Skip to Main Content

1-15 of 15 Results

eFigure 35–17. Life cycle of Entamoeba histolytica. Transmission of infective cysts of this abundant amebic parasite of humans (with pathologic results in 10–20% of infections) may involve fecally contaminated flies, water, fingers, or food. The damage caused by these parasites can involve ulceration of the colon or passage through the intestinal mucosa and spread to other organs. Ingested cysts are acted on by stomach and duodenal enzymes and rapidly excyst. The tetranucleated trophozoite (1) separates into four uninucleated amebae (2), each of which divides mitotically, resulting in eight uninucleated amebulae (3). Occasionally, trophozoites penetrate the mucosa, ingest red blood cells (4), and initiate the ulceration process, which may spread extraintestinally. Normally, the trophozoites multiply in the lumen of the colon and form a commensal colony, feeding on fecal bacteria (5). The trophozoites complete their vegetative phase and begin cyst formation by a process consisting of loss of water and rounding up (6), formation of a central vacuole with chromatoidal particles (7), an early cyst maturation stage with development of typical round-ended chromatoidals (binucleated stage) (8), and final maturation, often completed after passage in the feces, to form the tetranucleated cyst (9), which loses its chromatoidals and becomes infective to 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

View in Context

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

View in Context

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

View in Context