A study published April 19, 2010 ahead of print in the peer-reviewed journal Environmental Health Perspectives (EHP) demonstrates that conditions that can support the establishment of Lyme disease have come together in southern Quebec, an area that until now has not sustained the disease.
A North American epidemic of Lyme disease began in the upper Midwest of the United States in the late 1970s. Caused by a bacterium, Borrelia burgdorferi, that is carried by the deer tick, Ixodes scapularis, Lyme disease risk was found in isolated locations on the northern shore of Lake Erie in Ontario in the 1980s. Although disease-carrying ticks brought north by migrating birds create a low risk of the disease across Canada, a complex life cycle involving the tick, the bacterium, rodents, and the adult tick’s preferred food source, the white-tailed deer, must be established before the disease can become self-perpetuating in a new environment. In southern Quebec, all the pieces for maintaining the disease have now fallen into place.
The study draws on data gathered beginning in 1990 as veterinary and medical clinics in Quebec submitted ticks to public health authorities at Quebec’s provincial public health laboratory for identification. Those identified as I. scapularis were sent on to the National Microbiology Laboratory of the Public Health Agency of Canada to be tested for B. burgdorferi infection. Genetic typing of B. burgdorferi from the collected ticks, published in this study, shows that the bacteria seen in clinics in Canada are almost all identical to strains found in people and ticks in areas afflicted with Lyme disease in the northeastern United States.
From 2004 to 2008, the number of ticks submitted increased exponentially, suggesting that ticks seen in clinics were being produced by breeding populations in Canada in addition to those being carried north by birds. Field studies in 2007 and 2008 in woodland sites in three regions of southern Quebec, Montréal, Estrie, and Montérégie, showed that the tick’s life cycle has been established and that the ticks are now breeding in the warmest regions of southern Quebec. The study suggests that a warmer climate helps the ticks to become established.
The bacteria in field-collected ticks, like those seen in ticks collected at clinics, were closely related to strains in the northeastern United States, but in the locally grown populations of ticks, fewer were infected with B. burgdorferi. Most likely this is because infected ticks arriving on migratory birds rarely contact the rodent hosts of B. burgdorferi, so it may take some years after the ticks establish for the tick and bacterial life cycles to become fully intertwined.
The article is a available free of charge here.
EHP is published by the National Institute of Environmental Health Sciences (NIEHS), part of the U.S. Department of Health and Human Services.