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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:

Life cycle of Schistosoma spp.

Eggs are eliminated with feces or urine The number 1. Under optimal conditions the eggs hatch and release miracidia The number 2, which swim and penetrate specific snail intermediate hosts The number 3. The stages in the snail include 2 generations of sporocysts The number 4 and the production of cercariae The number 5. Upon release from the snail, the infective cercariae swim, penetrate the skin of the human host The number 6, and shed their forked tail, becoming schistosomulae The number 7. The schistosomulae migrate through several tissues and stages to their residence in the veins (The number 8, The number 9). Adult worms in humans reside in the mesenteric venules in various locations, which at times seem to be specific for each species The number 10. For instance, S. japonicum is more frequently found in the superior mesenteric veins draining the small intestine The letter A, and S. mansoni occurs more often in the superior mesenteric veins draining the large intestine The letter B. 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 The letter C, 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 The number 1. 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.

Life cycle image and information courtesy of DPDx.

 
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