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eFigure 35–28. Life cycles of Fasciola hepatica and F gigantica (liver flukes). A grazing cow ingests vegetation contaminated with metacercariae. Young flukes excyst, penetrate through the intestine, and pass directly into the liver parenchyma and bile ducts, where maturation of flukes follows. Undeveloped eggs are excreted via bile into stool. If this occurs in marshy or wet areas inhabited by host snails (Lymnaea), the miracidia within the eggs develop, hatch, and infect the snails by penetration. In the snail, miracidia become sporocysts that produce rediae that multiply and finally produce swarms of cercariae. These motile larvae emerge, swim from the snail, and encyst on nearby aquatic vegetation. 1–10: After excystation (1), the fluke passes through the gut wall directly into the liver and bile ducts (3). The mature fluke (2) produces large numbers of undeveloped eggs (4) that pass in feces from the host, mature in water (5), hatch, and release miracidia (6) that invade a suitable snail in which multiplication occurs through sporocyst formation and successive generations of rediae (7, 8), resulting finally in shedding from the snail of great numbers of cercariae (9). These swimming larvae encyst on aquatic or immersed vegetation (10), the source of infection of cattle and other herbivores or of humans. (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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eFigure 35–29. Life cycle of Clonorchis sinensis (Chinese liver fluke). Ingestion of raw fish infected with viable metacercariae is followed by development of mature flukes in hepatic bile ducts and then passage of fluke eggs in feces with access to water in which the appropriate snail hosts (Parafossarulus) are found. A suitable freshwater fish host such as the carp then becomes infected with metacercariae. 1–10: After excystation (1) in the human duodenum of cysts ingested with raw or undercooked freshwater fish, an adult fluke (2) in the bile duct (3) lays eggs (4) in bile that pass to the intestine and are deposited with stool into a fishpond. Feces are eaten by a snail, and the ingested egg hatches within the snail, releasing a miracidium (5) that penetrates snail tissues and forms a mother sporocyst (6) which produces a number of rediae (7). The rediae proceed through successive generations, ultimately filling much of the snail. The final redial generation produces numbers of large-tailed cercariae (8) that leave the snail, swim to a fish, crawl between its scales (9), and encyst in the tissues (10) to be eaten by another human or piscivorous mammalian reservoir host to sustain the 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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