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How did we inherit the word hydatid? It was derived from the Greek word, hudatid, that means watery vesicle. Hydatid disease or echinococcosis is a parasitic infection caused by the larval stage of the dog tapeworm Echinococcus granulosus or, less frequently, Echinococcus multilocularis. The adult tapeworm is approximately 2–11 mm in length and lives in the upper small bowel of the definitive host, usually a domestic dog. Intermediate hosts including sheep, cattle, pigs and humans become infected by ingesting eggs that are passed in the feces of the definitive host. In the intermediate hosts, digestive processes lead to the release of larvae (embryos) that penetrate the bowel mucosa and pass to the liver via the portal vein. Most larvae (70%) are trapped in the liver but a minority pass through the liver and develop in other organs, including the lungs (20%), spleen, kidney and brain. The disease is endemic in many parts of the world including the Mediterranean region, the Middle East, South America, New Zealand and Australia.

Larvae that evade host defence mechanisms become encysted and grow at a rate of approximately 1 cm per year. This is well-tolerated by humans until the cyst becomes large enough to cause pain and/or pressure effects. The longevity of cysts appears to vary widely but most cysts (80%) are non-viable in patients over 60 years of age. The appearance of an hydatid cyst on magnetic resonance imaging using T2-weighted sequences is shown in Figure 1. There is some calcification of the cyst wall, a hyperintense (white) appearance caused by fluid and hypointense structures within the cyst representing floating internal membranes. The patient was treated by surgical resection and the gross appearance of the cyst is shown in Figure 2. The outer layer is a fibrous capsule (pericyst) formed by the host response to the infection. The second layer (ectocyst) consists of an acellular ‘laminated’ membrane, while the third inner layer (endocyst) is a thick nucleated membrane called the germinal membrane. From the inner layer of the latter membrane, brood capsules expand into the cystic cavity and produce multiple protoscolices by asexual budding.