A tissue infection caused by the larval stage of the Echinococcus granulosus worm. In the U.S., the disease has been reported in the lower Mississippi valley, Alaska, and northwestern Canada. Humans become infected when they swallow eggs passed in dog feces. The infection is carried to the liver where cysts form.
Some cysts form in the lung.
Infrequently cysts form in the brain, bones, skeletal muscles, kidney, spleen, or other tissues. A liver cyst may produce no symptoms for 10 to 20 years until it is large enough to be felt by physical examination or to produce symptoms. Risk factors include exposure to cattle, sheep, pigs, deer, or exposure to the feces of dogs, wolves, or coyotes. The incidence is 1 out of 100,000 people. In endemic areas, health education and routine worming of dogs to remove tapeworms help prevent the disease.