Personal Health Navigator

How climate change is spreading mosquito-borne diseases

Written by Paul Taylor

Question: I’ve read news reports about an outbreak of Eastern Equine Encephalitis in the United States. It’s spread through mosquito bites and it can be deadly for people. Should I be avoiding travel to the U.S.?

Answer: Although the viral infection – known as “EEE” or “Triple E” for short – can be quite serious, the actual number of human cases is still relatively small. Public health officials have not issued any warnings to stay clear of outbreak areas, but they urge people to take precautions to prevent mosquito bites.

Even so, this outbreak represents a worrisome development. And, in the foreseeable future, Canadians might not be able to dodge the risk of getting this infection, or other foreign mosquito-borne illnesses, by simply staying at home.

In fact, climate change and various human activities are creating ideal conditions for the establishment of invasive disease-carrying pests in Canada. Some of them are capable of transmitting EEE, Zika, dengue, chikungunya, yellow fever as well as West Nile virus, which is already well established in parts of Canada.

But before looking at the bigger picture, it’s worth taking a deeper dive into the EEE virus, which is found in the Eastern United States, plus around the Great Lakes and Gulf Coast.

The virus is carried by a mosquito species called Culiseta melanura, which breeds in swampy areas and feeds primarily on birds, says Dr. Fiona Hunter, a professor of medical and veterinary entomology at Brock University in St. Catharines.

This mosquito can transmit the virus to a bird when it takes a blood meal.

To spread further, another type of mosquito usually needs to get involved – one that feeds on both birds and other creatures. So, a second mosquito species, such as Culex erraticus, can acquire the EEE virus by feeding on an infected bird, and then transmit the illness to a mammal, for example, when it dines again. Horses are especially vulnerable to the infection – hence “equine” is in the name of the virus.

Most infected people seem to experience only mild flu-like symptoms or feel no ill effects. However, some individuals – perhaps two per cent – develop encephalitis, a serious inflammation of the brain, says Dr. Samira Mubareka, a virologist at Sunnybrook Health Sciences Centre in Toronto.

Of those who contract encephalitis, 30 per cent will die and survivors are often left with permanent disabilities. “Triple E can have a significant clinical impact for the few patients with encephalitis,” says Dr. Mubareka.

It’s highly likely that people who suffer a severe reaction have some pre-existing vulnerability to the virus, says Dr. Mark Loeb, a professor in the department of pathology and molecular medicine at McMaster University in Hamilton.

He has been studying possible risk factors and thinks a number of genetic variants could lead to a “biological predisposition” to certain mosquito-borne diseases.

In a typical year, there are an average of seven EEE cases in the entire United States. (One human case was reported in Canada in 2016.)

But this year, there’s been a surge in EEE activity. More than 30 cases – including 11 deaths – have been reported across seven states.

Experts are at a loss to explain why 2019 could end up being one of the worst years on record for EEE.

Many of them are concerned that this may be a harbinger of things to come for a wide range of mosquito-borne infections.

Dr. Hunter and her research colleagues have been closely monitoring the encroachment of new mosquito species into Canada. Over the past two decades, at least a half dozen foreign mosquito species – capable of spreading sicknesses – have been found at numerous sites in Southern Ontario.

“With climate change, our habitat is becoming more conducive to southern mosquitoes moving in and flourishing here,” says Dr. Hunter. Warmer temperatures also enable the virus to replicate faster in the insects.

Global trade is another source of invasive pests, which can hitch a ride in goods destined for North America.

Aedes albopictus, or the Asian Tiger Mosquito, is breeding quite happily near Windsor, Ontario,” says Dr. Hunter. “We first picked it up in 2016 and the population has expanded further afield each year.”

It’s not only a potential West Nile virus transmitter, but Aedes albopictus can also spread Zika, dengue and chikungunya – illnesses once considered to pose little or no peril to Canada.

“At this point, there is no need to alarm people,” she says, adding that the current risk is small. “But 20 years from now, it could be a different story.”

If more mosquitoes capable of transmitting illnesses become well established and widespread, we could have outbreaks of diseases with foreign origins. A tourist, for instance, could pick up an exotic virus abroad and bring it back to Canada where it might then be spread locally by the invasive insects, says Dr. Hunter.

In 2007, she notes, health officials recorded cases of chikungunya fever being spread this way in Italy.  The virus was likely introduced into Italy from someone who had recently been in India. The Italian outbreak marked the first time that chikungunya was transmitted within a temperate country.

So how will Canadians guard against these foreign illnesses?

Dr. Hunter points out that vaccines can be costly and challenging to develop. Furthermore, government mosquito control programs are of limited effectiveness because there are so many different places where the insects can breed and flourish.

That means the onus will likely fall on people to adopt their own defensive measures against mosquito-borne diseases, such as covering up with appropriate clothing and using insect repellent.

“It’s just good to protect yourself from mosquito bites because we never know what threat might be around the corner,” says Dr. Hunter.

About the author


Paul Taylor

Paul Taylor, Sunnybrook’s Patient Navigation Advisor, provides advice and answers questions from patients and their families, relying heavily on medical and health experts. Email your questions to
and follow me on Twitter @epaultaylor