Life cycle of schistosome parasites causing schistosomiasis in humans
The life cycles of the schistosomes causing schistosomiasis are complex. Eggs are eliminated from a human host with feces or urine (1). Under optimal conditions, the eggs hatch and release miracidia (2), which swim and penetrate specific snail intermediate hosts (3). The life stages within the snail include two generations of sporocysts (4) and the production of cercariae (5). Upon release from the snail, the infective cercariae swim, penetrate the skin of the human host (6), and shed their forked tail, becoming schistosomulae (7) (human contact with water is necessary for infection by schistosomes). The schistosomulae migrate through several tissues and stages to their residence in the veins (8,9). Adult worms in humans reside in the mesenteric venules in various locations, which at times seem to be specific for each species (10). For example, S. japonicum is more frequently found in the superior mesenteric veins draining the small intestine (A) and S. mansoni occurs more often in the superior mesenteric veins draining the large intestine (B). 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 (C), 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) or of the bladder and ureters (S. haematobium) and are eliminated with feces or urine, respectively (1).
Various animals--including dogs, cats, rodents, pigs, horses, and goats for S. japonicum and dogs for S. mekongi--serve as parasite reservoirs. 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 and spinal cord.