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eFigure 35–25. Life cycle of Schistosoma mansoni (blood fluke). Contact with water exposes skin to cercarial penetration. Worms mature in the liver and migrate to mesenteric vessels, where females lay eggs that work through into the lumen of the large intestine and pass out in human wastes. Eggs captured in intestinal tissues or liver are encapsulated in a granulomatous response (inset). Susceptible intermediate host snails (Biomphalaria) are penetrated by miracidia that hatch from eggs that reach snail-inhabited water. Sporocyst multiplication in the snail results in large numbers of infective cercariae that leave the snail and seek to penetrate human skin. 1–9: The skin is penetrated by a cercaria, which breaks off its forked tail and passes into the dermis through a hair follicle (1). A mature pair of worms in the liver (2) migrates up mesenteric vessels (3) to egg-laying sites near or within villi. The eggs cytolyze or mechanically work their way through villi into the gut lumen (4), enter water with feces (5), and quickly hatch, releasing ciliated miracidia (6) that penetrate a host snail. The penetrating larva passes through two or perhaps several generations of sporocysts (7). Cercariae produced by the final sporocyst generation pass into the water (8) and collect near the surface, suspended by their forked tails (9), awaiting the stimulus of nearby human skin for penetration and onset of a new cycle. (Reproduced, with permission, from Goldsmith R, Heyneman D [editors]. Tropical Medicine and Parasitology. Originally published by Appleton & Lange. Copyright © 1989 by The McGraw-Hill Companies, Inc.)

Current Medical Diagnosis & Treatment 2018 > Protozoal & Helminthic Infections

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