Giardia lamblia is the most common cause of parasite-induced diarrhoea in immunocompetent hosts in Canada and Alaska. This organism has been found in a wide variety of wild

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and domestic animal species throughout this region; therefore human cases may be either indigenous (locally called 'beaver fever') or imported. The prevalence of G. lamblia carriage and symptomatic illness varies widely in Canada and Alaska with the highest rates reported in children (2-31%), adult staff of daycare facilities (8%), native communities, travellers, immigrants (4-67%), the immunocompromised, and the institutionalized. It has been shown that within 6 years of arrival in Canada, levels in immigrant populations decrease to that of the general Canadian population (estimated to be approximately 4%) (Gyorkos et al. 1987, 1992). The presence of G. lamblia cysts in virtually all of the recreational waterways of these regions also puts hikers, campers, boaters and hunters, and others using these waters at risk (Isaac-Renton and Philion 1992). In Canada, the number of giardiasis cases reported between 1985 and 1994 has varied from 6 500 to 9 500 per year, figures which probably reflect symptomatic cases and which would therefore underestimate the prevalence in the general population.

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