Sarcocystis hominis is a unicellular parasite with a complex life cycle. These parasites (as well as the related S. suihominis) use humans as definitive hosts and cause intestinal sarcocystosis in the human host.
Sarcocystis species are intracellular protozoan parasites with a two-host (intermediate host and definitive host) life cycle. These parasites are typically transmitted from an intermediate to a definitive host by the latter eating the former and from a definitive to an intermediate host by the intermediate host consuming oocysts (excreted in definitive host feces) via contaminated food, water, or bedding. In some cases, the same species may have the potential to serve as both intermediate and definitive host. Asexual stages develop in an intermediate host after it ingests the oocyst stage from definitive host feces and terminate with the formation of intramuscular cysts (sarcocysts) in muscle tissue of the intermediate host. Sexual stages develop in the intestine of the definitive host after it ingests sarcocysts in meat of infected intermediate hosts and terminate with the excretion of oocysts in the feces. (Fayer 2004 and references therein) Sarcocystis hosts are mainly mammals, but some species infect birds and reptiles.
Apparent cases of Sarcocystis infection of humans as intermediate hosts, with the formation of sarcocysts, number fewer than 100 and are of unknown origin (humans are presumably dead ends for the parasite in these cases unless they have the misfortune of subsequently being eaten by a definitive host carnivore). More commonly, humans acquire intestinal sarcocystosis from eating Sarcocystis-contaminated meat (i.e., by consuming a contaminated intermediate host and then developing intestinal sarcocystosis as a definitive host). Most cases of infection with S. hominis (and S. suihominis) have been reported from the Asian tropics or subtropics. (Fayer 2004 and references therein)
Sporocysts of S. hominis use cattle (and water buffalo), but not pigs, as intermediate hosts, whereas sporocysts of S. suihominis infect pigs but not cattle. Humans, baboons, and Rhesus Monkeys that eat raw or undercooked meat from infected cattle (intermediate hosts) can serve as definitive hosts for S. hominis, and humans, chimpanzees, Rhesus Monkeys, and Long-tailed Macaques that eat raw meat from infected pigs (intermediate hosts) can serve as definitive hosts for S. suihominis. No other definitive hosts have been identified for S. hominis or S. suihominis. (Fayer 2004 and references therein) Sarcocystis hominis and S. suihominis are found worldwide, but are more common in areas where livestock are raised (Centers for Disease Control Parasites and Health website).
Intestinal sarcocystosis can be prevented by thoroughly cooking or freezing meat to kill bradyzoites in the sarcocysts in muscle tissue. To prevent infection of (intermediate host) food animals in the first place, they must be prevented from ingesting water, feed, or bedding contaminated by the feces of an infected definitive host. To minimize the chance of humans being infected as intermediate hosts, water should be boiled and food should be thoroughly washed or cooked before being eaten. The prevalence of intestinal sarcocystosis in humans is apparently low (although the actual incidence of infection is likely far higher than currently recognized) and is only rarely associated with illness. (Fayer 2004 and references therein)