myelitis-like infection (Theiler, 1941; Gard, 1943) and virus-induced demyelinating disease (Dal Canto and Lipton, 1975; Lipton, 1975; Lipton and Dal Canto, 1979a,b).

Agent

TMEV is a RNA virus, family Picornaviridae, genus Enterovirus. Other members of this genus include polioviruses, coxsackieviruses, and enteroviruses. Strains of TMEV are divided into two groups based on pathogenicity for mice and in vitro growth characteristics. The small-plaque strains are Theiler's original (TO) and similar strains (DA, WW, TO4, Yale, and BeAn 8386). These produce small plaques in L929 cells, are relatively avirulent, cause chronic demyelinating disease in mice, do not hemagglutinate human erythrocytes, and are not readily cultivated in embryonating eggs. The large-plaque strains are FA, GDVII, and others that produce large plaques in L929 cells, are highly virulent for mice, may or may not hemagglutinate human erythrocytes, and grow readily in eggs (Lipton, 1980). The MHG strain was isolated from rats (Hemelt et al., 1974). All strains are antigenically related in the cross-neutralization, hemagglutination inhibition, and complement fixation tests (Hemelt et al., 1974).

The virus particles are naked icosahedral nucleocapsids, measuring 2030 nm in diameter, and have a single-stranded RNA genome. The virions contain four major structural polypeptides: VP1, VP2, and VP3, each with a molecular weight of 27,000-58,000, and VP4, with a molecular weight of 6,000. In the large-plaque strains, VP1 is slightly heavier than in the less virulent small-plaque strains (Stroop and Baringer, 1981; Downs, 1982, Matthews, 1982).

TMEV can be propagated in several continuous cell lines, but BHK-21 cells are most commonly used (Lipton, 1978a). TMEV is rapidly inactivated by heating at 50-55°C for 30 minutes and by treatment with 50% acetone or alcohol, but not by treatment with ether (Theiler and Gard, 1940a).

Hosts

Laboratory mice (Downs, 1982) and rats (McConnell et al., 1964).

Epizootiology

Natural TMEV infection has been reported most frequently in laboratory mice (Downs, 1982) and on one occasion in laboratory rats (McConnell et al., 1964), but not in wild Mus musculus (Downs, 1982). The prevalence of TMEV infection in mice has been reported to be 5% of commercial barrier breeding colonies (Lindsey et al., 1986a) and 62% of other mouse populations (Parker, 1980) in the United States. The prevalence of TMEV infec-



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