DNA Polymorphisms Associated with Murine Virulence of Toxoplasma gondii Identified by RAPD-PCR

Abstract

There are large differences in virulence among the various strains and isolates of T. gondii, although only one species of the genus Toxoplasma has been observed to exist so far. This virulence diversity among isolates may have considerable impact on epidemiology, immunology, pathology and the parasite-host relationship. Evidence that there is more than one strain of T. gondii is based on the observation of differences in virulence among different isolates for laboratory animals (Krahenbuhl and Remington 1982). Virulence can be estimated as either the time taken before infected animals die and the percentage of mortality, or the number of parasites needed to kill infected animals (Kaufman et al. 1959; Dubey and Frenkel 1973). Some strains such as the extremely virulent RH strain have lost the ability to form oocysts in the cat, and as few as ten tachyzoites can be lethal for a mouse within a week when injected i.p. Other strains such as the avirulent S-1 strain form cysts in the brains of mice injected with 1000 oocysts (equivalent to 8000 tachyzoites) and these mice survive without ill effects (Dubey and Frenkel 1973). Early studies showed that differences in virulence correlated with tachyzoite generation time in tissue culture (Kaufman 1958; Kaufman et al. 1959). Virulent strains rapidly destroy the cells while avirulent strains grow slowly causing minimal cell damage. The pathogenicity of some avirulent strains has been observed to increase following frequent i.p. passage of the tachyzoites in mice (Frenkel 1956).