Trichinosis in Canada and Alaska is caused by two distinct species, Trichinella spiralis and T. nativa. Trichinella spiralis has a life cycle that involves pigs and a variety of other carnivores in the southern regions of Canada. A common source of infection for humans is undercooked pork or bear meat. Trichinella nativa has a life cycle that involves a variety of carnivores, including northern bears and walrus. It differs biologically from T. spiralis in its behaviour in laboratory animal models and also, most remarkably, in its resistance to freezing. Poorly cooked bear has been the main source of human trichinosis in Alaska while the walrus has been the main source in the eastern Canadian Arctic region. While bear is eaten undercooked unintentionally, walrus is eaten raw by choice.
Trichinosis is a reportable disease in the United States and Canada with estimated incidence rates of 0.05-0.06 cases/100 000 per year. The regions with the the highest rates are Alaska and northern Canada (Northwest Territories and northern Québec) with rates of 1.8 cases/100 000 and 11 cases/100 000, respectively (MacLean et al. 1989). Focal serological studies have reported rates as high as 18% for Trichinella antibodies in native communities in northern Québec. Since Trichinella antibodies are thought to persist less than 5 years after a single exposure, these data suggest significant ongoing exposure in these communities. The clinical presentation of trichinosis in northern communities is often a prolonged diarrhoea (e.g. six weeks) rather than the classical myopathy and fever seen in T. spiralis infections around the world. Recent research suggests that the diarrhoeal presentation occurs upon reinfection of individuals who have pre-existing immunity to Trichinella (MacLean et al. 1989, 1992).
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