Sarcocystosis life cycle

Both sporulated oocysts (containing two sporocysts) and individual sporocysts can be passed in stool (1). Sporocysts contain four sporozoites and a refractile residual body. Sporocysts ingested by the intermediate host (cattle for Sarcocystis hominis and pigs for S. suihominis) rupture, releasing sporozoites. Sporozoites enter endothelial cells of blood vessels and undergo schizogony, resulting in first-generation schizonts. Merozoites derived from the first generation invade small capillaries and blood vessels, becoming second-generation schizonts. The second-generation merozoites invade muscle cells and develop into sarcocysts containing bradyzoites, which are the infective stage for the definitive host (2). Humans become infected when they eat undercooked meat containing these sarcocysts. Bradyzoites are released from ruptured cysts in the small intestine (3) and invade the lamina propria of the intestinal epithelium (4). There, they differentiate into macro- and microgametocytes. Fusion of male and female gametes (5) results in the formation of oocysts (6). Oocysts sporulate in the intestinal epithelium and are shed from the host in stool. Due to the fragile nature of the oocyst wall, individual sporocysts may also be detected in stool (7). Humans act as accidental hosts when sporocysts or oocysts passed in the feces of a definitive host (8) are inadvertently ingested (9). Asexual development occurs in the intestine and then the vasculature (small arteries, arterioles, capillaries, venules, and veins) prior to forming sarcocysts in skeletal muscle (10).