The large cystic lesion of the liver is a hydatid cyst of Echinococcus granulosus, a cestode (tapeworm) infection in which man becomes the intermediate host. Domestic farm animals including sheep, cattle, and hogs are the typical intermediate hosts, with carnivores (including domestic dogs) the definitive hosts. Tapeworm eggs, when ingested, hatch to form embryos (oncospheres) that penetrate the intestinal wall and migrate to various tissues, often liver or lungs, where hydatid cyst development occurs over many years. The laminated wall of the cyst seen here is lined by a germinal epithelium from which daughter larvae develop by the thousands and float into the clear fluid filling the unilocular cyst of Echinococcus granulosus. These larvae (several of which are seen here microscopically with characteristic hooklets of the scolex) settle out and form the so-called "hydatid sand".
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