The larval stage of the cestode (tapeworm) Echinococcus granulosus causes cystic echinococcosis, the form of human echinococcosis (hydatidosis, or hydatid disease) most frequently encountered. Echinococcus granulosus occurs practically worldwide and more frequently in rural, grazing areas where dogs ingest organs from infected animals.
The adult Echinococcus granulosus (3 to 6 mm long) resides in the small bowel of the definitive hosts, dogs or other canids. Gravid proglottids (bisexual reproductive segments) release eggs that are passed in the feces. After ingestion by a suitable intermediate host (under natural conditions: sheep, goat, swine, cattle, horses, camel), the egg hatches in the small bowel and releases an oncosphere that penetrates the intestinal wall and migrates through the circulatory system into various organs, especially the liver and lungs. In these organs, the oncosphere develops into a cyst that enlarges gradually, producing protoscolices and daughter cysts that fill the cyst interior. The definitive host becomes infected by ingesting the cyst-containing organs of the infected intermediate host. After ingestion, the protoscolices evaginate, attach to the intestinal mucosa, and develop into adult stages (7) in 32 to 80 days.