How risky is a mosquito bite in Canada? What to know about West Nile virus this summer

Dr. Gerald Evans, Chair of Infectious Diseases at Queen's University, speaks about how Canadians can protect themselves from West Nile virus.

A dead crow in Guelph, Ont., tested positive for West Nile virus, a sign the mosquito-borne illness is circulating in the province once again.

But infectious disease experts says Canadians should keep the risk in perspective: while the virus returns every summer and can cause severe illness in rare cases, most people infected after a mosquito bite never develop symptoms at all.

Health Canada says West Nile virus remains the leading domestically acquired mosquito-borne disease in Canada. It is spread through the bite of an infected mosquito that has previously fed on an infected bird.

According to the Public Health Agency of Canada (PHAC), birds — particularly crows, ravens, jays and magpies — act as reservoirs for the virus, while humans and horses are considered incidental hosts.

“We do see cases of West Nile every year in Canada,” Dr. Gerald Evans, chair of infectious diseases at Queen’s University, said in a Zoom interview with CTVNews.ca Tuesday.

“Just be careful when you’re outdoors, especially as the warm weather arrives and mosquitoes begin to feast on people,” he added.

While West Nile virus is found across the country, Evans noted that most symptomatic cases in recent years have occurred in Western Canada, particularly in Alberta, Saskatchewan and British Columbia. Ontario continues to see cases, but less frequently.

The timing of infections is also important, Evans explained. Although mosquitoes are already active in many parts of Canada, the risk of contracting West Nile virus is generally highest later in the summer.

That’s because the virus first needs to circulate among bird populations before it can spread more widely through mosquito bites, Evans said.

Mosquitoes that feed on infected birds can then pass the virus to humans during subsequent blood meals.

“The principal times you need to be worried about West Nile virus is actually from about mid-to-late summer,” Evans said. “You have to get that amplification in the bird reservoir before you can see that spillover into humans.”

Evans said the discovery of infected birds, such as the crow recently found in Guelph, is not unusual and serves part of Canada’s surveillance system for tracking the virus.

“That can tell us the virus is present and beginning its amplification within those bird populations,” Evans said.

Federal and provincial health agencies monitor West Nile virus activity through mosquito testing, reports of infected birds and animal surveillance. Public health officials then use that information to identify areas where the virus is circulating and issue warnings where needed.

‘Iceberg’ disease

Evans describes West Nile virus as an “iceberg” disease, claiming the small number of patients hospitalized with sever neurological complications represent only the visible tip.

According to PHAC, fewer than one per cent develop severe neurological disease — invasive West Nile disease — which can impact the central nervous system.

“The people who get really sick from West Nile, who can get encephalitis, a brain inflammation, they’re the tip of the iceberg,” he said.

Infectious disease expert Dr. Isaac Bogoch told CTVNews.ca in a phone interview Tuesday that inflammation around the area of the brain — known as meningitis and more rarely paralysis syndrome that looks like polio — can occur.

Below that are people who develop a short-lived fever and flu-like symptoms before recovering — accounting for about 20 per cent of infected people.

Bogoch said the incubation period is just a few days. Mild symptoms like a fever, headache, muscle aches, pains and fatigue can last anywhere from two to five days and people generally recover in the comfort of the their home.

“We have the diagnostic tests for this in Canada, if need be. There’s no drug that we use to treat West Nile. The virus runs its course,” Bogoch said.

“We won’t see the vast majority of cases in the health-care sector, because most cases are mild,” he added.

The largest group consists who people who become infected, but never experience symptoms at all. According to PHAC, 70 to 80 per cent of people infected with West Nile virus do not develop symptoms.

Evans said researchers are still trying to understand why some people develop serious complications while others experience no illness.

Evans also added that people with weakened immune systems may face a higher risk of severe disease, including those undergoing cancer treatment or taking medications that suppress the immune system for conditions such as rheumatoid arthritis, inflammatory bowel disease or psoriasis.

Still, he cautioned against assuming a West Nile infection will automatically result in a severe illness.

“The danger from West Nile virus in Canada is not as great as it is for some of the other things we see,” Evans explained.

In Eastern Ontario, for example, Evans said tick-born illnesses such as Lyme disease, anaplasmosis and babesiosis are encountered more frequently than mosquito-borne infections.

Preventive measures

That doesn’t mean Canadians should ignore West Nile virus. Instead, Evans recommends taking simple precautions to avoid the virus, including insect repellent, wearing protective clothing and limiting exposure during dawn and dusk when mosquitoes are most active.

Bogoch said the more effective mosquito repellent contains either 30 per cent DEET or 20 per cent ICARIDIN — both types of repellents.

Bogoch advised that emptying freestanding water where mosquitos tend to breed is important.

For Canadians, heading outdoors this summer Evans said be aware and not alarmed. While West Nile virus remains a seasonal health concern, the chances of developing severe illness after a mosquito bite remains low, he clarified.