Echinococcosis or hydatid disease is rare in the United States but frequent in Sicily, Turkey, South America, Australia, and New Zealand. It is caused by the larval or cystic stage (metacestode) of the dog tapeworm. Four species of the parasite have been identified: Echinococcus granulosus (by far the commonest), E. obligarthrus, E. patagonicus, and E. multilocularis. Its definitive hosts are dogs, wolves, cats, and other carnivores. The intermediate or cystic stage usually affects sheep, hogs, and cows, but man or other mammals can become infected.
The most common sites of echinococcus cysts are the liver (60% to 70% of patients), brain, and lung, but they may occur in many locations, including spleen, soft tissue, bone, breast, heart, and spinal extradural space. When the cyst is viable, the skin and complement fixation tests are often positive, and eosinophilia is frequent. Death of the parasite is accompanied by collapse of the wall and calcification. At this stage, the skin test is of little value, and eosinophilia is present in less than 5% of the cases. The laboratory diagnosis can usually be made by hydatid serology and confirmed or established by ultrasound or computed tomography. Reliable antibody detection tests have been developed for E. multilocularis but not for E. granulosus. Clinical manifestations of involvement of the liver include hepatomegaly, obstructive jaundice, and cholangitis.
Communication with the biliary tract and superimposed infection are frequent in echinococcosis of the liver. Rupture of the cysts into the peritoneal cavity may result in a fatal anaphylactic reaction or in the formation of innumerable small granulomas grossly resembling peritoneal tuberculosis. The diagnosis is made by identifying fragments of germinal membrane or scolices in their center. Hepatic echinococcus cysts also can rupture inside the gallbladder or through the diaphragm into the pleural space and lung.
Grossly, hydatid cysts are solitary in about two thirds of cases. The cysts average 1 to 7 cm. Histologic examination of the cyst wall shows an outer chitinous (or fibrous laminar) layer and an inner germinal layer; this may be surrounded by either granulation tissue or a fibrous capsule (so-called pericyst layer). Calcification in the latter layer signifies that the cyst is dead. The neighboring parenchyma often shows pressure atrophy and a moderately intense infiltrate in which eosinophils may be prominent. The viable cyst is filled with colorless fluid, which contains daughter cysts and brood capsules with scolices. In some cases, daughter cysts are present outside the fibrous laminar layer of the cyst; these are referred to as extracapsular or satellite cysts. The scolices can be easily identified after macerating a portion of the germinal layer in saline solution. They have characteristic hooklets, 20 to 40 µm in length.
The surgical treatment options for uncomplicated hydatic cyst are evacuation, scolicidal irrigation, primary closure, and radical excision of the cyst by either pericystectomy or resection.