Reduction of transmission stages concomitant with increased host immune responses to hypervirulent Sarcocystis singaporensis, and natural selection for intermediate virulence.

Parasite virulence (pathogenicity depending on inoculum size) and host immune reactions were examined for the apicomplexan protozoan Sarcocystis singaporensis. This parasite is endemic in southeastern Asia and multiplies as a proliferation (merozoite) and transmission stage (bradyzoite) in rats. Virulence in wild brown rats of parasites freshly isolated in the wild (wild-type) was surprisingly constant within the endemic area and showed an intermediate level.