Hydatid cyst of the right atrium wall

Hydatid disease (echinococcosis) is caused by the larvae of Echinococcus granulosus. Sheep are the intermediate hosts of the cystic stages of these tapeworms. Dogs, another host, defecate outside, and further spread may be via streams, effluent or flies. Thick egg shells protect them from environmental extremes. Sheep acquire strong resistance against new cysts developing but this has little effect on existing cysts. The cycle is perpetuated as carcasses of infected sheep are eaten by dogs. Small cysts are susceptible to desiccation but large hydatid cysts are very resilient. Cool, moist conditions are most favourable for survival. Freezing is not likely to be severe enough in the field to kill a significant number of cysts. After ingestion, larvae pass the intestine and reach the right side of the heart through the thoracic duct and superior vena cava; from the right ventricle the embryo passes through the pulmonary vascular bed into the left ventricle, from where it could reach any part of the body through systemic circulation. The first successful surgical intervention on hydatid cyst was reported by Long in 1932.