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Echinococcus granulosis – a re-emerging threat (Proceedings)

May 01, 2011

Echinococcus granulosus is a tiny (~3 mm) tapeworm responsible for cystic echinococcosus in humans. In many regions of the world, this is a major public health problem as well as an important economic issue. As with other tapeworms, the life cycle involves both definitive and intermediate hosts. Definitive hosts include dogs and other canids while intermediate hosts include domestic and wild ungulates, the latter of which are infected with the hydatid cyst. This fluid-filled, spherical, unilocular cyst consists of an inner germinal layer supported by an acellular laminated layer of variable thickness. This is surrounded by an fibrous adventitial layer of host origin. Small vesicles bud internally from the germinal layer, producing multiple protoscolices by asexual division. In humans, cysts localize primarily in the liver and lungs, although they can be found in other organs such as the brain or eye. Cysts grow slowly over many years and can attain a volume of several liters containing thousands of protoscolices. Because of this slow gorwth, unless localized in the brain or eye, infections acquired in childhood generally do not become symptomatic until adulthood with only 10-20% of cases diagnosed in patients <16 years old.

Geographically distinct strains of E. granulosus have been identified with different host affinities. Molecular genetics have identified 10 distinct genotypes, designated G1-G10. G1 and G2 are sheep strains, G3 and G5 are bovid strains, and G4 is a horse strain. Additional strains include a camelid strain (G6), two pig strains (G7 and G9), and two cervid strains (G8 and G10). Phylogenetic analysis, coupled with biological differences, indicates these strains likely represent distinct species. Echinococcus granulosus sensu stricto consists of strains G1-G3. Strain G4 has been designated Echinococcus equinus, G5 Echinococcus ortleppi, G6, G7 and G9 Echinococcus intermedius, and G8 and G10 Echinococcus canadensis. However, the species status of genotypes G6-G10 is still ambiguus.

Within the North America, two variants have clearly been recognized – the northern variant (G8, E. canadensis) indigenous to Holarctic tundra and boreal forests, and a domestic or southern variant (E. granulosus (G1-G3)). The latter form is the most cosmopolitan form as a result of European migrants moving with their livestock throughout the world, although it is speculated introduction into the US occurred through the importation of infected sheep dogs from Australia. This form also seems to be the form most commonly associated with human infections. Primary hosts for E. granulosus in the US are domestic dogs and domestic sheep, although a dog-swine cycle existed at one time. Primary hosts for E. canadensis are canid definitive hosts (wolves and dogs) and a cervid intermediate host (moose, elk, deer and caribou). This form does not readily infect domestic ungulates. As with E. granulosus, the link to humans is through the dog. Feeding of viscera of moose, caribou and other cervids to dogs leads to frequent opportunities for transmission to dogs and subsequent exposure of humans. Human infection with this strain is characterized by predominantly pulmonary location, slower growth with abnormal development and a decreased frequency of clinical complications. Often, infections resolve via rupture and expulsion. In contrast, E. granulosus is mainly associated with sheep-rearing areas of the US. Autochthonous human cases are uncommon and are generally restricted to high-risk groups associated with sheepherding dogs. A third strain, probably representing G4 (E. equinus), has also been reported to in the mid-Atlantic area of the US and appears associated with horses and dogs engaged in fox hunting.

Recently, E. granulosus was reported from a mountain goat in central Idaho and wolves in Idaho and Montana. Although no genetic work was performed, it is likely this represents the northern variant (G8, E. canadensis) because of it's association with sylvatic ungulates and wild canids. The source of this new endemic foci is not known. Wolves were reintroduced into central Idaho and Yellowstone National Park in 1995 and 1996. Although treated with praziquantel prior to release, some speculate treatment may not have eliminated 100% of the tapeworms. Another possibility is that the natural movement of wolves south into Montana from Alberta and British Columbia may have brought the parasites with them, resulting in infection of local sylvatic ruminants. Finally, it is also possible that the parasite already existed in the area at low levels and not until the discovery of the infected mountain goat was a dedicated surveillance study initiated and the distribution finally realized. Regardless of how it became established, its presence has raised numerous questions such as the risk to domestic livestock and public health. If this does prove to be the northern biotype, the risk to domestic livestock would be low as this form does not readily infect domestic livestock. Although the human health hazards associated with the northern variant is less compared to the domestic variant, they cannot be totally ignored. The link for most humans is going to be the domestic dog. Therefore, control efforts need to focus on preventing infection in pets. Eliminating scavenging behaviors and not feeding raw viscera are exceedingly important as this breaks the life cycle. Routine anthelmintic treatment with praziquantel may be necessary in areas where the lifestyle of the working dog may not allow for complete dietary control.