Hydatid disease is a parasitic infestation by a tapeworm of the genus Echinococcus. The liver is the most common organ involved,followed by the lungs. The life cycle of Echinococcus granulosus includes dogs (and other canines) as the definitive host,and a variety of species of warm-blooded vertebrates (sheep,cattle,goats,and humans) as the intermediate host. The adult worms are very small,usually consisting of only three proglottids (total length = 3-6 mm) that live in the animal's small intestine. Eggs are liberated in the host's feces,and when the intermediate host ingests these eggs,they hatch in the host's small intestine. The larvae in the eggs penetrate the gut wall and enter the circulatory system. The larvae can be distributed throughout the intermediate host's body (although most end up in the liver) and grow into a stage called a hydatid cyst. Hydatid cysts have the ability to grow quite large; cysts the size of golf balls are not uncommon,and cysts the size of basketballs are reported on rare occasions. One or two small cysts in the liver of a host might go unnoticed for years. However,a single large cyst in the liver could prove fatal. Hydatid disease is far more serious when the cysts are found in other locations,particularly the brain |