Echinococcosis

Echinococcosis is a zoonotic parasitic disease caused by cestodes of the genus Echinococcus, primarily Echinococcus granulosus and Echinococcus multilocularis. The two major clinical forms are cystic echinococcosis, also known as hydatid disease, and alveolar echinococcosis.

Humans are accidental intermediate hosts, typically infected via ingestion of eggs shed in the feces of definitive hosts, such as dogs or foxes. In cystic echinococcosis, ingested eggs release oncospheres that migrate through the intestinal wall into the bloodstream and localize in organs, most commonly the liver or lungs, where they form unilocular fluid-filled cysts. In alveolar echinococcosis, larvae form invasive, tumor-like masses, primarily in the liver, which can infiltrate adjacent structures and metastasize, resembling malignancy. Host immune response and cyst integrity determine disease progression and complications.

Cystic echinococcosis, caused by Echinococcus granulosus sensu lato, is endemic in pastoral regions of the Middle East, North and East Africa, South America, Central Asia, and parts of China, where close contact between dogs and livestock facilitates transmission. It is also found worldwide in rural grazing areas where dogs consume infected offal. Genotypes G1 and G3, associated with sheep, are the most prevalent. The global burden is several hundred thousand cases, with most human infections occurring in endemic regions or among immigrants from these areas. Alveolar echinococcosis, caused by Echinococcus multilocularis, is less common but more lethal due to its infiltrative growth. It occurs across the northern hemisphere, particularly in Central and Eastern Europe, Russia, China, and Japan, with rare cases in North America, including Alaska, Canada, and the U.S. Midwest. E. vogeli and E. oligarthrus, which cause polycystic echinococcosis, are restricted to Central and South America.

Close contact with infected dogs or wild canids, poor hygiene, residence in endemic rural regions, involvement in livestock farming or herding, and consumption of contaminated food or water increase the risk of infection.

Echinococcosis often follows an indolent course, with both cystic and alveolar forms remaining asymptomatic for years. Clinical manifestations typically emerge when cysts enlarge or infiltrate tissue, causing mass effect or organ dysfunction. In cystic echinococcosis, the liver and lungs are most commonly affected, and symptoms may include abdominal discomfort, hepatomegaly, jaundice, cough, or chest pain, depending on cyst size and location. Brain or other visceral involvement can result in neurologic or systemic complications. Alveolar echinococcosis presents with progressive hepatic invasion, potentially leading to cholestasis, portal hypertension, or liver failure, and may mimic malignancy. Rupture of cysts can provoke allergic reactions or anaphylaxis. In immunocompromised patients, particularly those with AIDS or post-transplant immunosuppression, both forms (especially alveolar echinococcosis) may progress rapidly and present atypically, delaying diagnosis and complicating management.

Outcomes vary by disease form, lesion location, and treatment response. In cystic echinococcosis, univesicular liver cysts (types 1 and 2) treated medically for 3-6 months show cure rates of up to 82%, though relapse occurs in over 25% of cases, typically within 2 years. Lifelong monitoring is recommended. Alveolar echinococcosis carries a more guarded prognosis due to its infiltrative behavior, particularly in advanced or immunocompromised patients, where rapid progression and treatment resistance are more common.