14. Hydatid pulmonary cyst (echinococcosis)

AETIOLOGY AND LIFE CYCLE

Pulmonary hydatid disease is caused mainly by the larval stage of Echinococcus granulosus.
Dogs and other canids are definitive hosts; sheep and other livestock are intermediate hosts.
Humans acquire infection by ingesting eggs from contaminated food, water or hands.
Oncospheres penetrate the intestinal wall and travel via the circulation; the liver is most commonly affected, followed by the lungs.
Pulmonary cysts may be solitary or multiple and may coexist with hepatic cysts.

A hydatid cyst has an outer host-derived pericyst, laminated acellular membrane and inner germinal layer that produces brood capsules and daughter cysts.
In the lung, cysts are often spherical and may enlarge because of the compliant parenchyma.
Rupture into a bronchus may produce cough, expectoration of cyst fluid or membranes, haemoptysis and allergic reactions.
Rupture into pleura can cause pneumothorax, hydropneumothorax, empyema or dissemination.
Anaphylaxis is possible but not inevitable.

Small cysts may be asymptomatic and found incidentally.
Larger cysts cause cough, chest pain, dyspnoea, recurrent infection or haemoptysis.
Rupture may cause sudden cough with salty fluid or membrane expectoration, fever, urticaria, wheeze or anaphylaxis.
Ask about residence/travel in endemic regions, animal exposure and previous hydatid disease.
Examine for respiratory compromise and signs of hepatic involvement.