perimental tumors (a Cysticercus fasciolaris-induced sarcoma in F344 rats and a transplantable leukemia in OM rats). In subsequent years research with this agent emphasized molecular biology of the virus (Tattersall and Cotmore, 1986) and experimental animal models for producing lesions such as cerebellar hypoplasia (Kilham and Margolis, 1975; Margolis and Kilham, 1975) while relatively little attention was given to understanding the natural history and pathogenesis of KRV in rat colonies (Jacoby et al., 1987).

Agent

KRV is a single-stranded DNA virus, family Parvoviridae, genus Parvovirus. KRV is the type species of the genus (Siegl et al., 1985). It is synonymous with rat virus (RV) and parvovirus r-1. Approximately a dozen strains of the virus have been isolated (Tattersall and Cotmore, 1986). The virion is non-enveloped, and measures 18-30 nm in diameter. The genus Parvovirus presently contains 13 distinct serotypes, three of which occur in rodents: KRV, H-1 virus, and Minute Virus of Mice. Serotyping is based on the neutralization (NT), complement fixation (CF), and hemagglutination inhibition (HAI) tests (Jacoby et al., 1979; Tattersall and Cotmore, 1986). KRV can be propagated in primary rat embryo, 324K, and BHK-21 cells (Jacoby et al., 1979, 1987).

Parvoviruses are remarkably resistant to environmental conditions. Infectivity is retained after heating at 80°C for 2 hours or 40°C for up to 60 days. They also are resistant to dessication, pH 2 to 11, chloroform, ether, and alcohol (Toolan, 1968; Jacoby et al., 1979; Tattersall and Cotmore, 1986).

Hosts

Laboratory and wild rats (Rattus norvegicus) are the natural hosts. Syrian hamsters and several other species have been infected experimentally (Kilham, 1966; Jacoby et al., 1979; Tattersall and Cotmore, 1986).

Epizootiology

KRV is a common natural infection of wild and laboratory rats. Kilham (1966) found antibodies to KRV in 40% to 62% of wild rats at four different locations near Hanover, New Hampshire, and in 89% of a population of laboratory rats. More recent serological surveys of laboratory rats reported prevalences of 44% (Lindsey et al., 1986a) and 71% (Parker, 1980) in the United States, 74% in Canada (Lussier and Descoteaux, 1986), 60% in the United Kingdom (Gannon and Carthew, 1980), and 32% in West Germany (Kraft and Meyer, 1986).

Transmission of KRV is primarily by the horizontal route, either through



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