Concerns rising over rare disease
Experts warn residents to limit their exposure to mosquitoes
It is extremely rare in humans but often deadly, killing more than half of those who become infected according to experts. And, like other mosquito-borne diseases, it could become more common in Ontario as the climate changes.
Amid rising concern about the rare eastern equine encephalitis, and confirmed cases of the infection in horses in Ottawa and eastern Ontario, Ottawa medical officer of health Dr. Vera Etches is warning residents to protect themselves against mosquito bites.
“This is a rare disease, but there are multiple reasons to try to limit mosquito bites.
“Taking the steps you can to reduce exposure to mosquitoes will help,” Etches said Friday while answering questions during an event marking overdose awareness day.
Parts of the northeastern United States have been on high alert in recent weeks after cases were reported in Massachusetts and one man died in New Hampshire.
That represents the first death in the U.S. from the disease this year.
Some towns in Massachusetts are advising people to avoid going outside when the risk of mosquito bites is the highest, especially at dusk, after cases were reported in humans and horses there. Other parts of New England and the northeastern U.S. are also on alert after cases have been identified in horses.
EEEV, as it is known, is the deadliest known mosquito-borne illness, killing between 50 and 75 per cent of people who become ill with it, according to Kingston infectious disease specialist Dr. Gerald Evans. The very young and the elderly are at higher risk of severe outcomes.
EEEV is normally transmitted between wild birds and mosquitoes but can occasionally infect horses and, rarely, humans, through bites from infected mosquitoes, according to OPH. Humans do not get infected from a horse or another human. Horses can be protected by a vaccine, but there is no vaccine for humans. The mosquitoes that carry the virus are usually found close to swampy hardwood forests.
“Anyone who lives, works or participates in outdoor activities in or around wooded swampy areas where the virus is active can be at risk,” OPH warned in a recent public safety advisory.
There have been just three human cases of EEEV in Ontario, none of them in Ottawa. The most recent Ontario case was in 2022, according to Ottawa Public Health.
Although it is rare, the disease can cause serious neurological disease or death, OPH said in the advisory about EEEV and West Nile disease, which is also spread by mosquitoes.
In addition to a positive EEEV test in a horse within the city, there have been two human cases of West Nile disease in Ottawa this year and a lab testing program has confirmed 14 positive mosquito pools in addition to the two human cases of West Nile. That is a notable increase over recent years, according to Ottawa Public Health. Several horses in rural eastern Ontario have also tested positive for EEEV, according to Evans.
That is part of a trend. Infectious diseases in general are on the rise, as are those spread by mosquitoes. Some mosquito-borne diseases previously limited to tropical climates have begun showing up in the southern U.S. and others, including West Nile and EEEV, appear to be becoming more common in parts of Canada.
Evans, who is chair of the division of infectious diseases and a professor at Queen's University as well as an infectious diseases physician, said the rise of mosquito-borne diseases in Ontario and elsewhere this year partly reflects the wet summer we are experiencing, but it is also a product of climate change.
“There is real evidence that climate change is driving an increasing range of mosquito borne diseases.”
EEEV, he said, is the most dangerous of the viral mosquito-borne diseases. “It makes West Nile look like a walk in the park,” he said.
“They are going to increase in frequency and there will be more mosquitoes able to transmit them. “
Evans also noted that mosquito-borne diseases such as EEEV and West Nile tend to show up later in the summer, which is why cases have been appearing since later in August. That will continue into the fall until there is frost — something that comes later in the fall than it once did in eastern Ontario.
Like OPH, Evans advises people to wear long sleeves and pants as well as insect repellent when they are out at dusk.
“I want people to not be frightened by this,” he added. “It is a risk, but don't freak out.”