As 海角社区官网recovers from this week’s storm, residents may soon have to contend with a new threat emerging from the stagnant remains of the flood: West Nile virus-spreading mosquitoes.
The city was already facing “concerning” numbers of a mosquito species capable of transmitting the illness, amplified by an inordinately warm and humid summer, experts said. Now, they’re expecting the species to multiply in the aftermath of a flood that’s turned the city into a “prime breeding habitat.”
On Tuesday, 海角社区官网Public Health announced West Nile virus had been detected for the first time in the city this year, found in a batch of mosquitoes in the High Park area.
“We’ve already got positives for West Nile, and it’s still early in the season,” said Fiona Hunter, a professor of veterinary and medical entomology at Brock University. “It’s because it’s been so hot聽鈥 and that’s a nice combination (for mosquitoes): heat and standing water.
“I think this might be a really bad year.”
Tuesday鈥檚 deluge, which dropped a month鈥檚 worth of rain in roughly three hours, may have been Toronto鈥檚 third 鈥100-year storm鈥 since 2013 鈥斅犫淲e
Tuesday鈥檚 deluge, which dropped a month鈥檚 worth of rain in roughly three hours, may have been Toronto鈥檚 third 鈥100-year storm鈥 since 2013 鈥斅犫淲e
Will the 海角社区官网flooding lead to more mosquitoes?
Toronto’s flooding is expected to boost the numbers of at least one type of mosquito. Unfortunately, “it’s not a nice species at all,” Hunter explained.
Aedes trivittatus (Och.聽trivittatus) 鈥 a species of floodwater mosquito that thrives in the aftermath of flooding 鈥 is not only capable of spreading West Nile virus and dog heartworm, but also tends to bite people during the day when other mosquito species are inactive, Hunter said.
“They’re little and they’re aggressive. They’re painful and they’ll bite during the day as well as at dusk,” she explained. “There was a lot of prime land that was flooded. That’s going to become a prime breeding habitat for the species.”
On the flipside, the flooding likely killed a great deal of other species, said Dr. Howard Shapiro, Toronto’s associate medical officer of health.
Danger such as E.coli, mould and airborne toxins can plague a city for weeks after a deluge.
Danger such as E.coli, mould and airborne toxins can plague a city for weeks after a deluge.
He explained the torrential downpour likely “washed away” thousands of developing mosquito babies hidden in puddles and catch basins, likely diminishing species such as Culex pipiens and Culex restuans 鈥 considered the main spreaders of West Nile virus in Toronto, Shapiro continued.
Och. trivittatus numbers have been climbing in Toronto; earlier this summer, their population was already double that of the same time last year, according to recent surveillance Hunter’s company, Entomogen Inc., did for 海角社区官网Public Health. She expects their numbers to multiply further in the coming days.
“You have the perfect storm where the grounds are saturated from the flood聽鈥 they love laying their eggs in flooded fields or lawns, or聽flooded areas along streams and creeks 鈥 and they can breed really, really quickly depending on the temperature, anywhere from five to eight days,” she said. “I think, within a couple of weeks, we’ll see those numbers go way up.”
What’s the risk of West Nile virus in Toronto?
On Tuesday, 海角社区官网Public Health confirmed a batch of mosquitoes had tested positive for West Nile virus for the first time in 2024. While Shapiro noted the bugs were caught “in the High Park area,” he warned “the risk is more general than just in High Park聽鈥 that just happens to be where we found it.”
“We know, as the summer progresses, we’re going to find mosquitoes carrying West Nile virus in different parts of the city, in different parts of the province.”
City surveillance teams set out 22 mosquito traps across 海角社区官网every week from mid-June to mid-September; the fact that only one trap tested positive for the virus means it likely isn’t widespread yet, Shapiro said.
Public health believes the threat level to be currently low聽鈥 “but that’s because it’s still early in the year,” Shapiro explained. “The risk of West Nile, it sort of builds over the summer. And so far we’ve only found this one positive batch.”
However, Hunter notes that because the virus was detected relatively early in the season聽鈥 the first positives historically appear in late July to early August聽鈥 the risk will likely be greater than previous years as the summer continues.
“We just had so much heat that the virus is amplifying very efficiently in the mosquitoes,” she said. “From the modelling that we’ve done, the hotter the early part of this season is, the greater the risk.”
West Nile virus has been a constant in 海角社区官网throughout the years, after the first Canadian case was discovered in Ontario in 2002. Last year, 海角社区官网reported 23 human infections and one death, according to city data shared with the Star.
Shapiro warns the real numbers may be far higher, as the majority of West Nile human cases are asymptomatic or lead to mild symptoms that may get dismissed.
What is West Nile virus?
West Nile virus is a potentially serious illness spread by certain mosquitoes after feeding on an infected bird. While roughly 80 per cent of infected humans show no symptoms, it can lead to serious health consequences 鈥 especially in immunocompromised individuals, people with underlying health conditions or those older than 50.
“About 20 per cent of the people who are infected will have symptoms,” Shapiro explained, noting these usually appear within two to 15 days of a bite and include hallmarks of viral infection such as “fever, headache, a rash, maybe aches and pains.”
“About one in 100 people who are infected will have infection or inflammation of their brain, or the lining of the brain, which can be quite severe and require hospitalization,” he continued. Signs of this include: a severe headache, high fever, stiff neck, nausea or vomiting, difficulty swallowing, drowsiness and confusion.
If you display any of these symptoms after a mosquito bite, see your health care provider immediately.
How to protect against聽mosquitoes
Two new studies shed light on why mosquitoes may be attracted to certain humans but not others.
Two new studies shed light on why mosquitoes may be attracted to certain humans but not others.
海角社区官网residents should empty out any pockets of standing water left over from the flood, said Hunter Murphy, general manager of mosquito control company Mosquito.Buzz.
“Check things like wheelbarrows, bird baths, even potted plants,” as well as unseen areas like the gutters on your roof, he said. “Within the amount of water that you could pour in a bottle cap, a mosquito can lay up to a thousand eggs.”
He also advised installing insect screens on windows and doors, and ensuring they’re tight fitting. You can also try to avoid going out in the evenings or early mornings, when mosquitoes tend to be more active.
Hunter recommended wearing light-coloured, long-sleeved shirts and pants when outdoors, or at least applying a .
“When people hear that the numbers are going up, absolutely people have to take responsibility and protect themselves,” he said.
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