Hydatid disease (echinococcosis) is caused by the larval forms of Echinococcus granulosus, Echinococcus multilocularis and Echinococcus vogeli. Echinococcus granulosus (unilocular cystic echinococcosis) is the most common form affecting man and is seen in the Mediterranean area, Eastern Europe, Africa, South America, the Middle East, Australia and New Zealand.

Humans are accidental hosts and acquire infection by ingesting ova from fomites or contaminated water and by direct contact with dogs. Cysts in the mediastinum, heart and pulmonary arteries are rare.

The clinical manifestations of pulmonary hydatidosis are non-specific. Complications occur because of cyst rupture. Hydatid cysts are usually solitary but may be multiple and/or bilateral in 10% of cases. They may be ruptured (two-thirds) or unruptured (one-third) at the time of presentation. Aggressive invasion of vascular structures such as bronchial and pulmonary arteries may result in massive haemoptysis and haemorrhage.

The radiological findings in patients with unruptured pulmonary cysts are one or more homogeneous, roughly spherical or oval, sharply demarcated mass lesions. Cyst rupture is usually associated with secondary infection and may spread into the airways or pleural space. 

The radiographic appearance resembles the air crescent of a mycetoma. Should there be disruption of the inner layers, a complex cavitary lesion results with one or more of the following radiographic features: an air–fluid level, a floating membrane (water lily sign, camalote sign) (Fig. 1), a double wall, an essentially dry cyst with crumpled membranes lying at its bottom (rising sun sign, serpent sign), and a cyst with all its contents expectorated (empty cyst sign). Secondary infection of a hydatid cyst may produce a lung abscess with or without surrounding lung opacity. Rupture into the pleural space causes an effusion or, if there is airway communication, a hydropneumothorax.

FIGURE 1 ■ Ruptured hydatid cyst. A 65-year-old male shepherd with abrupt onset of expectoration and pruritus. Close-up view of the right upper lung shows a cystic lesion surrounded by a parenchymal consolidation due to a massive aspiration of intracystic content. Note a rounded opacity immediately above the fluid level ('water lily' sign) (arrows).

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