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.

Biological control of rodents using Sarcocystis singaporensis.

Parasites have been identified as an important factor in regulating vertebrate populations. In replicated field experiments (plots up to 4 ha) performed in Thailand we tested whether commensal and field rodents could be artificially infected and controlled with the host-restricted apicomplexan protozoon Sarcocystis singaporensis which is endemic in Southeast Asia. When bait-pellets containing high numbers of these parasites were consumed by rodents of three species (Rattus norvegicus, Rattus tiomanicus, Bandicota indica) in different agricultural habitats (chicken farm, oil palm plantation, ricefield), we observed a parasite-induced mortality ranging from 58% to 92%.

Sarcosporidiasis in rodents from Thailand.

One to six Sarcocystis spp. were identified in the skeletal muscles of 41 (33%) of 124 wild rodents (Rattus spp. and Bandicota indica) mainly captured in the central plains of Thailand throughout the year in 1995.