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eFigure 35–33. Life cycle of Paragonimus westermani (lung fluke). Ingestion of cysts in uncooked freshwater crab or crayfish meat is followed by excystation of juvenile worms in the duodenum and migration to the lungs, where they form worm pairs enclosed in host tissue. Eggs from adult worms are passed to the environment in sputum or stool and develop in the shell. Miracidia hatch and penetrate an appropriate snail (Semisulcospira). Massive multiplication of rediae occurs in the snail, which in turn produces many cercariae that leave the snail and crawl on the bottom of a pond in search of a crustacean intermediate host. 1–11: Metacercariae (1) in crab tissue undergo excystation (2) in the human duodenum; (3) and (4) show an adult worm in the lung and a cross section of a worm pair in a lung capsule. Undeveloped eggs (5) are carried to the mouth, swallowed, and passed in stool. Hatching of a miracidium (6) in suitable fresh water after development in the eggshell is followed by penetration of a snail by the miracidium, which changes into a mother sporocyst (7) in snail tissue. Rediae leave the sporocyst and initiate successive generations of redial progeny (8), which fill the snail. Cercariae emerge from the final redial generation (9) and move along the bottom of the pond, using an adhesive substance in a round bunny tail, toward an intermediate host, which they penetrate, aided by a penetration stylet (10, 11), whereupon a metacercaria encysts in the flesh of a crab. (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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