Hydatid disease, caused by the Echinococcus granulosus tapeworm, is often found in cattle- and sheep-raising regions of the world. It should therefore be considered in a patient presenting with a well-defined round pulmonary density who has spent time in an endemic area. The organism is usually found in dogs, coyotes, and wolves that ingest echinococcal cysts from the intestines of their prey. The ova from the cysts are released in the feces, thereby contaminating water and vegetation.

In humans, once ingested, the parasite typically penetrates the small bowel mucosa, entering the portal system. However, entry by way of intestinal lymphatics or direct inhalation has also been suggested. The most common site of lodgment is the liver. The lung may be involved in 10% to 30% of cases and is usually accompanied by synchronous hepatic lesions. As the cysts rupture, ova enter the venous circulation and can result in secondary cyst formation.

Pulmonary echinococcosis usually affects the lower lobes and is more common on the right side. Cysts are multiple in 30% of cases and bilateral in 20% of cases and may develop more quickly and extensively in children.

Two types of cysts exist: unilocular and alveolar. The unilocular type is found in the lung. These cysts are thick walled and are filled with clear hydatid fluid. The wall has three layers: a germinal layer and laminated membrane from the parasite, surrounded by a pericystic zone or adventitia from the host. The inner germinal layer is a thin, transparent layer lined with capsules containing larvae, which can form daughter cysts suspended in the fluid. The laminated membrane is 1 to 3 mm in thickness and forms the plane for surgical dissection. The outer pericystic layer is formed by local reaction against the parasite, with infiltration by leukocytes and fibro blasts. Hepatic cysts usually calcify, although this is rarely seen in pulmonary cysts.