Hydatid disease

Hydatid disease is an important zoonosis in populations engaged in hunting in Canada and Alaska. Although two species of Echinococcus are widely distributed in this region (E. granulosus, E. multilocularis), human disease is caused almost exclusively by E. granulosus (cystic echinococcosis) with rare cases caused by E. multilocularis (alveolar echinococcosis). Two biotypes of E. granulosus are found in Canada and Alaska based on the host specificity of the larval stage. The northern or cervid biotype is likely to be the ancestral form of this parasite and is widely distributed in the holoarctic and northern boreal forest (taiga) regions. This biotype cycles between northern herbivores (e.g. moose, caribou) and wild canines (principally wolves).

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The European or sheep biotype likely evolved from the cervid biotype in association with the development of animal husbandry. Echinococcus multilocularis also has a wide circumpolar distribution cycling between rodent intermediate hosts (e.g. voles, lemmings) and canines (e.g. foxes, coyotes). Cystic hydatid disease caused by the cervid biotype has long been recognized as a significant health problem in native and northern populations. Some data suggest that the clinical presentation of the northern biotype differs from the European form of E. granulosus in that cysts are more frequently found in the lungs and the course of infection may be more benign (Rausch 1986). Surveys in wild animals have demonstrated high prevalence rates in moose (19-59%), caribou (9.5%), and wolves (47%) in northern regions (Curtis et al. 1987; Messier et al. 1989). High rates of human infection across northern Canada and Alaska have also been demonstrated by autopsy studies (1% of Alaska natives; Arthaud 1970) and serosurveys in native communities (2-4% seropositive; Curtis et al. 1987). Alveolar hydatid disease is one of the most fatal helminth infections of man with historical mortality rates as high as 63% (Wilson and Raush 1980). The prevalence of E. multilocularis in natural hosts is influenced by a variety of factors, including the relative densities of fox and rodent populations, periodicity of rodent numbers, and the diversity of prey species eaten by foxes. Nevertheless, very high infection rates have been well documented in arctic foxes (40-100%), voles (2-80%), and deer mice (15-22%). Despite the wide distribution of the parasite and high prevalence rates in the natural hosts, human cases of alveolar disease have rarely been reported and are localized in western Alaska (Stehr-Green et al. 1988). No cases have been recorded in humans living in the extensive sub-Arctic and Arctic regions of Canada where the parasite is enzootic. Although the likelihood of infection with E. multilocularis is also decreasing with lifestyle changes in native and northern communities, a recent survey in western Alaska has demonstrated seroprevalence rates ranging from 7-8/100 000 to as high as 98/100 000 (Schantz et al. 1995). Controlling E. multilocularis infection in domestic dogs as well as improved control of the dog population itself in these communities is probably the key to eliminating human alveolar hydatid infection (Stehr-Green et al. 1988).

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