Etiology and Pathogenesis

Cystic echinococcosis is a zoonosis caused by larval forms of the tapeworm Echinococcus granulosus and transmitted by dogs. In humans, the disease is characterized by slowly growing cyst commonly occurring in liver and lungs. Clinical features of hepatic hydatid cyst are mainly right upper quadrant pain, feeling of lump and enlarged tender liver. The cyst may be complicated by infection or rupture and may lead to anaphylactic reaction. Ultrasonography supported by serology is the main diagnostic modality. Treatment of univesicular cyst is predominantly medical or percutaneous. Percutaneous treatment (PAIR) is safe and effective and complications are infrequent. For multivesicular and complicated cyst surgery remains the mainstay of treatment. © parenchyma by scolices and the disease behaves like a malignant disease. ETIOLOGY AND PATHOGENESIS Adult tapeworm lives in the upper small bowel of the domestic dogs, a definitive host. Other definitive hosts are wolves, jackals, domestic cats, and reindeer etc. Sheep, cattle, pigs and humans contain larval stage and are intermediate hosts. They are infected feco orally by Echinococcus eggs shed in the environment with feces of infected dogs. Eggs hatch in intestinal mucosa and transform into oncospheres which penetrate bowel wall, enter portal venous blood, and are carried to the liver. The embryos become encysted and grow at the rate of about 1 cm/yr. The hydatid cyst is composed of a fibrous and inflammatory outer layer of host origin, a laminated cuticle and a germinal membrane where protoscoleces grow and secrete clear fluid. From germinal layer, daughter cysts may be formed by invaginations. The life cycle is completed when the definitive host is infected by viable cyst containing organs of intermediate host. In the intestine of the dog, protoscoleces develop into adult tapeworms. Man is a dead end host and becomes fecoorally infected by echinococcal eggs.