Different strains of T. gondii cause different degrees of pathogenecity in different hosts (Suzuki et al. 1989, 1993). Generally, humans, cattle, horses, rats, and old-world monkeys belong to the resistant species, whereas mice, guinea pigs, hamsters, and new-world monkeys are sensitive (Darcy and Zenner 1993).

Virulence of T. gondii has been traditionally measured in a susceptible host (e.g. mice). Based on studies in mice, some T. gondii strains are considered more virulent than others. Virulence is influenced by the stage of the parasite (tachyzoite, bradyzoite, or sporozoite), the route of inoculation (oral, intraperitoneal, and sub-cutaneous), and the susceptibility of the host. In mice, oocysts from the M-7741 isolate needed an inoculum size approximately 10-100 times lesser than tissue cysts, producing both earlier symptoms and more deaths than tissue cysts with inoculums of the same size (Dubey and Frenkel 1973). However, according to Dubey and Beattie (1988) there are no truly avirulent strains of T. gondii: 100000 oocysts of all strains of T. gondii tested were lethal to mice by the oral route. More severe infections are found in pregnant or lactating mice than in non-lactating mice. Concomitant infection may make the host more susceptible or resistant to T. gondii infection (Remington 1970). Newborn kittens were more prone to severe toxoplasmosis and death compared to adult, non-immune cats (Dubey and Frenkel 1972).

The most well-known virulent RH strain of T. gondii was isolated in 1939 from a 6-year-old boy (with initials R. H.) in mice (Sabin 1941). Five of the eight mice inoculated with the brain of this boy died within 21 days and three mice were not infected. Thus, the RH strain was virulent for mice on its first isolation. Although the RH strain of T. gondii can be virulent in many hosts, including humans, it is avirulent for adult rats and adult dogs (Dubey and Beattie 1988). The virulence of the RH strain has been changed after passages in mice (Yano and Nakabayashi 1986). Virulence in mice has been linked to expression of the SAG1, where it was found that virulent stain expressed higher amounts of SAG1 compared to avirulent isolates, and this was related to the number of a 27-bp repeat in the SAG1-promoter region (Windeck and Gross 1996). Others have found that the virulent RH isolate has increased levels of DNA-polymerase activity compared to mice -avirulent isolates (Makioka and Ohtomo 1995). High expression of a heat shock protein, HSP, has been linked to virulence (Lyons and Johnson 1995).

Mice with severe combined immuno deficiency, SCID, or nude mice die from acute infection with T. gondii, but survive if reconstituted with spleen cells from immune mice or if kept covered with sulfadiazine ( Johnson 1992).

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