENTS 63 levels. Resistant strains, including DBA/2, B;lO.A, C57BL/6, B~4LB/c, and SJL/J, eliminate the infection in 5 weeks. Susceptible strains and stocks, including C3H/ He and A/J and outbred Crl:ICR (CD@-1), require 10 weeks to eliminate the infection. Highly susceptible athymic (nu/nu) mice have prolonged infections. Resistance during the acute phase of infection is thought to be controlled by several genes not linked to the H-2 locus, while resistance during the elimination phase is inherited as a dominant trait. Protective immunity is probably dependent on both antibody- and cell-mediated mechanisms. The milk of immune mice contains both IgA and IgG antibodies against Giardia muris and conveys passive protection. In uncomplicated G. muris infection, morphological changes in the small intestine are usually minimal. The villus to crypt ratio may be reduced, and variable numbers of lymphocytes may be present. Diagnosis. Infection by other possible primary or contributing pathogens must be excluded. The organism is diagnosed histologically by identifying character- istic "monkey-faced" trophozoites in sections of the small intestine. Trophozoites also can be recognized in wet mounts of intestinal contents by their characteristic shape and their rolling and tumbling motion. Cysts can be demonstrated in wet mounts of feces. Control. The most practical approach to controlling infection is to procure rodents from breeding populations shown by health surveillance testing to be free of G. muris and to maintain them in a barrier facility. Cesarean derivation is re- quired to eliminate the parasite from infected stocks. Metronidazole can be used for treatment of infected animals but does not completely eradicate infection. Interference with Research. Infection with G. muris can increase the severity and mortality of wasting syndrome (presumably caused by mouse hepatitis virus) in athymic (nu/nu) mice. The organism causes a transient reduction in immunoresponsiveness of mice to sheep erythrocytes during the second and third weeks of infection. It also alters intestinal fluid accumulation and mucosal im- mune responses caused by cholera toxin in mice. Suggested Reading Belosevic, M., G. M. Faubert, E. Skamene, and J. D. MacLean. 1984. Susceptibility and resistance of inbred mice to Giardia muris. Infect. Immun. 44:282-286. Boorman, G. A., P. H. C. Lina, C. Zurcher, and H. T. M. Nieuwerkerk. 1973. Hexamita and Giardia as a cause of mortality in congenitally thymus-less (nude) mice. Clin. Exp. Immunol. 15:623-637. Brett, S. J.1983. Immunodepression in Giardia muris and Spironucleus muris infections in mice. Parasitology 87:507-515. Brett, S. J., and F. E. G. Cox.1982. Immunological aspects of Giardia muris and Spironucleus muris infections in inbred and outbred strains of laboratory mice: A comparative study. Parasitology 85:85-99. Hsu, C.-K. 1982. Protozoa. Pp. 359-372 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press.
64 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Stevens, D. P. 1982. Giardiasis: Immunity, immunopathology and immunodiagnosis. Pp. 192-203 in Immunology of Parasitic Infections, 2nd ea., S. Cohen and K. S. Warren, eds. Oxford: Blackwell Scientific. Hymenolepis nana Agent. Tapeworm, order Cyclophyllidea, family Hymenolepidae. Life Cycle. Direct or indirect. The life cycle includes adult, egg (with embryo or oncosphere), and larval (cercocystis) stages. In direct transmission, eggs hatch in the small intestine. Larvae penetrate and develop as cercocystis in the intestin- al villi, then return to the lumen to become mature adults. The cycle requires only 1-16 days. In indirect transmission, the eggs are ingested by an arthropod interrnedi- ate host such as a flour beetle, and the cercocystis develops in the intestine of the beetle. The intermediate host is eaten by the definitive host, and adult H. nana develop in the lumen of the small intestine. The entire life cycle by indirect transmission requires 20-30 days. Animals Affected. Mice, rats, hamsters, other rodents, nonhuman Inmates, and humans. Epizootiology. Weanling and young adult rodents are most frequently infected. The duration of infection by adult worms in the small intestine is usually only a few weeks. Clinical. Most infections are subclinical. Severe infections have been reported to cause retarded growth and weight loss in mice and intestinal occlusion, intestin- al impaction, and death in hamsters. Pathology. Presence of adult worms in the small intestine is usually associated with mild ententis. Larval stages occasionally reach the lymph nodes, liver, or lung, where they incite a granulomatous inflmnmatory response. Diagnosis. Diagnosis is made by demonstration and identification of adult tapeworms in the small intestine. Eggs can be demonstrated in feces. Also, histologic sections occasionally are successful in demonstrating the cercocystis in intestinal villi and lymph nodes. Control. The most practical method of control is to obtain rodents from stocks demonstrated to be free of H. nana. Cesarean derivation and barrier maintenance are the most effective methods for eliminating infection. Interference with Research. H. nana is a potential zoonotic infection to humans. It can interfere with studies involving the intestinal tract. Suggested Reading Flynn, R. J. 1973. Cestodes. Pp. 155-202 in Parasites of Laboratory Animals. Ames, Iowa: Iowa State University Press. Hsu, C.-K. 1979. Parasitic diseases. Pp. 307-331 in The Laboratory Rat. Vol. I: Biology and Diseases, H. J. Baker, J. R. Lindsey, and S. H. Weisbroth, eds. New York: Academic Press.
