Contributors: J. Oscar Croxatto and Luis Crovetto
Hydatid disease (echinococcosis,  human cystic echinococcosis, Echinococcus infection) is an infection caused by cestodes of the genus Echinococcus. The metacestode forms are called hydatid cyst, and the disease they cause, hydatid disease. The disease is more prevalent in rural areas of  the Middle East, Central and South America, North Africa, Central Europe, and Mediterranean countries. Echinococcus granulosus lives in the small intestine of many domestic and wild carnivores, the dog been the most important. The intermediate host, most often sheep, ingests eggs discharged in the feces. Humans act as intermediate hosts when they ingest ova accidentally. Children and adults may be affected. The eggs hatch in the intestine and release the oncosphere which pass through the mucosa into the portal circulation. Cyst formation represents the degeneration of the oncospheral stage and emergence of the metacestode stage. Life cycle is completed when viable cysts are ingested by the definitive host.
The clinical manifestations of hydatid disease are determined by the location of the cysts and their size. The cysts are most frequently located in the liver [liver disease] and the lungs, and less frequently in the spleen, kidney, heart, central nervous system, and elsewhere. The most common ophthalmic manifestation is an orbital cyst [cyst - orbit] and orbital involvement accounts for 1-2% of all hydatid cysts. In certain geographic locations orbital hydatid cysts account for 5-20% of orbital tumors. Occasionally, hydatid cysts have been reported in the vitreous, subretinal space and anterior chamber. Hydatid cysts are fairly characteristic when combined with demographic data. In older patients hydatid cysts of the orbit may resemble large orbital mucoceles [mucocele].