More frightening than its bite are the pathogens this tick could be carrying — and the rate at which the blacklegged tick is spreading across Canada. In the 1970s, there was only one known colony of blacklegged ticks in Canada, in Long Point, Ont. By the 2000s, ticks could be found throughout southern Ontario, Quebec, Manitoba and the Atlantic provinces. Now, they lurk as far west as Alberta and Saskatchewan. And they’re steadily moving north — hitchhiking on deer, mice and birds — as climate change creates chunks of warm forest that are suitable for the ticks to establish and complete their life cycles. Their range is expanding north by an estimated 46 kilometres per year.

Blacklegged ticks can spread Lyme disease, anaplasmosis, babesiosis and Powassan virus. But the disease that’s rapidly becoming a concern to public health is Lyme (see chart on map), caused by the bacterium Borrelia burgdorferi. The bacteria had been silently circulating in North American forests for about 60,000 years before Lyme disease was described in Lyme, Connecticut, in the 1970s. The reasons for its relatively recent infection of humans — and its spread — are more complicated than simply climate change.

One genome study from Yale University points to the ecological transformation of North America that created the optimal conditions for the spread of blacklegged ticks (also known as deer ticks). Forests were fragmented and hunting was intense during early colonization. By the 20th century, with fewer wolves and decreased hunting pressures, the population of white-tailed deer, the main carriers of ticks, exploded into suburban landscapes. Deer passed the baton (or tick) to highly mobile animals like white-footed mice and robins, which thrive in suburban areas and are ideal hosts for Borrelia burgdorferi. Humans are just collateral — an incidental host for the bacteria.

As climate change enters the scene, Lyme risk maps are getting hotter and hotter. And yet, hope is not lost. Last summer, one Lyme disease vaccine entered Phase 3 of clinical trials. In the meantime, if you go down to the woods today: do a tick check.