It’s summertime, and the living is easy—at least for Ixodes scapularis, the species of ticks now out in droves spreading Lyme disease throughout the Northeast, the mid-Atlantic, the West Coast, and the upper Midwest. Cases of Lyme disease have increased sharply over the last 15 years: Between 1995 and 2009, the number of infections jumped from 15,000 to 40,000. While most cases of Lyme disease now occur in a few specific regions of North America (and mainly in the U.S.), scientists believe that people elsewhere in North America will someday have cause for concern as well: The disease-carrying ticks may soon expand their territory northward. The question, of course, is why.
And, as with many questions related to nature these days, the answer is because of climate change. A 2005 study by Yale Medical School’s John S. Brownstein, Theodore Holford, and Durland Fish found that, as the planet warms, an increasingly large swath of the continent will become suitable habitat for Ixodes scapularis. Identifying this expansion of habitat as a “public health concern,” the authors note that, while the overall distribution area may initially contract (as parts of the American South become too hot for the disease-bearing ticks to thrive), the ticks will ultimately expand their suitable habitat into currently unaffected regions. Among these regions will be the central U.S., which will become home to ticks migrating north from states like Texas and Florida. But the most dramatic change could take place in Canada, where, by the 2080s, climate change will expand the total suitable habitats for I. scapularis by 213 percent.
At the time of writing, the scientists noted that Canada faced little risk from I. scapularis: “established populations … are currently limited to a small number of foci in southern Ontario.” But, over the next several decades, the study “specifically forecasts the emergence of a tick-borne infectious disease in Canada” as suitable habitats expand to northern Ontario and Manitoba. Events seem to confirm this prediction: In 2010, Canada began to require doctors to report cases of Lyme disease, and Canadian health officials now warn that “the incidence of Lyme disease in Canada is increasing.”