Warming climate pushes vectors farther north, increasing risk of Lyme disease in Canada

Anyone who traveled west to Lake Erie in the early 2000s might recall the warnings of black-legged ticks. Driving through Haldimand County, passing Caledonia, to the reaches of Port Dover and Turkey Point Provincial Park, signs with illustrations of the ominous 8-legged oval shaped insects lined the highways the same way you would see a warning for deer or turtle crossings.

These particularly nasty vectors, a part of childhood lore for the fear they invite, engorging on blood while latched onto one’s scalp, spreading disease, have long been part of our Canadian summer warnings.

Campers know not to keep food lying about to avoid attracting bears. Forays into tall grass bring our eyesight into sharper focus, on the lookout for the distinctive three-leaf green or reddish pattern of poison ivy. And those traveling southwest a couple decades ago to the tip of Long Point Provincial Park knew to tuck their pants into their socks, use bug spray, and check themselves for the dreaded black ticks.

A blacklegged dear tick. 

(Pestworld.org)

But now, the potential for these resilient vectors to interact with Ontarians has spread across many other parts of the province, a direct consequence of climate change, as these scary parasites thrive in further reaches for extended parts of the year.

“Climate change has now tipped that ecological threshold that many areas are now automatically suitable for the life cycle [of ticks], which has allowed them to survive and reproduce,” Dr. Katie Clow, a registered veterinarian and assistant professor specializing in epidemiology at the University of Guelph, tells The Pointer.

The isolated population of black-legged deer ticks (not to be confused with the brown tick or dog tick which do not carry Lyme disease) has survived in the area surrounding Long Point Provincial Park for hundreds of years, scientists estimate. But due to the unique conditions required for the species to thrive, there was never any spread outward.

Ticks are constantly on the move. Populations are often transported on the backs of migratory birds and deposited in various locations across the province and the rest of Canada’s southern areas. This has not changed until recently as rising temperatures have made it suitable for tick populations to spread and complete their life cycle in places where they were previously not found, or where their numbers were much smaller than what they are now.

“There are plenty of refugees for the ticks to hide from extreme cold, and also extreme heat and extreme drought,” Dr. Nick Ogden, senior research scientist and director of the Public Health Risk Sciences Division at Health Canada, says. “Woodlands provide them a protective environment. What they can't get away from though is the overall impact of climate, which has an impact on the duration of their lifecycle.”

Historically, the harsh winters that plague much of Ontario and other parts of Canada have kept ticks, and the disease they spread, away. While ticks can survive extreme cold temperatures when burrowing in the forest floor, they only become active at a certain temperature, an estimated four to seven degrees celsius, before they need to find blood to survive.

“If the temperatures aren't suitable, they'll stay in sort of a quiet period and actually starve out because the temperatures haven't become suitable again, for them to come out and feed,” Clow says. “So with climate change, we're seeing earlier warmer temperatures and longer periods of [suitable] temperatures. So we meet that threshold for them to feed and develop into the next stage.”

Public Health Ontario uses monitoring measures to map at-risk areas for Lyme disease annually. Much of southern Ontario, along the St. Lawrence and Great Lakes, is considered to be at high risk.

(Public Health Ontario)

As average seasonal temperatures begin to creep up, ticks deposited in different areas of Ontario have been able to survive. In the early 2000s, populations began to establish themselves along the shores of Lake Erie and Lake Ontario, as well as the St. Lawrence River valley. But more recently, Clow says there has been an explosion across southern Ontario, and now the vast majority of the province’s population is routinely exposed to the threat of ticks.

Ideally, these invertebrates prefer forested and rocky areas where humidity is higher and natural shelter protects them. There are areas of southern Ontario where temperatures are suitable for ticks to survive but the habitat is not present.

“You're not gonna find ticks in a parking lot or something like that,” Clow says. “But we have many areas, in eastern Ontario and most of southern Ontario that we know are automatically suitable.”

While we may not see ticks travel as far as some of the northern reaches of the province — Ogden says it is unknown whether our boreal forests provide a suitable habitat for their survival — we can expect to see populations continue to expand and increase in the south.

One of the most reliable indicators of the encroachment of the black-legged tick is the augmented report of cases of Lyme disease. Lyme disease is a zoonosis that lives in the reservoir of small mammals and is carried by ticks after feeding on an infected host. The bacteria can then be transmitted to humans through an infected tick bite. It is a bacterial disease which can cause mild symptoms ranging from a rash, to fever, dizziness and fatigue, often mimicking the flu or another viral infection. But, if left untreated, Lyme disease can cause chronic problems such as partial or full facial paralysis, neck stiffness, heart palpitations and severe headaches. There is no cure for Lyme disease, but if caught early, it can be treated with antibiotics.

Between 2009 and 2022, provincial public health units across the country reported 17,080 cases of Lyme disease in humans. The actual case count is estimated to be much higher.

(Health Canada)

According to data from the Health Canada website, cases of Lyme disease have been increasing dramatically over the past one to two decades. In 2009, the first year for which data is provided, there were 144 reported cases of Lyme disease across the country. In 2021, that number was 3,147. While there is variation in the years in between, this equates to approximately a 95 percent increase in cases over 12 years.

