Sarcocystis Species

Pathogen

Humans are the natural definitive hosts for 3 known Sarcocystis spp.: S. hominis and S. heydorni with cattle being the natural intermediate hosts and S. suihominis with pigs as the intermediate host ( Fig. 272.2 ). ^{, ,} Humans acquire infection by eating raw or undercooked sarcocyst-containing meat. Only sexual stage parasites are found in definitive-host infections. In the small intestine, motile bradyzoites released from sarcocysts enter cells of the mucosa, where they transform into either a macrogamont (female) or a microgamont (male). Once fertilized, each macrogamont develops into an oocyst that sporulates, to form a pair of sporocysts, each containing 4 sporozoites ( Fig. 272.3 ). Oocysts enter the intestinal lumen and fracture into individual sporocysts, either before or after being shed in the feces.

Humans are aberrant intermediate hosts for S. nesbitti (a species with a reptile as its likely definitive host) and possibly for other, unidentified Sarcocystis spp. ( Fig. 272.4 ). ^, In intermediate-host infections, only asexual stage parasites are found. ^, Infection is acquired by ingesting food or water contaminated with feces from a sporocyst-shedding definitive host. In the small intestine, sporocysts excyst, releasing their 4 motile sporozoites, which penetrate the gut wall and enter the vasculature. There the sporozoites disseminate and undergo cycles of asexual development (merogony or schizogony) within the vascular endothelium of all parts of the body, first in small arterioles, then in capillaries, and finally in veins. Ultimately, the parasite enters myocytes of skeletal, smooth, or cardiac muscle where it forms bradyzoite-containing sarcocysts ( Fig. 272.1 ).