In early 2024, Canada’s average temperature was 3.1 °C above the reference value, matching 2010 as the warmest year on record since 1948. Though this warming trend is not unique to Canada, it is a harbinger of an increase in mosquito-borne diseases like West Nile virus (WNV).

WNV, which is transmitted when mosquitoes feast on infected birds and then bite humans, is endemic in Canada. The season runs from April through November, and human transmission is most common in late August through the first frost (in late September or October). But climate change is forcing public health experts to beef up modeling, surveillance, and warning systems, especially in provinces like Ontario, where Canada’s first human case of WNV was reported in 2002.

“Mosquitoes are very, very sensitive to climate; their production rate and capacity to amplify the virus in their bodies when they are infected is directly correlated to temperature,” Antoinette Ludwig, DVM, PhD, veterinarian and epidemiologist with the National Microbiology Laboratory at the Public Health Agency of Canada, told Medscape Medical News.

“The higher the temperature, the more rapid the cycle of transmission. The length of their reproduction cycle diminishes, and the Culex pipiens mosquito, the main vector for WNV, is spending the winter at the adult stage.”

Ludwig said that these adult mosquitoes are intent on staying in urban areas such as Ottawa, Toronto, and York, where the temperature does not fall precipitously. They shelter in storm drains and other places where standing water collects.

“They’re very comfortable breeding in urbanized areas, residential areas, which is the first aspect of the transformation we’re seeing,” said Ludwig, adding that development and deforestation also have helped maintain the transmission cycle.

The mosquitoes are “well adapted to breed in areas around people’s homes, and we do see that the distribution and hot spots where mosquito species are found tend to be in urban environments already, at least in eastern Canada,” said Manisha Kulkarni, PhD, professor of epidemiology and public health at the University of Ottawa and University Research Chair in Climate Change and Emerging Diseases, both in Ottawa.

“We have new, potentially invasive mosquitoes (such as Aedes albopictus or tiger mosquito) that have arrived in Canada due to climate change that can also transmit WNV and are also adaptable to more urban environments,” she said. “We could be seeing more WNV amplification in these urban centers, which is where the population density is the highest.”

New Cases Confirmed, More Likely

The reporting of human cases has begun almost like clockwork. As of September 8, public health authorities had confirmed two known human cases in the most recent week in Ontario’s urban centers (one in Toronto and one in Windsor-Essex, Ontario). On September 2, mosquitoes in Lambton County tested positive for WNV, although no human cases have been reported yet.

“It’s tough to predict what each season will hold,” said Isaac Bogoch, MD, infectious disease specialist, general internist, and professor of medicine at the University of Toronto, Toronto. In addition to temperature and climate patterns, “which foster the amplification of the virus, a lot of this is related to migratory bird patterns,” he said.

“We have scientific evidence that the climate is influencing bird reproduction, the capacity to find meals, and their migratory routes,” said Ludwig. “For transmission to occur, you need some sort of reservoir, so the birds would stay in our latitudes for longer periods of time to amplify the virus,” she said.

Data suggest that a shift in mosquito feeding preference to human blood meals at the end of the summer might lead to an increase in human cases if birds acting as reservoirs delay their migration, which amplifies the virus locally.

Shifting Surveillance, Intervention Strategies

“In the early 2000s, there was an initial ramped-up effort for surveillance using the One Health approach, which looked at animal reservoirs, human cases, and the mosquito population,” said Kulkarni. This collaborative approach involved a range of partners that sought to reduce the risk for human WNV infections and improve public awareness.

But national efforts stalled, mostly due to the annual variability in WNV activity, case numbers, and weather patterns.

“A lot of provincial WNV surveillance programs tapered off over time. But we do still have ongoing surveillance in Ontario starting in May, where mosquito traps are put out to detect WNV and other virus activity in mosquitoes,” said Kulkarni. Virus activity in mosquitoes is a good indicator of human cases and helps identify hot spots for transmission, she added.

Vector control programs include larvicide in catch basins and other measures to diminish the adult population in affected areas. Still, the system is imperfect, especially when it comes to responding to climate change.

“We need to rethink the surveillance strategy for mosquito-borne disease in Canada, not only because of the changes we expect for transmission of WNV but also because we expect that other viruses may be more frequent, like Eastern equine encephalitis,” said Ludwig. A horse in Ottawa tested positive for the latter disease in late August.

At the same time, ongoing surveillance is in the process of transformation, especially in Ontario, said Ludwig. “In Ontario, the entomological surveillance is being enhanced, and we are trying new approaches (eg, citizen science approaches over traps, DNA sampling in water, and pictures of mosquitoes to encourage the identification of invasive species),” she said.

“Scientists and public health are quite ready for this new threat.”

Boosting Public Awareness

Though most WNV cases in humans are asymptomatic, “manifesting as a nonlocalizing febrile illness lasting a few days, with people recovering in their homes, the virus has the potential to cause rare but still significant neurologic illness: encephalitis, meningitis, and a paralysis-type syndrome,” said Bogoch. “For the foreseeable future, we are going to see the expansion of vector-borne illnesses in this country, and it’s going to get progressively worse.”

It’s important that three groups be aware of this, Bogoch continued. “The first is the public, because a well-informed public can take steps to mitigate illness. Number 2 is healthcare providers, especially frontline healthcare providers, who need to be aware of emerging, vector-borne diseases so they can more readily identify and manage cases. And the third are public health professionals so systems can be set up to track the expansion and mitigate the impact through multiple initiatives,” he said.

When Bogoch speaks to patients, he reminds them that no one wants a mosquito bite and that practical steps (such as using insect repellent) can reduce the risk for bites. The best insect repellents contain 30% DEET or 20% picaridin. Others are available but don’t tend to last as long.

Providers also can direct patients, especially the most vulnerable to serious infections, to prevention information provided by the Public Health Agency of Canada. Vulnerable patients include those who are older, have certain chronic diseases, are immunocompromised, are socioeconomically deprived, or live in older buildings.

Ludwig, Kulkarni, and Bogoch reported having no relevant financial relationships.

Liz Scherer is an independent health journalist who frequently reports on Canadian and global health news. She regularly covered Zika when it was first reported and has also written about other mosquito- and tick-borne diseases.