After the liver the lung is the most frequent site of the localization of hydatid cysts. Those hexacanth embryos which are able to pass through the hepatic filter cross the right side of the heart and reach their second filter, the lung; here they can attach themselves and develop into hydatid cysts.
Hydatid cysts of the lung can be central or peripheric. The latter, through their ever increasing growth, can come into contact with the pleura. Whether central or peripheric, a pulmonary hydatid cyst may empty into a bronchus and be expelled through a hydatid vomica.
Sometimes, when the entire parasite (fluid and membranes) is expelled, spontaneous healing of the disease occurs. If only the fluid is eliminated, the membranes will remain in the adventitial cavity where the retraction of the cavity's walls and the narrowness of the bronchial opening make the expulsion of the membrane almost impossible (incarcerated membrane).
ARCE J. HYDATID DISEASE (HYDATIDOSIS)HYDATID CYST OF THE LUNG. Arch Surg. 1941;43(5):789-802. doi:10.1001/archsurg.1941.01210170056005