Causal Agents:
Schistosomiasis is caused by digenetic blood trematodes.  The three main species infecting humans are Schistosoma haematobium, S. japonicum, and S. mansoni.  Two other species, more localized geographically, are S. mekongi and S. intercalatum.  In addition, other species of schistosomes, which parasitize birds and mammals, can cause cercarial dermatitis in humans.

Life Cycle:

Eggs are eliminated with feces or urine .  Under optimal conditions the eggs hatch and release miracidia , which swim and penetrate specific snail intermediate hosts .  The stages in the snail include 2 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 .  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 instance, 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 is not possible to state unequivocally that one species only occurs in one location.  S. haematobium most often occurs in the venous plexus of bladder , but it can also be found in the rectal venules.  The females (size 7 to 20 mm; males slightly smaller) 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 and S. japonicum 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.  Pathology of S. haematobium schistosomiasis includes: hematuria, scarring, calcification, squamous cell carcinoma, and occasional embolic egg granulomas in brain or spinal cord.

Human contact with water is thus necessary for infection by schistosomes.  Various animals, such as dogs, cats, rodents, pigs, hourse and goats, serve as reservoirs for S. japonicum, and dogs for S. mekongi.

Geographic Distribution:
Schistosoma mansoni is found in parts of South America and the Caribbean, Africa, and the Middle East; S. haematobium in Africa and the Middle East; and S. japonicum in the Far East.  Schistosoma mekongi and S. intercalatum are found focally in Southeast Asia and central West Africa, respectively.