It’s mosquito season in Ontario and that means it’s time to protect yourself against West Nile virus.
Several public health units in southern Ontario have now reported mosquitoes testing positive for the virus this year.
No human cases have been reported in Ontario so far in 2026.
Case numbers typically jump in August and September, according to government data.
Which areas found West Nile-positive mosquitoes?
The Durham Region Health Department said on Friday, July 17, a batch of adult mosquitoes collected in Ajax tested positive for the virus in the region’s first positive test of the season.
One day earlier, York Region Public Health reported its first West Nile virus-positive batch of mosquitoes this year.
York Region’s public health unit said batches of mosquitoes in two traps, both in Markham — one at Warden Avenue and Highway 7 East and the other at Kennedy Road and 16th Avenue — tested positive for the virus.
These reports follow a similar discovery in Halton earlier this month.
Halton Region reported on July 2 that a batch of mosquitoes trapped in Oakville tested positive for the virus.
Toronto Public Health confirmed on July 17 “one batch of mosquitoes” in that city tested positive for West Nile virus.
Dead crow tests positive in June
Back in early June, Wellington-Dufferin-Guelph Public Health reported that a dead crow that tested positive for the virus was discovered in Guelph.
A positive test in a local bird suggests “mosquitoes carrying the virus may be present and residents should take precautions to reduce the risk of bites,” the health unit said at the time.
What is West Nile virus?
West Nile virus (WNV) was first found in 1937 in the West Nile district of Uganda, the Alberta Health Services said. Since then, outbreaks happened in Egypt, Israel, South Africa and in parts of Europe and Asia.
The virus then made its way to North America, with the first recorded case in humans here in 2002.
The virus is normally found in birds. It transfers to some mosquitoes after they feed on birds and/or other animals infected with WNV.
Because weather patterns impact the mosquito breeding season, the level of risk can vary from summer to summer.
The typical prime time for mosquitoes is July to September.
Symptoms can take time to appear
According to Health Canada, the first symptoms of West Nile virus usually appear within two to 15 days after infection.
Up to 80 per cent of people infected with West Nile virus will have no symptoms. Others may experience mild symptoms.
Mild symptoms include:
- Fever
- Headache
- Body aches
- Mild rash
- Swollen lymph glands
With mild symptoms, recovery usually takes a week.
Adults aged 50 and older, and those with underlying health conditions or weaker immune systems, are at greater risk to develop more severe symptoms, which impacts about one per cent of patients.
More serious symptoms include:
- Rapid onset of severe headache
- High fever
- Stiff neck
- Nausea or vomiting (sometimes both)
- Difficulty swallowing
- Drowsiness
- Confusion
- Loss of consciousness
- Lack of co-ordination
- Muscle weakness
- Paralysis
In the case of more severe symptoms, treatment can take months to years for recovery. In some circumstances, it can be fatal.
There is no vaccine or specific treatment for West Nile virus.
How many Canadians had the virus in 2025?
In 2025, Canadian health officials reported 296 human cases of West Nile virus among people who had not travelled outside the country, with 169 of those being reported in Ontario.
That’s up from the 177 reported cases across the country in 2024, which included 81 Ontario cases.
There were additional travel-related cases reported as well.
Across the country, there were nine deaths reported in 2025 among people who had not travelled outside of Canada.
How can I protect myself?
WNV is directly tied to mosquitoes, so the best way to protect yourself is to protect yourself from mosquito bites.
Tips include:
- Cover up when going outside and wear light-coloured, long-sleeved shirts and pants with tightly woven fabric.
- Get rid of all water-filled containers and objects. Bird baths and other still pools can be breeding grounds for mosquitoes.
- Use an insect repellent, such as one containing DEET or Icaridin.
- Keep mosquitoes out of the home by ensuring windows and door screens are tight and without holes, cuts or other openings.
Urban areas tend to be more likely to have WNV-infected mosquitoes. Not all mosquitoes are able to carry the virus, and the types of mosquitoes that transmit WNV to humans most commonly breed in urban areas such as bird baths, plant pots, old toys and tires that hold water.
Learn more through Public Health Ontario
Public Health Ontario has more resources available about West Nile virus. It’s available online.