DISEASE AGENTS 65 Kunstyr, I., and K. T. Fr~edhoff. 1980. Parasitic and mycotic infections in laboratory animals. Pp. 181-192 in Animal Quality and Models in Biomedical Research, A. Spiegel, S. Erichsen, and H. A. Solleveld, eds. Stuttgart: Gustav Fischer Verlag. Wescott, R. B. 1982. Helminths. Pp. 373-384 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Spironucleus musts Agent. Flagellated protozoan, order Diplomonadida, family Hexamitidae, subfamily Hexamitinae. Formerly called Hexamita muris. Life Cycle. Direct. Trophozoites reproduce by longitudinal fission and form highly resistant cysts. The minimal infectious dose for a mouse is one cyst. Animals Affected. Mice, rats, and hamsters. Epizootiology. Two- to 6-week-old mice are most susceptible to infection with S. muris. Trophozoites usually inhabit the crypts of Lieberkuhn in the small intestine, but in young animals the lumen can also contain large numbers of troph- ozoites. In older mice and in rats there are very few trophozoites, and those that are present can be found only in glands of the gastric pyloris. Transmission is by ingestion of cysts that are shed in the feces. The greatest numbers are shed by young or immunocompromised hosts. Cysts are inactivated by some disinfectants and high temperature (45°C for 30 minutes) but are highly resistant to most other environmental conditions. Infectivity is retained for 6 months at-20°C, for 1 day at pH 2.2, for 14 days at room temperature, or for 1 hour in 0.1% glutaraldehyde. Clinical. Infection is usually subclinical in immunocompetent hosts. In athy- mic (nu/nu) and lethally irradiated mice, S. muris infection has been associated with severe chronic enteritis with weight loss. Pathology. After ingestion of cysts, trophozoite (and cyst) numbers in the intestines of immunocompetent rodents peak at 1-2 weeks and decline to low numbers by 4-5 weeks in BALB/c mice; 7-9 weeks in CBA, SJL/J, and C3H/He mice; and 13 weeks in A and B.B10 mice. Numbers in athymic (nu/nu) mice persist indefinitely at high levels. In severe infections, the small intestine may appear reddened and contain watery fluid and gas. Smears of the intestinal contents con- tain numerous motile trophozoites, and cysts can be demonstrated in the cecum and colon. The best indicator of S. muris infection in hematoxylin and eosin-stained sections of the small intestine is distension of the crypts of Lieberkuhn by masses of granular-appearing trophozoites. Trophozoites can cause shortening of micro- villi on the crypt epithelium and increased turnover of enterocytes. There is usual- ly little or no inflammatory response in immunocompetent animals, but heavily parasitized, immunodeficient animals can have moderate to severe enteritis. Diagnosis. Other possible causes of digestive tract disease (e.g., enterotrophic strains of mouse hepatitis virus) must be ruled out. Characteristic trophozoites can be demonstrated in the contents of the small intestine, or cysts can be demonstrated in the contents of the large intestine or feces using wet mounts under reduced light.
