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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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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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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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