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The Disease The control of rabies has been hampered by our very limited under- standing of the disease in wild animals. Only fragmentary data are available on the distribution of the disease in wild animals, the means by which these animals are infected, and the subsequent host response to infection. Data on biological properties of the known wild-animal hosts of the disease, including pertinent information on population dynamics, movement, and inter- and intraspecies interaction are inadequate. PATHOGENESIS Rabies virus is believed to be pathogenic for all mammals, and experi- mental infection has been accomplished in the preponderance of mammalian species tested. Rabies virus is currently classified as a member of the rhabdovirus group. Its single-stranded, helical, RNA nucleocapsid is contained within a lipid envelope that is essential to infectivity. Until very recently, it had been agreed by most that rabies virus was a single antigenic entity and that antigenic variation was never very marked. 1
2 CONTROL OF RABIES Recent studies with two African viruses, the Lagos bat and Ibaden shrew viruses, indicate that these are morphogically and serologi- cally related to rabies virus, but that, similar to the European "mouse viruses," they differ significantly enough in certain aspects to threaten some of our basic concepts of rabies epidemiology (Shope etal, 1970). Rabies is generally considered to be a neurotropic virus. Available evidence suggests that the neural route is probably the natural path- way for dissemination of the virus within an organism (Johnson and Mercer, 1964; Baer et al, 1965). It is apparent, however, that the virus is not totally dependent on neural tissue for replication, inas- much as it is readily propagated in nonneural tissue in vitro. While the central nervous system is considered the optimal site for isolation of virus from animals infected with rabies, after death it frequently can be isolated from visceral organs, glandular secretions, and body excretions. Until recently, rabies was thought to be transmitted exclusively by the introduction of virus-laden saliva into a bite wound or open lesion. Other portals of infection have now been demonstrated. Transmission can occur by ingestion of infected material, by intra- nasal and rectal instillation of virus suspensions, and by inhalation of contaminated air. The implications of nonbite transmission of rabies are of considerable epidemiologic significance. The laboratory demonstration that skunks become infected follow- ing ingestion of a single rabid rodent suggests the oral route as a poten- tial mechanism for dissemination of the disease in nature (Bell and Moore, 1971). Thus, while there are no data on this type of transmis- sion for other carnivores, ingestion of infected material has been proven a mechanism by which laboratory rodents can be infected. Still further, oral immunization of foxes with attenuated rabies virus has been described (Baer et al, 1971). It appears, then, that the po- tential for oral infection by ingestion of virus may be far greater than generally recognized. Transmission by inhalation of infective aerosols has special signifi- cance in the potential role of bats as reservoirs for rabies in terrestrial mammals. Experimental studies have shown that wild carnivores ex- posed only to the contaminated atmosphere in bat caves are readily infected by the airborne virus (Constantine, 1967). Other studies have described the extensive use of bat caves by carnivores and have pro- vided some statistical analyses that seek to correlate increased carni-
THE DISEASE 3 vore rabies with airborne infection in bat caves (Frederickson and Thomas, 1965). Inhalation has also been suggested as a possible mechanism for transmission between animals other than those associated with bat caves. Communal denning and individual contact between animals, wherein considerable "sniffing" is part of the behavioral pattern, have both been implicated as means of dissemination for non-bite- transmitted rabies (Kauker, 1967). Aerosol rabies infection has been shown to be a hazard to humans in only one cave in the United States. Two human beings died of rabies apparently as a result of nonbite exposures in that particular cave where millions of bats lived each summer. In addition, all the sentinel animals (foxes and coyotes) placed in that cave and pro- tected against bat bites also died of rabies infection. There was a high rabies infection rate among those bats, and the environmental conditions were conductive to aerosol transmission. Other caves in the United States are of questionable public health significance for risk of rabies infection by aerosol. EPIDEMIOLOGY Only two orders of mammals, Carnivora and Chiroptera, have been demonstrated to be important in the maintenance of the disease in nature (Held et al, 1967). Emphasis on broad susceptibility to the rabies virus is misleading in an epidemiological sense, because a dis- tinctive feature of rabies epidemics in wildlife is remarkable com- partmentation, whereby epidemics tend to persist within a single population (Verts and Storm, 1966; Chalmers and Scott, 1969). For example, in the early 1970's in North America, rabies was known to be endemic within skunk, fox, raccoon, and bat populations, repre- senting numerous individual endemics in noncontiguous populations. Although interspecies transmission does occur and is occassionally re- sponsible for changing the pattern of disease, endemic rabies tends to remain confined to a single reservoir species in a given area and ap- pears to establish itself in other species only after prolonged exposure. Conditions that permit the transmission of endemic rabies from one species to another are not well understood, although high contact rates between the old and new host species are presumed to be important. The geographic distribution of rabies in foxes, skunks, and raccoons,
4 CONTROL OF RABIES though not actually static, appears to be fairly well defined (Held, et al, 1967). Fox rabies is endemic throughout most of the United States east of the Mississippi River and in the south central states west of the river. The disease is most prevalent in the montane and pied- mont areas from Maine to Tennessee. Rabies is endemic in skunks throughout the Mississippi River drainage, the midwestern tall grass prairie areas being most heavily infected. It is endemic in racoons only in Florida and Georgia. Smaller foci of infection are known, such as rabies in skunks in areas of the Far West. No geographic distributional patterns have been described for rabies in bats. Within the broad areas of involvment, the incidence of infection fluctuates in a somewhat cyclic pattern (Verts, 1967). Seasonal cycles have been attributed to biological parameters of the host species; cycles of longer periodicity have been attributed to broad population fluctuations (Davis and Wood, 1959). Although the origin in North America is unknown, it is probable that the patterns of rabies now being observed in wildlife have existed for many years but have become recognizable only after canine rabies was controlled (Parker, 1969).
