
Lyme disease is a complex multi-system inflammatory illness that's caused by the bite of a tick infected with one of several closely related species of borrelia bacteria, including Borrelia burgdorferi which is responsible for the majority of cases in Canada. Lyme disease is, by far, the most common tick-borne illness in Canada and is pretty much the only one most Canadians can name.
That said, our understanding of Lyme disease in Canada is in its infancy and, much to the frustration of a great many patients, the unknowns far outweigh the knowns.
What we do know is that blacklegged ticks are the primary transmitter of Lyme disease in all of Canada east of the Rocky Mountains while western blacklegged ticks (Ixodes pacificus) and Ixodes angustus (no common name) are the primary ticks responsible for transmitting Lyme bacteria to people in several regions of southern British Columbia. Lyme bacteria has also been found in other Ixodes spp. ticks in this country and although these lesser known domestic ticks don't play a major role in transmitting Lyme bacteria to people, they likely play an important role in maintaining Lyme disease in local transmission cycles in Canada's wild spaces where a wide variety of small mammals and ground foraging birds act as reservoir hosts.
The documented history of Lyme disease in Canada is relatively brief. The first breeding colony of blacklegged ticks (Ixodes scapularis) was discovered at Long Point, Ontario on the north shore of Lake Erie back in the early 1970s, but it wasn't until the late 1980s that Lyme bacteria was documented within that colony. Over the past 25 years, breeding blacklegged tick colonies have been identified throughout much of southern Canada, with hotspots turning up in British Columbia, Manitoba, New Brunswick, Nova Scotia, Ontario, and Quebec. Locally acquired cases of Lyme disease cases have been reported in every province and, every year, more Lyme-endemic regions are added to risk maps.
Lyme disease is projected to continue spreading rapidly across Canada over the coming decades and any province not yet known to have a permanent breeding population of blacklegged ticks is expected to be colonized sooner rather than later. Many factors are driving this expansion, including climate change, forest fragmentation, declining biodiversity and more. Migrating birds are estimated to distribute as many as 175 million blacklegged ticks across Canada each spring from two distinct populations in the United States -- the Northeast (whose ticks primarily wind up in Ontario, Quebec and the Maritimes) and the Midwest (whose ticks primarily wind up in the prairie provinces) -- as well as from established colonies within Canada, making them a major force in introducing hitchhiking ticks to new locations.
Lyme disease has been making headlines in recent years for another reason: it is notoriously difficult to diagnose. Partly this is due to diagnostic tests that have some highly publicized accuracy issues that make unreliable during the first weeks of infection when treatment is most effective and partly because Lyme patients can present with a range of symptoms, many of which are considered non-specific (meaning they can be the result of any number of illnesses or no disease at all). Those symptoms can include chills, fatigue, fevers, headaches and joint and muscle pain. For many, Lyme disease feels like the worst flu they ever had. If not diagnosed and treated early, it can progress to a complex illness that can impact the heart and neurological system, and some patients go on to develop long-term symptoms that can persist for months or years. Although a distinctive expanding oval rash that sometimes takes the form of a bull's eye has long been considered diagnostic of Lyme disease, there's some debate over what percentage of patients actually develop such a rash. These days the thinking is somewhere around 30 percent although there are some who would argue it's lower than that.
It should be noted that Lyme disease can arrive with co-infecting organisms such as anaplasma, babesia, and/or Borrelia miyamotoi. These pathogens infect the same ticks and small mammal hosts as Lyme bacteria and can be transmitted to people simultaneously, resulting in a complex illness that is often more severe than what's seen when Lyme bacteria arrives alone.
It's hard these days to avoid newspaper headlines announcing that Lyme disease cases are on the rise in Canada, and the statistics back them up. Between 2004 and 2006, roughly 69 domestically acquired cases were officially recorded in this country. Compare that to 2023 when 2,544 cases were officially recorded. The true number of cases acquired in Canada each year is unknown and attempts to come up with a firm number have been mired in controversy. Nearly everyone agrees the official number is far too low. But whether the true number is 2 or 10 or 20 times the official statistics is the subject of a great deal of disagreement.
Whatever the true number is, the expectation is that Lyme disease cases numbers will continue to climb as blacklegged ticks expand their geographic range across Canada and the percentage of infected ticks within existing colonies rises.