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. In contrast, serially passaged parasites either became avirulent or virulence increased markedly (hypervirulence). Production of transmission stages was maximal for the wild-type whereas numbers were significantly reduced for hypervirulent and avirulent (shown in a previous study) parasites. Analyses of B and T cell immunity revealed that immune responses of WKY rats to the transmission stage were significantly higher for hypervirulent than for wild-type parasites. These results suggest that it is the immune system of the host that is not only responsible for reduction of transmission stages in individual rats, but also could act as a selective force that maintains intermediate virulence at the population level because reduction of muscle stages challenges transmission of S. singaporensis to the definitive host. Collectively, the presented data support evolutionary theory, which predicts intermediate rates of parasite growth in nature and an 'arms race' between host immunity and parasite proliferation.

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%. Detection of merozoites of S. singaporensis in lung tissue samples of rats collected dead at the experimental sites using a species-specific monoclonal antibody confirmed that S. singaporensis was the causative agent of mortality. As observed with brown rats, the parasite's effect on the host was not related to sex. These experiments demonstrate for the first time that artificial infection of rodents with an endemic protozoon has the potential for effective population control.

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. Included were S. singaporensis, S. villivillosi, and S. murinotechis-like cysts all of which showed a striated cyst wall at the light microscopical level, and Sarcocystis cymruensis, S. sulawesiensis, and S. zamani which possessed smooth cyst walls. The ultrastructure of the cyst wall and other morphological characteristics used to distinguish species are described. By inoculation of muscle cysts from wild-caught rodents into coccidia-free pythons (Python reticulatus, P. molurus bivittatus), we confirmed that P. reticulatus is a suitable definitive host for S. singaporensis and S. zamani in Thailand. Furthermore, we showed by fecal examination of reticulated pythons collected in the wild and subsequent experimental infection of laboratory rats that these hosts also are naturally infected with both species. Sarcocystis cymruensis is reported for the first time from Southeast Asia. This parasite was prevalent in brown rats (Rattus norvegicus) and bandicoot rats (B. indica) which were captured near human habitations; it is likely to be transmitted to rats via cats. The definitive hosts of S. sulawesiensis and S. murinotechis are unknown. Hence, at least three Sarcocystis spp. (S. singaporensis, S. zamani, S. villivillosi) are likely to cycle between snakes and rodents in agricultural areas in Thailand. Among these, S. singaporensis appears to be the most prevalent species.