Gastrointestinal ImagingHydatid disease
(also called echinococcosis), parasite infection caused by the larvae of
Echinococcus granulosus, and less frequently by
Echinococcus multilocularis (alveolaris) or
Echinococcus vogeli.
E. granulosus infection is prevalent in geographical areas where cattle are raised in association with dogs: South-America, Australia and New Zealand, Central Europe, the Near and the Middle East.
E. multilocularis is found in arctic and subarctic regions, including Canada, Siberia and Northern Europe. Like other cestodes, echinococcal species have both intermediate and definitive hosts. The definitive hosts are dogs that pass eggs in their feces. After the ingestion of eggs, cysts develop in the intermediate hosts: sheep, cattle, horses for
E.granulosus and mice and other rodents for
E. multilocularis. Humans are accidental intermediate hosts for all species. When a dog ingests beef or lamb containing cysts the life cycle is complete.
The liver and the lungs are the organs most commonly affected but the larvae may lodge in any other organ of the body. Larvae develop into fluid-filled cysts ("endocyst") that contain an external membrane composed by a chitin-like substance and an inner germinal layer. Daughter cysts develop from the inner germinal layer as do germinating cystic structures called brood capsules. New larvae called scolices develop within the brood capsule. A third wall the "eco-cyst" is formed as the cyst grows, causes an inflammatory reaction and compresses the surrounding tissue. It is composed by host granulation tissue and fibrosis. The cysts grow slowly over a period of years at an average rate of about 13 cm annually. About 30% of the cysts die spontaneously, rupture and collapse and become partially or completely calcified. Complete peripheral calcification of the pericyst should indicate that the cyst is not growing anymore.
The cyst of E. multilocularis is quite different in that the larval form remains in the proliferative phase, the cyst is always multilocular, and vesicles progressively invade the host tissue by peripheral extension. The process is infiltrative without a limiting membrane and macroscopically it is very similar to malignancy. Extensive calcification may be present.
Many slowly expanding echinococcal cysts will not cause symptoms and may be an incidental findings on imaging studies. Symptoms may be cough, haemoptysis, pain in the right upper quadrant, jaundice, bone pain and fracture. Occasionally, internal rupture of the cyst, usually following trauma, may cause acute illness characterized by fever, urticaria, eosinophilia and anaphylactic shock. The clinical course of E. multilocularis cyst is more malignant in failure to respond to medication, in recurrence and in final outcome. Also, see hydatid disease hepatic, hydatid disease pancreatic and hydatid disease splenic.
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