Histopathology of the neglected zoonosis: cystic echinococcosis

Hydatid disease (cystic echinococcosis), is a parasitic disease caused by the intermediate stage of echinococcus granulosus in humans. The adult parasite is a small tapeworm living in the intestines of canidae. The dog-sheep cycle besides pastured cattle is important in many geographic areas. Hundreds of surgical operations to remove hydatid cysts and related complications take place in each year. Cysts occur mostly in the liver and lungs, and are usually less than 10 cm in diameter when clinically diagnosed but can attain larger sizes. Rupture and spillage of cyst fluid during surgery or trauma are important. If the hydatid cyst content has viable protoscolices (about 50% among resected surgical cases in our region), these structures result in numerous new cysts in the spillage area, besides a risk of anaphylaxis to the cyst fluid. Hydatid cysts in humans have a thick acellular laminated membrane with a tiny inner germinative layer yielding protoscolices which helps to distinguish it from other cestode cysts. A related parasite, echinoccus multilocularis (alveolaris) lesions mimic malignancy with invasive minute cysts without germinative layer and protoscolices in humans. Another cystic tissue parasite of humans, cysticercosis shows minute cysts (commonly less than 2 cm) with a larval form in each. Hydatid cysts may loose their germinative layer and protocolices in time, however, a periodic acid-Schiff (PAS) stain may reveal the thick laminated membrane of the parasite differentiating it from an epithelial cyst or amebic abscess readily.