Cestode Zoonoses - ECHINOCOCCOSIS

Echinococcus granulosis - causes "cystic" disease. Echinococcus multilocularis - causes "alveolar" disease. E. vogeli - causes polycystic disease.

RESERVOIR AND INCIDENCE

The definitive host for E. granulosis is a carnivore (all of which, except for the lion, are Canidae) that harbors the adult tapeworm in the small intestine. Human infection with E. granulosus occurs principally where dogs are used to herd grazing animals, particularly sheep. The disease is common throughout southern S. America, the Mediterranean and Middle East, central Asia, and East Africa. Foci of endemicity are in eastern Europe, Russia, Australasia, India, and the UK. In North America, endemic foci have been reported from the western USA, the lower Mississippi Valley, Alaska, and northwestern Canada. The life cycle for E. multilocularis involves foxes as definitive host and microtine (e.g., voles and meadow mice) rodents as intermediate host. Domestic dogs and cats can also become infected with the adult tapeworm when they eat infected wild rodents. The disease in humans has been reported in parts of central Europe, much of Siberia, northwestern Canada, and western Alaska. One case has been reported in Minnesota. The principle definitive host for E. vogeli is the bush dog; the main intermediate hosts are the paca and spiny rat. Domestic hunting dogs are also definitive hosts, and serve as an important source of human infection. Cases have been reported in South America.

TRANSMISSION:

E. granulosis: Human infection occurs when eggs passed in dog feces are accidentally swallowed. E. multilocularis: Human infection is by accidental ingestion of tapeworm eggs passed in fox or dog feces.

DISEASE IN ANIMALS:

Usually no clinical signs except for enteritis in heavy infestations. In sheep, hydatid cysts cause considerable condemnation of meat and loss of production.

DISEASE IN MAN:

E. granulosis - Cystic hydatid disease A liver cyst may remain silent for 10-20 years or more until it becomes large enough to be palpable, to be visible as an abdominal swelling, to produce pressure effects, or to produce symptoms due to leakage or rupture. There may be right upper quadrant pain, nausea, and vomiting. The effects of pressure may result in biliary obstruction. If a cyst ruptures, anaphylaxis and death may result. If fluid and hydatid particles escape slowly, allergic manifestations may result. Rupture can occur into the pleural, pericardial, or peritoneal space or into the duodenum, colon, or renal pelvis. Dissemination of germinal elements may be followed by the development of multiple secondary cysts. Pulmonary cysts cause no symptoms until they leak; become large enough to obstruct a bronchus, or erode a bronchus and rupture. Brain cysts produce symptoms earlier and may cause seizures. Cysts in the bone marrow may present as pain or spontaneous fracture. The bones most often affected are the vertebrae and paraplegia may develop due to compression of the spinal cord. 20% of patients have multiple cysts. 15% of untreated patients eventually die.
E. multilocularis - Alveolar disease The primary localization of alveolar cysts is in the liver, where they may extend locally or metastasize to other tissues. The larval mass has poorly defined borders and behaves like a neoplasm; it infiltrates and proliferates indefinitely by exogenous budding of the germinative membrane, producing an alveolus-like pattern of microvesicles. 90% of untreated cases die within 10 years.
E. vogeli - Polycystic form of human hydatid disease. Symptoms are variable according to cyst size and location. The polycystic hydatid is unique in that the germinal membrane proliferates externally to form new cysts and internally to form septi that divide the cavity into numerous microcysts. Brood capsules containing many protoscolices develop in the microcysts.

DIAGNOSIS:

Immunoblot assay.

TREATMENT:

Currently the definitive treatment is surgical removal of cysts. Newly available chemotherapy (albendazole or mebendazole) may alter this position.

PREVENTION/CONTROL:

In endemic areas, prevention is by prophylactic treatment of pet dogs with praziquantel and prevention of feeding dogs offal.