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Here's what you need to know about West Nile virus as you head outdoors this weekend.

WHAT IS WEST NILE VIRUS?

The first human case of West Nile virus appeared in Canada in 2002.

The virus was originally identified in the West Nile region of Uganda, said Dr. Isaac Bogoch, an infectious diseases specialist at the University Health Network in Toronto, but was carried to many places around the world by migratory birds.

When mosquitoes bite infected birds, they in turn become infected and then can pass on the virus to humans and other animals when they bite them.

The type of mosquitoes that transmit West Nile virus usually bite around dusk and at night, Bogoch said.

Human infections usually occur in mid to late summer, petering out as the temperature cools down.

The Public Health Agency of Canada says humans don't spread the virus to other humans, except for very rare cases including blood transfusions, organ or tissue transplants, mother-to-child transmission during pregnancy or breastfeeding.

WHAT ARE THE SYMPTOMS?

"The vast majority of people who are bitten by a mosquito harbouring West Nile virus will have no symptoms whatsoever," said Bogoch.

For the 20 to 30 per cent of people who get sick, most will have a few days of symptoms that resolve on their own.

Symptoms usually begin between two and 14 days after the mosquito bite. They can include fever, muscle aches, headaches, fatigue, nausea, vomiting, skin rash, swollen lymph glands and a stiff neck.

The concerning part of West Nile virus is that up to two per cent of people infected will get neuroinvasive disease, including encephalitis or meningitis, Bogoch said.

Encephalitis is inflammation of the brain and meningitis is inflammation of the area around the brain and spinal cord.

In "very rare" cases, people can also develop paralysis, he said.

Although anyone can get these severe West Nile virus infections, they tend to happen more in older adults, Bogoch said.

HOW IS WEST NILE VIRUS TREATED?

There is no antiviral medication to treat West Nile virus. Most people won't need medical attention and can manage aches and pains with over-the-counter acetaminophen, rest and fluids.

For the rare cases of neuroinvasive diseases, hospitals provide supportive care, including fluids and electrolytes, and provide rehabilitation if needed.

IS THERE A VACCINE FOR WEST NILE VIRUS?

There is no vaccine for West Nile virus.

HOW CAN I PREVENT WEST NILE VIRUS?

The best way to prevent West Nile virus is to avoid being bitten by a mosquito in the first place.

Using insect repellent is "very effective," Bogoch said.

Both Bogoch and the Public Health Agency of Canada recommend using bug sprays and lotions containing the chemicals DEET or Icaridine. Health Canada says those products should not be used on infants under six months old and parents can use mosquito nets over cribs or strollers when babies are outside instead.

Wearing long sleeves and long pants, socks and a hat can also protect against mosquito bites. People can also wear mosquito nets over their hats. Choose light-coloured clothing, because mosquitoes are attracted to dark colours.

Making sure all open windows have screens to keep mosquitoes and other bugs out is important.

Getting rid of any standing water around your home or cottage also helps because that's where mosquitoes lay eggs. Drain buckets, planters, old tires, pool covers, wading pools and other objects where water accumulates.

HOW COMMON IS IT?

The number of reported cases annually varies greatly since PHAC started tracking domestic infections in 2003, ranging from a handful of cases per year to a peak of 2,401 cases in 2007.

In 2024, there were 166 reported cases, according to PHAC's preliminary data.

"For most Canadians, the risk of getting infected is low because relatively few mosquitoes in Canada are infected with West Nile virus," said Mark Johnson, a spokesperson for PHAC, in an email Friday.

This report by °µÍø½ûÇø was first published Aug. 2, 2025.

Canadian Press health coverage receives support through a partnership with the Canadian Medical Association. CP is solely responsible for this content.