Lyme disease: a zoonotic disease of increasing importance to Canadians.

Lyme disease, which is caused by the bacterium Borrelia burgdorferi, is transmitted by the bite of an infected tick vector. There are 2 reasons Lyme disease has not been common in Canada. First, although populations of the western black-legged tick, Ixodes pacificus, are established throughout parts of southern British Columbia, this tick infrequently bites humans and relatively few ticks are infected.1 Second, the black-legged tick, Ixodes scapularis (Figures 1 and 2), is currently established in only a few specific locations in central and eastern Canada (known as endemic areas), including areas within southeastern Manitoba, southern and eastern Ontario, southeastern Nova Scotia, and, as preliminary data suggest, southern Quebec and New Brunswick. However, these black-legged ticks are more likely to bite humans and a relatively high proportion are infected.1 There is a low but nonetheless real risk of infection in people across Canada, even in areas where ticks are not established, owing to ticks that are dispersed from endemic areas by migratory birds. Studies suggest that the range of the black-legged tick is expanding in some parts of eastern and possibly central Canada, and the scope of endemic areas is thought to be increasing—a process that could accelerate with climate change.1,2 Lyme disease will be nationally notifiable in Canada by the end of 2008.