formation of cysts, or hydatids, at the site of infestation by the larval form of Echinococcus granulosus, a tapeworm common in sheep, cattle, camels, dogs, and many other mammals. The disease can develop in humans upon ingestion of the eggs, which may be present in the tissues of infected animals or on food contaminated by their excreta. The emergent larvae become enveloped in a cyst, most commonly in the liver, that may grow for 5 to 20 years without being detected. Surgical excision of the cyst is the only effective treatment, but in many cases the disease recurs because the contents of the cyst may escape during the operation.