Schistosoma mansoni is one of the three main Schistosoma trematode flatworms that infect humans and cause schistosomiasis (=bilharziasis), the other two being S. haematobium and S. japonicum (in some regions, S. mekongi and S. intercalatum also infect humans and cause schistosomiasis). Other schistosome species, which parasitize birds and non-human mammals, can cause cercarial dermatitis in humans.
The life cycle of S. mansoni and related schistosomes is complex. Eggs are eliminated from a human host with feces or urine. Under optimal conditions, the eggs hatch and release miracidia, which swim and penetrate specific snail intermediate hosts. The life stages within the snail include two generations of sporocysts and the production of cercariae. Upon release from the snail, the infective cercariae swim, penetrate the skin of the human host, and shed their forked tail, becoming schistosomulae (human contact with water is thus necessary for infection by schistosomes). The schistosomulae migrate through several tissues and stages to their residence in the veins. Adult worms in humans reside in the mesenteric venules in various locations, which at times seem to be specific for each species. For example, S. japonicum is more frequently found in the superior mesenteric veins draining the small intestine and S. mansoni occurs more often in the superior mesenteric veins draining the large intestine. However, both species can occupy either location, and they are capable of moving between sites, so it cannot be stated unequivocally that either is found only in one location or another. Schistosoma haematobium most often occurs in the venous plexus of bladder, but can also be found in the rectal venules. The females (7 to 20 mm in length, slightly larger than males) deposit eggs in the small venules of the portal and perivesical systems. The eggs are moved progressively toward the lumen of the intestine (S. mansoni and S. japonicum) and of the bladder and ureters (S. haematobium), and are eliminated with feces or urine, respectively.
Pathology of S. mansoni schistosomiasis includes: Katayama fever, hepatic perisinusoidal egg granulomas, Symmers' pipe stem periportal fibrosis, portal hypertension, and occasional embolic egg granulomas in brain or spinal cord.
Schistosoma mansoni is found in parts of South America and the Caribbean, Africa, and the Middle East.