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eFigure 35–40. Life cycle of Diphyllobothrium latum (broad fish tapeworm). Ingestion of raw or inadequately cooked fish containing plerocercoid larvae is followed by development of a tapeworm in the small bowel and passage of feces containing operculated eggs. Eggs deposited in a freshwater pond or lake hatch and infect the first intermediate host, a copepod (water flea). The infected copepod conveys the early larval form to a small fish, which can in turn infect a number of piscivorous fish until a human or other piscivorous mammal feeds on the larger fish and acquires the infection. 1–6: The scolex (1) of an adult worm attaches by sucking grooves to the wall of the small intestine. Mature segments (2) deposit eggs in the gut lumen that are passed in stool (3). Eggs that reach a freshwater pond hatch after a period of development, releasing the ciliated coracidium (4), which develops in the first intermediate (copepod) host into the procercoid (5). Fish—often minnows—feed on the copepods and digest the procercoids free. The procercoids penetrate the gut, pass to the fish musculature, and mature into a nonencysted plerocercoid (6) capable of passing from the gut of one transport fish host to the flesh of a larger piscivorous host. The final transfer occurs when a human or other piscivorous mammal feeds on the infected fish and digests the plerocercoid free. The young worm attaches by its scolex and grows into an adult tapeworm, often 8 m or more in length and up to 2 cm in breadth. (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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