Larval stages of the tapeworm Echinococcus granulosus, the most common Echinococcus, cause cystic echinococcosis or hydatid disease. Hydatid cysts enlarge slowly and require several years for development. Developed cysts range from 1–15 cm in diameter, but may be larger. Infections may be asymptomatic until cysts cause noticeable mass effect; signs and symptoms vary according to location, cyst size, cyst type and numbers. Ruptured or leaking cysts can cause severe anaphylactoid reactions and may release protoscolices that can produce secondary echinococcosis. One or several cysts, typically spherical, thick-walled and consisting of a single cavity (unilocular), are most frequently found in the liver and lungs, although they may occur in other organs.

Clinical diagnosis is based on signs and symptoms compatible with a slowly growing tumor, a history of residence in an endemic area, and association with canines. Differential diagnoses include benign tumor, malignancies, amebic abscesses, and congenital cysts. Ultrasonography, computerized tomography and serological testing are useful for supporting diagnosis, with ultrasonography the method of first choice. WHO has developed a classification of ultrasound images of liver cystic echinococcosis for diagnostic and prognostic purposes and determination of the type of intervention required (see Specific treatment, 9B7). Definitive diagnosis in seronegative patients, however, requires microscopic identification from specimens obtained at surgery or by percutaneous aspiration; the potential risks of this (anaphylaxis, spillage) can be avoided by ultrasound guidance and anthelminthic coverage. Species identification is based on finding thick laminated cyst walls and protoscolices as well as on the structure and measurements of protoscolex hooks. Molecular techniques are now available to identify the species from biopsies.