Although not the most ubiquitous tick species in this country, the blacklegged tick is easily the best known of all Canada's ticks thanks to its role as the primary vector of Lyme disease in Canada east of the Rocky Mountains.

Blacklegged ticks are three-host generalist feeders, meaning that over the course of their journey from egg to larvae to nymph to adult, these ticks must dine on three host vertebrates in order to acquire the blood they need to fuel their transition from one stage to the next.

Stages of engorgement of a nymphal blacklegged tick. Image courtesy of CDC.

And they aren't terribly picky about which vertebrates they dine on.

Although long associated with white-footed mice, the immature stages of the blacklegged tick (larva, nymph) like to feed on the blood of a wide variety of small vertebrates including many species of birds, chipmunks, shrews, squirrels, mice and voles. They also target larger mammals such as deer, dogs, or humans when the smaller creatures they prefer are in short supply. Adult females (the males don't feed) prefer to get their blood meals from dogs, foxes, horses, opossums, raccoons, skunks and, in some cases, humans.

Blacklegged ticks are rapidly expanding their geographical range.

For decades they were thought to be limited to strongholds along the northern shores of Lake Erie and Lake Ontario in Canada. Blacklegged ticks are now known to be distributed across a wide swath of the Canadian landscape and infected ones have been found as far north as the Northwest Territories although it's unlikely they have formed permanent colonies there.

Lifecycle of the blacklegged tick. Images courtesy of the CDC.

Each year, during spring migration, birds carry literally millions of immature blacklegged ticks into Canada from from hotspots in the US in addition to those that have already formed colonies here. And although some regions of Canada are considered to be at higher risk for Lyme disease than others, in recent years it's been recognized that there is really no location in Canada that can be considered 100% risk-free thanks to the important role that migrating birds play in its geographical spread.

Blacklegged ticks are frequently encountered in deciduous or coniferous woodlands and, importantly, in the zones where woodlands and grasslands meet. They are also frequently found in long grass and shrubs in suburban neighbourhoods, so even though we have a tendency to associate blacklegged ticks with the wilderness, many Canadians have found out the hard way that they can become infected with Lyme bacteria after being bitten by ticks in their own backyards or in their neighbourhood parks.

And Lyme disease isn't the only infection that can be acquired from blacklegged ticks although it is by far the most common.

Blacklegged ticks in Canada also carry several other infectious organisms that can cause significant illness and even death in humans. So while most blacklegged tick bites are harmless, some bites deliver disease-causing organisms either singlely or in combination and research has shown that patients infected with multiple tick-borne pathogens tend to become significantly sicker than those who contract a single infectious organism and their infections last much longer.

Diseases transmitted: Anaplasmosis, babesiosis, Borrelia miyamotoi disease, Lyme disease, Powassan virus encephalitis.

Where found: Widespread in southern, eastern and northwestern Ontario, southern Manitoba, southern Quebec, New Brunswick and Nova Scotia. Endemic regions are spreading rapidly and migrating birds transport blacklegged ticks to many locations beyond endemic areas marked on maps.