Map shows the spread of blacklegged ticks across Canada

Blacklegged ticks are spreading across Canada — and they’re bringing Lyme disease with them

Blacklegged ticks are unable to fly or jump so they will instead position themselves on greenery with outstretched legs, ready to grab on to a potential host. (Photo: Erik Karits/Unsplash)
Expand Image

Perched on the edge of a leaf somewhere, perhaps in a field or forest near you, a tiny, eight-legged creature waves its front legs to and fro, sensing minuscule changes in heat, carbon dioxide and pheromones. It’s a black- legged tick — and it’s waiting for its next meal to walk by.

Its host likely won’t even notice this sneaky parasite until it’s too late. The tick attaches itself with barbed mouthparts, then buries its head into flesh to create a feeding lesion full of blood. Next, it unleashes its saliva: a chemical cocktail that kills pain, stimulates blood flow, prevents clotting and inhibits immune response. In fact, it changes up the protein composition of its saliva so its cover isn’t blown as it feeds for days. It’s the perfect crime.

A female blacklegged tick “quests,” holding onto leaves or grass with her third and fourth sets of legs, while her first pair of legs is held outstretched, waiting for a potential host to brush by. (Illustration: Iza Valle/Can Geo)
Expand Image

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.

Data credits: current risk,; Eastern Canada Risk 2000-2015,; future scenario,
Expand Image

Are you passionate about Canadian geography?

You can support Canadian Geographic in 3 ways:

This story is from the May/June 2023 Issue

Related Content

historic disease map


Q&A: Tom Koch on disease mapping and medical geography

‘Maps aren't magic,’ says University of British Columbia prof — but during disease outbreaks, they can help us sort good information from bad

  • 778 words
  • 4 minutes


Wildlife Wednesday: The land-to-sea evolution of the whale eye

Plus: the ins-and-outs of a Vancouver zoo wolf break-out, caribou conservation controversy in Quebec, more marmots on Vancouver Island and the tick-busting pine needle discovered in Nova Scotia

  • 882 words
  • 4 minutes
An underground, dark, tunnel


Sewage surveillance: How scientists track and identify diseases like COVID-19 before they spread

Sewage testing can be used for early detection of disease

  • 866 words
  • 4 minutes
The New York Times COVID-19 map


Mapping COVID-19: How maps make us feel

Canadian Geographic cartographer Chris Brackley continues his exploration of how the world is charting the COVID-19 pandemic, this time looking at how artistic choices inform our reactions to different maps

  • 1145 words
  • 5 minutes