As summer heats up in Canada, the mosquito population is on the rise, bringing with it an increased risk of West Nile virus, a potentially deadly disease.
This season, the most recent West Nile virus case in Canada was reported in Ottawa. On July 19, Ottawa Public Health confirmed the infection in a resident. The agency also confirmed the first positive mosquito testing pool in the area since the annual monitoring program began on June 1.
“West Nile virus is a mosquito-borne virus that was introduced to Canada in the early 2000s. It flared up quite a bit for the first little bit as it was spreading through the region and has since kind of died back a little bit, but it’s still prevalent throughout, most of Eastern Canada and into the Prairies,” Morgan Jackson, a Montreal-based entomologist, told Global News.
“And it’s a virus that primarily infects birds … like blue jays and ravens and crows. But when it does get into people and also some livestock, it can cause us to get sick as well.”
Since the virus arrived in Canada in 2002, it has spread in all provinces except for Prince Edward Island and Newfoundland and Labrador, according to Infection Prevention and Control Canada (IPAC). Since then, the number of cases each year has varied widely, ranging from more than 2,400 cases across the country to as few as five.
The latest preliminary data from Health Canada shows that in 2022 there were 47 cases.
However, the agency noted that surveillance captures only a fraction of West Nile virus cases in Canada. The actual number of infections is likely higher.
“This summer, the numbers are very low across Canada. But it’s still being found in mosquito populations and is still being found in dead birds across the country So there is still a prevalent and ongoing risk,” Jackson said.
Although West Nile virus cases occur annually in Canada, experts say there are still ways to enjoy the outdoors without being overly concerned about mosquitoes carrying the disease.
Here’s what you need to know about the disease and how to stay safe to make the most of your summer activities.
West Nile virus is considered the most widely distributed vector-borne disease in North America, according to the BC Centre for Disease Control (BCCDC). It is closely related to the virus that causes dengue fever and yellow fever.
The illness was first identified in the West Nile Valley region in Uganda in 1937, and the first cases appeared in North America in 1999.
“It’s transmitted by mosquitoes. So a mosquito takes blood from a bird or another mammal. The viruses replicate within the mosquito for a little while, and then when they come back and bite somebody else a day later, they can transmit it to them,” Jackson said.
“So most of the transmission is going amongst birds. But if there’s a mosquito that can’t find a bird and finds you in your backyard or out at the soccer field, then that’s how you can get it.”
The virus propagates in temperate zones of the world, and new evidence suggests that it may over-winter in northern climates in birds and animals, IPAC said, adding that the disease is fatal in domestic and wild birds.
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West Nile virus primarily infects birds (particularly crows, ravens, blue jays and magpies), although it has been found in other mammals such as horses and bears, IPAC said. Human cases are still very rare.
Most people infected with West Nile virus will experience no symptoms at all, according to BCCDC. About 20 per cent will develop flu-like symptoms. In rare cases, fewer than one per cent, more severe complications can occur, including brain inflammation, meningitis or paralysis.
During the West Nile virus season (mid-April to November), Canada conducts ongoing human case surveillance across the country, the Public Health Agency of Canada (PHAC) says.
National data indicates that provinces like British Columbia and Alberta have reported no new cases of West Nile virus as of July 5. In Quebec, no human cases have been reported, though the virus has been detected in three dead wild birds in the province.
Other provinces report their case numbers independently.
In Saskatchewan, there was an increase in the number of mosquitoes from July 7 to 13, the latest provincial data shows. However, all mosquito pools in the province tested negative for West Nile virus the same week. There also have been no reported cases of West Nile virus so far this year.
Saskatchewan stated the risk of West Nile virus transmission is currently low but is “expected to increase in the coming weeks.”
In Manitoba, 2024 surveillance data shows two human cases of West Nile virus so far this year. However, these cases were related to travel outside of Canada, the province added. Additionally, 11 adult mosquitoes have tested positive for the virus this year. In 2023, this number was 21, and in 2022, it was one.
There has been one reported case of West Nile in Ontario so far this year. Ontario’s latest data (July 7 to 13) said there have also been eight West Nile-positive mosquito pools.
Nova Scotia has reported no cases of West Nile virus as of 2022, 2023 and 2024, a government spokesperson said in an email to Global News on Monday. The province does not participate in mosquito trapping or mosquito testing for the virus.
Most West Nile virus infections are mild, with symptoms that can include a sudden onset of fever, headache, body aches, nausea, vomiting and occasionally a skin rash on the trunk of the body, along with swollen lymph glands, according to IPAC.
These symptoms can last around three to six days.
More severe infections include a headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis and, rarely, death.
The incubation period of West Nile virus encephalitis is usually three to 14 days, IPAC said.
Mosquito breeding grounds
Mosquito species vary across different regions in Canada, but the Culex species are the most significant, as they are the primary transmitters of West Nile virus and are the ones most people are likely to encounter, Jackson explained.
These types of mosquitoes seek out small pools of water to lay their eggs. Mosquitoes thrive in warm, nutrient-rich standing water, with even a small amount, such as a teaspoon, being sufficient for them to breed, he said.
“These are mosquitoes that live primarily in and around cities. This is a mosquito that breeds in containers; little pools of water around your yard are exactly what it loves,” he said.
“That’s why it really likes urban environments because there’s a lot of little pools of stagnant water … pop cans that have got filled up with rainwater. It loves children’s toys that are sitting out, gutters that haven’t fully drained.”
The population of this species also spikes when there is a lot of “cyclical rain,” he added.
The best way to avoid becoming infected with West Nile virus is to prevent mosquito bites.
Eliminating any standing water on your property can help prevent mosquito infestations.
To prevent mosquito breeding around your home, the BCCDC recommends checking for standing water outdoors at least once a week. Empty any water that accumulates in containers like flower pots, wheelbarrows, old tires, barrels, tin cans and even bottle tops.
Drill holes in the bottom of used containers so water can’t collect and change water in bird baths every few days.
If you have a swimming pool, immediately remove water that collects on pool covers and make sure the pool pump is circulating. Turn over wading pools when not in use.
Health Canada advises covering exposed skin when outdoors by wearing long pants, loose-fitting long-sleeve shirts, socks and a hat. For added protection, consider using a mosquito net over your hat. It’s also recommended to wear light-coloured clothing, as mosquitoes are more attracted to dark colours. Additionally, using insect repellents containing DEET or Icaridin can help protect you from mosquito bites.