Chest ImagingHydatid disease, pulmonary involvement
a parasitic infection caused by the tapeworms
Echinococcus granulosus and
Echinococcus multilocularis (alveolaris). The pastoral form of the disease is more common and occurs in sheep, cows or pigs and humans as the intermediate hosts (
Echinococcus granulosus), and dogs as the definitive host. The disease is endemic in sheep farming areas of South America, the Mediterranean Basin and in Australia. The so-called sylvatic form has the dog, wolf or arctic fox as definitive hosts (
Echinococcus multilocularis). It occurs in northern Europe and America. Larvae develop in the
duodenum of the new host, where they enter the blood stream and travel to the liver and lungs. Human disease is due to the cysts that form around the parasite. The hydatid
cyst itself is composed of two layers, an exocyst and an endocyst, within which the daughter cysts develop. The
pulmonary cysts may grow rapidly and rupture into the surrounding lung and
bronchial tree. Secondary infection of the ruptured
cyst is common. Occasionally the
cyst ruptures into the
pleural cavity. Uncomplicated cysts are asymptomatic. Rupture may result in an acute allergic reaction, sometimes inducing life-threatening
hypotension.
The cardinal radiographic features are one or more well-defined round or oval-shaped smooth masses, of homogeneous density in the lungs, usually in the mid or lower zones. CT scanning reveals the fluid content of the cyst with a density close to that of water (Fig.1). The daughter cysts, when present, appear as curved septations. Calcification of a pulmonary hydatid cyst is very rare. Surrounding inflammation may cause the margins of the cyst to be ill-defined. If communication develops between the cyst and the bronchial tree, air may enter between the fibrotic lung forming the pericyst and the exocyst, producing the appearance of a crescent of air at the periphery of the cyst. This sign is known as a meniscus or air crescent sign. If the cyst itself ruptures an airfluid level results, and daughter cysts may even be seen floating in the residual fluid. The floating ruptured membrane may also float giving rise to the classic water lily sign (Fig.2). All these signs are particularly well demonstrated at CT. With secondary infection, the membranes may be destroyed and the walls thickened, the appearance being indistinguishable from bacterial lung abscess. Primary or secondary hydatid cyst may also be present in the pleura. Mediastinal cysts are relatively rare. They may compress adjacent mediastinal structures or erode the bone of the thoracic cage. For a general description of the disease, see hydatid disease.
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