Health Canada is currently undertaking studies to determine the rate of cases that go unreported for various reasons. Lyme disease is hard to diagnose. As a relatively novel phenomenon, especially in Canada, with symptoms that mimic many other illnesses, Lyme disease often falls under the radar of healthcare practitioners. But even the cases that are diagnosed do not always get reported to health authorities.

Ogden says we can look to the United States to get an idea of the scale of the problem. When Lyme disease became more common south of the border in the early 2000s, it was estimated that one in three cases were reported. 

“Something different, though, has happened as Lyme disease has become more and more and more of a problem in the US,” he says.

In 2014, the Centre for Disease Control (CDC) used insurance records to count case totals and found that approximately one in ten cases were being reported. As case counts have increased, the proportion of reported cases has decreased, meaning the true spread of the disease is exponentially greater than what is known.

Ogden says he estimates Canada is sitting between the one in three and one in ten figures reported from the States.

“If you look at the Lyme disease numbers, we've seen a huge increase over the last several years, which I think is just an indication to many of us that that risk is growing and changing and we need to continue to be really cognizant of it,” Clow says. “Because there could still be many cases that aren't being recognized or aren't being reported. And so anyone working in this space in the medical community is cognizant that this is a risk for both people and animals.”

Lyme disease is not the only disease to look out for. Other vector-borne diseases, carried by mosquitoes, pose a threat to Ontarians. In 2002, the first case of West Nile Virus was identified in Canada. In the 21 years since, it has become endemic across the country. Peel Public Health identified the first West Nile Virus positive mosquitoes in the Region on July 24. In 2022, they did not appear until August 9. Warmer temperatures are increasing the duration of time that these diseases are present, which will inevitably increase disease transmission.

With increasing temperatures, there is the potential for other mosquito-borne diseases to make their way across the border and into Ontario and the rest of Canada. Across the world we hear about Zika, Chikungunya, Dengue, Yellow Fever and Malaria, all mosquito-borne diseases. While the threat may seem worlds away, warming temperatures could create suitable habitats for them to thrive.

Canada already has the vector that can cause malaria and the disease was endemic in Canada during the 19th and 20th centuries. While it can lead to kidney failure, seizures, coma and death, and was eradicated in the 1950s with the use of DDT and other chemical pesticides, the Anopheles mosquito is still buzzing around.

Climate change is only half the problem. As humans push their urban boundaries and encroach on natural habitats, we are putting ourselves in closer contact with animals increasing the chances for zoonoses to make the jump to us. 

When entering wooded and grassy areas, people are advised to take precautionary measures to avoid ticks and mosquitoes.

(Alexis Wright)

Ogden says that while the risk of Lyme disease and potentially other vectors are increasing, he hesitates to sound the alarm, for fear that people will shelter themselves indoors.

“It's important that we carry on doing outdoor activities for our health, but to take precautions before you go out into the woods.”

Precautionary measures include wearing long, light coloured pants tucked into socks and shoes, and long sleeves to limit the tick’s ability to attach to skin. Bug spray can also hinder the chances of getting bit. It is also advised to keep dogs and children on trail paths and out of long grass and wooded areas.

“It takes a while once they've attached to you to actually transmit [Lyme disease], so you've got a window of opportunity to find the ticks and pull them off,” he says.

In 2020, the Region of Peel’s public health unit released its priorities for the next decade, including plans to tackle problems associated with climate change and human health.

“A changing climate will impact human health through increasing temperature-related morbidity and mortality; intensifying the harmful effects of poor air quality; increasing the risk of injury and loss of life from extreme weather; increasing illness through food and water contamination, as well as vector-borne disease (e.g. disease carried by mosquitoes and ticks); increasing stress and harming mental health; and displacing communities (e.g. due to flooding),” the plan reads.

But the Region cannot tackle this problem alone. It requires a collaborative effort by all levels of government, if we want to avoid widespread disease spread associated with warm-weather insects.

Peel Public Health (PPH) works in coordination with the overarching provincial body, Public Health Ontario (PHO), to monitor and survey for black-legged ticks and the presence of Lyme disease throughout the province. The three methods for monitoring include human case interviews, active surveillance and passive surveillance. 

Human case interviews include recorded cases of Lyme disease. In Canada, the disease is reportable, meaning both the provincial and federal governments track case numbers in order to get a better picture of the impact on the population.

“When a Lyme disease laboratory report for a Peel resident is received, PPH staff interview the case to confirm diagnosis, collect epidemiological information, and identify the geographical location(s) where exposure to an infected tick may have occurred,” a spokesperson for PPH told The Pointer in an email statement. 

Public Health, as well as Health Canada, also partake in active and passive surveillance measures. Active surveillance refers to going out and collecting ticks and testing them for the bacteria that causes Lyme disease. The method of collection involves using a white flannel blanket and dragging it around in areas where it is believed ticks may be present. The ticks latch on to the cloth and are then collected and tested.

Passive surveillance involves citizen science efforts of monitoring. If you have found a tick, you can upload a picture to the Public Health monitored website etick.ca and receive notification of the species of tick which will then be added to a public tracking database. It is crucial to report diagnosed Lyme disease cases or apparent ticks in order to contribute to official monitoring efforts that keep all Ontarians safe.

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Twitter: @rachelnadia_

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