The summer's high rainfall amounts combined with heat and humidity means this summer could be the worst for West Nile virus in Ontario since 2012.

Public Health Ontario reports on its weekly West Nile virus surveillance website that across 24 different health units, 172 mosquito pools have tested positive for West Nile virus.

At this time last year, there were just 58 pools that tested positive for the virus.

As well, last week the number of positive pools in the province reached 59. That's the highest one-week total since 2012, which reached a high of 85 during the week of Aug. 5-11.

In a post on its website, Wellington-Dufferin-Guelph Public Health said the "abundance of rain" this summer means there are lots of spots for the insects to breed and the mosquito population "shows no sign of dying off."

"It's common to see an increase in the number of mosquitoes following significant rainfall. Stagnant water that pools on the ground becomes an ideal location for mosquitoes to lay their eggs," the health unit said.

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This graph from Public Health Ontario shows the number of positive mosquito pools as well as human cases of West Nile Virus in the province since 2002. (Public Health Ontario)

'Temperature really drives them'

Dr. Curtis Russell, an entomologist with Public Health Ontario, said the wet weather plays a factor, but so does the heat.

"The big thing with these mosquitoes is temperature. Temperature really drives them," he told CBC News.

"The hotter it is, the faster the mosquito can develop and the hotter it is, the faster the West Nile Virus can develop inside the mosquito for them to transmit it."

The mosquitoes that carry the virus are Culex pipiens, or northern house mosquitoes. Most mosquito species overwinter as eggs, which are cold tolerant, but northern house mosquito adults can last throughout the winter.

He noted the last "high-level year" was 2012 and "we are still quite a bit lower than that" in terms of positive tests.

This year, there have been eight reported — confirmed or probable — cases of West Nile virus in humans. That is the same as in 2016 by mid-August. In 2012 there were 105 at this point in the year. In 2002, the last big spike of positive test results in the province, there were 580 positive tests and 395 human cases.

"Based on projections, I would expect we do have human cases already in Ontario. I would expect to see more human cases but I don't know if we would get to the high levels that we saw in the past," Russell said.

General trends show that positive mosquito pools will peak in another week or two, then decline starting at the end of the month, he noted.

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Health officials say people should take steps to protect themselves from being bitten by mosquitoes. That includes clearing standing water from around your home, wearing light-coloured, long-sleeved shirts and pants and using a repellent containing DEET or icaridin. (Canadian Press)

Protect yourself

Brenda Miller is the manager of health protection and investigation with the Region of Waterloo Public Health and Emergency Services. The region just reported its first two positive test results of the summer. Miller said she expects there to be more.

"Provided conditions remain as they are, kind of those warm, humid days, warmer evenings, a little bit of water out there where they're able to lay their eggs, we could continue to see traps that are positive," Miller said.

She noted people should take note of the positive results and take action. That includes clearing standing water around their homes and covering up with long-sleeved shirts, pants and using a repellent containing DEET or icaridin — particularly at dawn and dusk.

"I think that the important thing for people to focus on is not that there's a trap that's positive this week or next week, but to know that any time when we have the conditions that we have during this summer period, we should be taking means to protect ourselves from West Nile Virus," Miller said.