Findings  -  Hydatid cyst in lower lobe right lung (with air pocket suggest endobronchial rupture of hydatid cyst).
Hydatid cyst within the spleen.

Chest radiograph findings

Unless the cyst ruptures, it appears of uniform density on the plain chest X-ray. The typical appearance of the ruptured cyst is the \'water lilly sign\' of an irregular margin of the partly collapsed cyst. This underlies the foreign nature of the tissue of the parasite. There are no vessels or fibrous strands that connect it to surrounding tissue. The elastic lung is always under tension (will collapse with a pneumothorax) and will hold open any other cystic lung disease that involves a direct connection to lung tissue. Hydatid disease in lung or liver can usually be \'shelled out\' because it is separate from and compresses neighboring tissue. There is rarely calcification; even then this tends to occur in the compressed lung around the hydatid cyst.
Discussion:

Hydatid disease is a parasitic infestation by a tapeworm of the genus Echinococcus. Of the 4 known species of Echinococcus, 3 are of medical importance in humans. These are Echinococcus granulosus, causing cystic echinococcosis (CE); Echinococcus multilocularis, causing alveolar echinococcosis (AE); and Echinococcus vogeli. E granulosus is the most common of the three. E multilocularis is rare but is the most virulent, and E vogeli is the rarest.

This disease occurs when humans ingest the hexacanth eggs of the dog tapeworm. Symptoms depend upon the size and site of the lesion. Slowly enlarging echinococcal cysts generally remain asymptomatic until their expanding size or their space occupying effects in an involved organ elicits symptoms (as in this case). Liver and lung are most common sites. They may discover incidentally on chest radiographs and abdominal ultrasonography. Pulmonary hydatid cyst may rupture into the bronchial tree (as in this case) and produce cough, chest pain and haemoptysis. Cyst may involve any organ, may involve bone, central nervous system and heart. In bone it may cause invasion of the medullary cavity with slow bone erosion producing pathologic fracture, in CNS it present as space occupying lesion and in heart it cause conduction defect and pericarditis. Radiological studies play a very important role in detecting and evaluating hydatid cysts. Ultrasonography, CT, echocardiography and MRI are of great value in diagnosing and determining the anatomic extent and relationship of the cyst. Chest radiograph are of great importance in diagnosing lung hydatid cyst.