What you need to know about mosquito-borne diseases

Belle River, Ont., teacher Melanie Klimkowski never worried much about mosquitoes, since they never seemed interested in biting her. 

After learning more about the bugs at a special workshop with her eighth grade class, she says she now carries small amounts of mosquito repellant at all times to make sure the arthropods stay away.

“Just like when we carry hand sanitizer, I have a little thing of the oil in my purse, just in case,” said Klimkowski. 

Experts say that infection rates in Canada for mosquito-borne diseases are extremely low, but evolving environmental factors — including climate change — are changing the way that mosquitoes live and breed, as well as providing extra opportunities for infected mosquitoes to bite people and increase the potential for disease. 

A closer look at mosquitoes in Canada

Canada is home to over 80 different species of mosquitoes, according to Brock University entomologist Fiona Hunter. They can be found in every province and territory. 

Not every mosquito carries diseases that infect humans. Mosquitoes of the Culex, Culiseta, Anopheles and Ochlerotatus genuses are the most common Canadian bugs that can. 

The typical mosquito breeding season is April to September, though Hunter says some mosquitoes can survive the winter.

The process is known as diapause — similar to hibernation in mammals — where they find hiding places like sewers or subway systems to slow down their development.

Adult mosquitoes can survive in winter, but Hunter says it’s more common for eggs to overwinter — live through the winter — than full-grown bugs.

“If you have a wood stove and then suddenly mosquitoes are flying around, they’ve actually been overwintering in your woodpile,” said Hunter. 

Mosquitoes typically breed in stagnant water, and experts advise dumping out unused buckets in garages or even emptying out saucers under flower pots in backyards to avoid providing the insects with a place to reproduce.

Of the many mosquito-borne diseases that affect humans around the world, Hunter says West Nile virus, eastern equine encephalitis (EEE) and the California serogroup viruses are the most prevalent in Canada, but remain rare. 

West Nile virus is the most common mosquito-borne disease in Canada, spread by mosquitoes of the Culex genus. 

Public health authorities report anywhere between 40 to 200 human cases each year, according to infectious diseases specialist Dr. Sumon Chakrabarti. That number is comparable to other parts of the world where the disease circulates more regularly due to tropical climate conditions, Chakrabarti says. 

“Notably in 2007, we saw 2,400 cases across Canada,” he told The Dose host Dr. Brian Goldman.

Weather conditions, a higher number of infected birds, as well as increased rainfall that year were likely contributors to the higher than normal case counts, Chakrabarti said.

In contrast, 13 human cases of West Nile virus have been confirmed in Canada in 2024, as of Aug. 26. 

Provinces and territories also track mosquitoes for infection by testing mosquito traps in pools set up by public health units. This year, more than 260 pools have tested positive for West Nile virus across Canada. 

Eastern equine encephalitis (EEE) is rarer than West Nile virus in Canada, with Hunter estimating one human case in the last two decades. 

The U.S. sees around 11 human cases of EEE per year, according to the Centres for Disease Control and Prevention. 

Mosquitoes spread diseases by consuming blood from infected animals like horses, sheep and birds. As the virus develops in the insect, the virus is transmitted through the mosquito’s saliva when it bites uninfected hosts, including humans.  

Chakrabarti says it’s not quite clear why some people are mosquito magnets while other people get bitten less often.

“Pheromones are part of it, the composition of the bacteria on your skin, your body heat,” he said, adding that body weight also could play a factor as well.

“People who tend to be a bit bigger, they’re putting out more [carbon dioxide],” he said, as female mosquitoes have receptors to detect the gas as we exhale.

Low-grade symptoms resemble cold and flu

People infected with West Nile virus typically present flu-like symptoms such as rashes or muscle aches, says Chakrabarti.

“When you see it in the hospital, it’s often people who are coming in with high fevers and particularly headaches,” he said.

Chakrabarti says many people who get infected with West Nile are mild cases and tend to recover normally. 

“That’s not to take away from the fact that this is something we should be aware of,” he said.

There are no specific treatments or vaccines for West Nile virus, nor is there an approved vaccine for EEE. 

“It’s something that generally you watch and give supportive treatment,” said Chakrabarti, including treating fever and muscle aches, as well as any seizures that may occur during more serious infections.

Climate change means longer breeding seasons

Despite low infection rates, Hunter says that climate change is driving a shift in Canadian mosquitoes and the introduction of new species. 

She pointed to the tiger mosquito as an example of a tropical species that has recently been spotted in Ontario’s Windsor-Essex region, adding that it’s a strong vector of transmission for more dangerous diseases like Zika virus.

The mosquito was first identified in Windsor-Essex in 2016, according to the Public Health Agency of Canada, and has been consistently reported in the region between 2018 and 2023. 

Zika virus is spread when a mosquito bites an infected human, and then transmits the virus to an uninfected person through another bite.

Hunter says public health units haven’t found positive Zika cases among humans or mosquitoes in Windsor-Essex, because “those mosquitoes haven’t been biting people who already have Zika.”

If a tiger mosquito bit someone who unknowingly brought over Zika virus from abroad, however, that could then lead to greater infection rates. 

“But it’s not a high risk,” Hunter clarified. 

How to protect yourself 

Chakrabarti recommends wearing long-sleeve shirts and pants, avoiding wearing bright colours that attract mosquitoes, as well using insect repellent to keep the annoying insects away.

Longer spring and fall seasons caused by human-influenced climate change also mean mosquitoes have more time to breed. At the same time, warmer weather leading to more precipitation means mosquitoes are more comfortable in Canada, despite the cold winters.

“Climate change has brought about those massive rain events that we’ve had, and when that happens, species that have always been here, their numbers just explode,” said Hunter. 

Hunter adds that floodwater mosquitoes bite during the day, as well as dawn and dusk when other mosquito species are typically more active. 

“All the public messaging of course is, protect yourself at dusk and dawn,” she said. 

“You should also protect yourself from that little floodwater mosquito in a year where it’s hot and you’ve had all that flooding.”