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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Cestodes

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Echinococcus granulosus - Parasitology

The adult E. granulosus inhabits the small bowel of dogs, wolves, and other canines, where it survives for a scant 12 months.

ECHINOCOCCUS

 

Echinococcosis is a tissue infection of humans caused by larvae of Echinococcus granu-losus and E. multilocularis. The former is a more common cause of human disease.

 

Echinococcus granulosus

 

PARASITOLOGY

 

The adult E. granulosus inhabits the small bowel of dogs, wolves, and other canines, where it survives for a scant 12 months. The scolex, like that of the genus Taenia, pos-sesses four sucking discs and a double row of hooklets. The entire strobila, however, mea-sures only 5 mm in length and contains but three proglottids; one immature, one mature, and one gravid. The latter segment splits either before or after passage in the stool, releas-ing eggs that appear identical to those of T. saginata and T. solium. A number of mammals may serve as intermediates, including sheep, goats, camels, deer, caribou, moose, and, most important, humans. When one of these hosts ingests eggs, they hatch. The embryos penetrate the intestinal mucosa and are carried by the portal blood to the liver. Here, many are filtered out in the hepatic sinusoids. The rest traverse the liver and are carried to the lung, where most lodge. A few pass through the pulmonary capillaries, enter the systemic circulation, and are carried to the brain, heart, bones, kidneys, and other tissues. Many of the larvae are phagocytosed and destroyed. The survivors form a cyst wall composed of an external laminated cuticle and an internal germinal membrane. The cyst fills with fluid and slowly expands, reaching a diameter of 1 cm over 5 to 6 months. Secondary or daughter cysts form within the original hydatid. Within each of these daughter cysts, new protoscol-ices are produced from the germinal lining. Some break free, dropping to the bottom of the cyst to form hydatid sand. When hydatid-containing tissues of the intermediate host are ingested by a canine, thousands of scolices are released in the intestine to develop into adult worms.


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