What causes hydatid disease (echinococcosis)?

Updated: May 07, 2019
  • Author: Imad S Dandan, MD; Chief Editor: BS Anand, MD  more...
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Answer

Echinococcosis is caused by larval cestodes of the phylum Platyhelminthes (tapeworms).

Their life cycle involves only two hosts, one definitive and the other intermediate. Humans act as an accidental intermediate host. The life cycle has three developmental stages, (1) the adult tapeworm in the definitive host, (2) eggs in the environment, and (3) the metacestode in the intermediate host. Metacestodes are ingested by the definitive host. The metacestodes mature into the tapeworm in the definitive host and, in turn, release eggs into the environment. The intermediate host ingests the eggs, which hatch into metacestodes, which infest the liver, lungs, muscles, and other organs of the intermediate host. [3]

Two biological forms of E granulosus have been recognized (depending on the geographic location and type of the intermediate host), (1) the northern type and (2) the European type, as follows:

  • The northern type is maintained in the tundra by a predator-prey relationship between the wolf and large deer, but dogs and coyotes can also become infested. Humans become infested in areas where reindeer are domesticated.

  • Intermediate hosts for the European type include camels, pigs, sheep, cattle, goats, horses, and many other animals. The definitive host for the European biotype is overwhelmingly the dog, but it also occurs in foxes, hyenas, and jackals. This is the most common biotype. The adult stage of E multilocularis occurs mainly in foxes and rarely in wolves, coyotes, lynxes, cats, and black bears.

The intermediate hosts for E multilocularis are eight families of rodents, including mice, rats, hamsters, gerbils, and squirrels.

E vogeli is a neotropical species maintained in the bush dog and the paca. It can easily infect other mammals that are exposed to its feces. It is the most rare form of the echinococci.

Exposure to food and water contaminated by the feces of an infected definitive host or poor hygiene in areas of infestation can lead to echinococcosis.


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