66 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS For routine health surveillance that includes histopathology, the examination of multiple histologic sections of the small intestine and gastric pylorus is probably superior to other methods because there may be very few parasites present, and they may be localized in distribution. Trophozoites can be stained by silver or periodic acid-Schiff methods. Control. Cesarean derivation and barrier maintenance are recommended for control of the organism. Treatment of mice with 0.04-0.1% dimetridazole in drinking water for 14 days can ameliorate clinical signs but does not completely eliminate the infection. Interference with Research. S. muris can increase the severity and mortality of the wasting syndrome (presumably due to mouse hepatitis virus) in athymic (nu/nu) mice. S. muris has been reported to increase mortality in cadmium-treated mice; alter macrophage function; reduce spleen plaque-forming cell responses to sheep erythrocytes; reduce lymphocyte responsiveness to mitogens such as phytohemagglutinin, concanavalin A, and pokeweed mitogen; and alter im- mune responsiveness to tetanus toxoid and type 3 pneumococcal polysaccharide. Whole-body irradiation increases susceptibility to S. muris infection and disease. Suggested Reading Brett, S. J. 1983. Immunodepression in Giardia muris and Spironucleus muris infections in mice. Parasitology 87:507-515. Brett, S. J., and F. E. G. Cox. 1982. Interactions between the intestinal flagellates Giardia muris and Spironucleus muris and the blood parasites Babesia microti, Plasmodiumyoelii and Plasmodium berghei in mice. Parasitology 85: 101-1 10. Keast, D., and F. C. Chesterman. 1972. Changes in macrophage metabolism in mice heavily infected with Hexamita muris. Lab. Anim. (London) 6:33-39. Kunstyr, I., E. Amme~pohl, and B. Meyer. 1977. Experimental spironucleosis (hexamitiasis) in the nude mouse as a model for immunologic and pharmacologic studies. Lab. Anim. Sci. 27:782-788. Stachan, R., and I. Kunstyr. 1983. Minimal infectious doses and prepatent periods in Giardia muris, Spironucleus muris and Tritrichomonas muris. Z. Bakt. Hyg. A256:249-256. Wagner, J. E., R. E. Doyle, N. C. Ronald, R. G. Garrison, and J. A. Schmitz. 1974. Hexamitiasis in laboratory mice, hamsters, and rats. Lab. Anim. Sci. 24:349-354. Syphacia obvelata (Mouse Pinworm) and Syphacia muris (Rat Pinworm) Agents. Roundworms, order Ascarida, suborder Oxyurina. Life Cycle. Direct; requires only 11-15 days for completion. Gravid females migrate from the large intestine to the perianal area, deposit their eggs, and then die. Eggs become infective in about 6 hours. Following ingestion by another host, eggs hatch in the small intestine, and the larvae reach the cecum in 24 hours. The parasites spend 10-11 days in the cecum where they mature and mate, thus continuing the cycle.
DISEASE AGENTS 67 Animals Affected. Laboratory mice, rats, hamsters, gerbils, and wild rodents. Epizootiology. Adults are found primarily in the cecum and colon of infected hosts. Eggs are efficiently disseminated from the perianal area of the host into the cage and room environments. The eggs can survive for weeks under most animal room conditions. Transmission is by ingestion of embryonated eggs. Clinical. Infections caused by Syphacia spp. alone are subclinical. Pathology. Pinworms of laboratory rodents are generally not considered pathogens. Pinworm burden in an infected rodent population is a function of age' sex, and host immune status. In enzootically infected colonies, weanling animals develop the greatest parasite loads, males are more heavily parasitized than females, and Syphacia numbers diminish with increasing age of the host. Athymic (nu/nu) mice have increased susceptibility to pinworm infection. Diagnosis. Diagnosis is made by demonstrating eggs on the perianal region using the cellophane tape technique or by finding adult worms in the cecum and colon at necropsy. Control. Cesarean derivation and barrier maintenance are effective methods of control. Hygienic methods, including frequent cage and room sanitization, can aid in controlling Syphacia in an infected rodentpopulation. Cage-to-cage transmission can be prevented by using filter-top cages. Several anthelminthics are effective in eliminating a high percentage of adult worms but are inefficient in clearing immature worms or eggs. Interference with Research. Pinwo~m infections in rats have been reported to reduce the occurrence of adjuvant-induced arthritis. Suggested Reading Flynn, R. J.1973. Nematodes. Pp.203-320 in Parasites of Laboratory Animals. Ames, Iowa: Iowa State University Press. Hsu, C.-K.1979. Parasitic diseases. Pp.307-331 in The Laboratory Rat. Vol. I: Biology and Diseases, H. J. Balcer, J. R. Lindsey, and S. H. Weisbroth, eds. New York: Academic Press. Pearson, D. J., and G. Taylor. 1975. The influence of the nematode Syphacia obvelata on adjuvant arthritis in rats. Immunology 29:391-396. Ross, C. R., J. E. Wagner, S. R. Wightman, and S. E. Dill.1980. Experimental transmission of Syphacia muris among rats, mice, hamsters, and gerbils. Lab. Anim. Sci. 30:35-37. Wescott, R. B. 1982. Helminths. Pp. 374-384 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Wescott, R. B., A. Malczewski, and G. L. Van Hoosier. 1976. The influence of filter top caging on the transmission of pinworrn infections in mice. Lab. Anim. Sci. 26:742-745. Trichomonas mur7s Agent. Flagellated protozoan, order Trichomonadida. Life Cycle. If acyst stage exists, transmission is probably primarily by ingestion of cysts.
