Cystic echinococcosis, or hydatid cyst disease, is a globally distributed zoonotic infection caused by the cestode species Echinococcus granulosus.1The adult parasite is a small tapeworm that infects the small intestine of carnivores, usually canids. Their eggs are shed in the feces of these definitive hosts and are inadvertently ingested by intermediate hosts such as sheep. In endemic areas, such as the Turkana district of Kenya, where people live in close proximity to their dogs and livestock, humans may accidentally ingest the eggs and also become infected. After ingestion, worm embryos, called oncospheres, are released from the eggs, migrate through the intestinal wall, and spread hematogenously. Once lodged in a capillary bed, the oncospheres develop into metacestodes, which are slow-growing, unilocular larval cysts also known as hydatid cysts. They have an outer acellular, laminated layer exclusive to the genus Echinococcusand an inner living, germinal layer from which tapeworm larvae, called protoscolices, bud in an asexual fashion.2The host-derived, fibrous capsule that forms around the outer cyst wall is sometimes considered a third cyst layer. The parasite's life cycle is completed when the protoscolices are ingested by a definitive host while it is feeding on intermediate host tissues. The protoscolices then evaginate, attach to the definitive host's intestinal mucosa, and mature into adult tapeworms.
Pathology Quiz Case 1: Diagnosis. Arch Otolaryngol Head Neck Surg. 2009;135(10):1056-1060. doi:10.1001/archoto.2009.124-b