68 COMPANION GUIDE TO INFECTIOUS DISEASES OF MICE AND RATS Animals Affected. Mice, rats, hamsters, and other rodents. Epizootiology. Trophozoites are found throughout the fecal mass in the cecum and colon. Clinical. Infections are subclinical. Pathology. T. muris is considered a commer~sal. Diagnosis. Diagnosis is by demonstration of trophozoites in wet mounts of contents from the cecum or colon. T. muris has characteristic wobbly or jerky movements. Trophozoites are found dispersed throughout the fecal stream in histologic sections of the cecum or colon prepared without disturbing the luminal contents. In hematoxylin and eosin-stained sections, the nucleus stains poorly, the nuclear membrane is indistinct, and the cell wall often appears wrinkled or folded upon itself. Control. Control measures are usually not necessary. Interference with Research. There have been no reports of interference with research results. Suggested Reading Hsu, C.-K. 1982. Protozoa. Pp. 359-372 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press. Kunstyr, I., B. Meyer, and E. Ammerpohl. 1977. Spironucleosis in nude mice: An animal model for immuno-parasitologic studies. Pp. 17-27 in Proceedings of the Second International Workshop on Nude Mice. Stuttgart: Gustav Fischer Verlag. Levine, N. D. 1974. Diseases of laboratory animals parasitic. Pp. 209-327 in CRC Handbook of Laboratory Animal Science, vol. II, E. C. Melby and N. H. Altman, eds. Cleveland: CRC Press. Other Endoparasites Numerous other endoparasites have been reported in wild mice and rats and are encountered occasionally in laboratory animals maintained by conventional methods. For information, comprehensive works on endoparasites should be consulted. Suggested Reading Flynn, R. J. 1973. Parasites of Laboratory Animals. Ames, Iowa: Iowa State University Press. 884 pp. Griffiths, H. J. 1971. Some common parasites of small laboratory animals. Lab. Anim. (London) 5: 123-135. Hsu, C. K. 1979. Parasitic diseases. Pp. 307-331 in the Laboratory Rat. Vol. I: Biology and Diseases, H. J. Baker, J. R. Lindsey, and S. H. Weisbroth, eds. New York: Academic Press. Hsu, C.-K. 1982. Protozoa. Pp. 359-372 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press.
DISEASE AGENTS 69 Levine, N. D. 1974. Diseases of laboratory animals parasitic. Pp. 209-327 in CRC Handbook of Laboratory Animal Science, vol. II, E. C. Melby and N. H. Altman, eds. Cleveland: CRC Press. Levine, N. D., and V. Ivens.1965. The Coccidian Parasites (Protozoa, Sporozoa) of Rodents. Urbana, Ill.: University of Illinois Press. 365 pp. Oldham, J. N. 1967. Helminths, ectoparasites and protozoa in rats and mice. Pp. 641-678 in Pathology of Laboratory Rats and Mice, E. Cotchin and F. J. C. Roe, eds. Oxford: Blackwell Scientific. Wescott, R. B. 1982. Helminths. Pp. 374-384 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press.