New numbers show Canada is experiencing its worst year for West Nile virus, and as strange as it sounds, one reason could be the lack of a certain type of mosquito.

A wet spring and hot summer in the Prairies provided the perfect breeding ground for the type of mosquitoes that transmit West Nile virus to humans.
(CBC)

So far this summer, 1,790 people are known to have become infected with West Nile, compared with the previous record of 1,481 cases in 2003.

This year, the vast majority of the reported cases are in the Prairies, where the deaths of seven people have been linked to the virus. Saskatchewan Health officials said Friday that a total of 1,054 confirmed or suspected cases have been reported, and in Manitoba, more than 500 cases have been reported.

Trying to determine why West Nile has hit the Prairies hardest has left experts scratching their heads.

"It is a little odd, and we're not sure exactly why that is," said Robbin Lindsay, a Winnipeg-based entomologist with the Public Health Agency of Canada.

One obvious factor is the weather, he said. A wet spring and hot summer were perfect for the development of Culex tarsalis mosquitoes, the type that transmits West Nile to humans.

Those same conditions weren't right for the breeding of nuisance mosquitoes, the biting bugs that, although annoying, don't carry West Nile.

When they aren't bothered by mosquitoes, people sometimes become complacent and forget to take precautions against West Nile, such as wearing bug spray, Lindsay said.

"If you ask people on the street if it was a bad year for mosquitoes, they'll say, 'God, no, it was beautiful. You really didn't get a lot of bites,'" he said. "That translated into not very many people thinking there was a big risk."

Heightened awareness could be a factor

Dr. Joel Kettner, Manitoba's Chief Medical Officer of Health, said a heightened awareness of the virus among both doctors and patients could be another reason why the province is seeing so many cases reported this year.

"We definitely have more testing for West Nile virus than ever before," Kettner said.

With more than 1,000 cases, Saskatchewan has the highest number of reported human cases of West Nile in Canada this year. Of those who are sick, 30 have West Nile Neurological Syndrome — the most severe symptoms.

Saskatchewan medical health officer Dr. Shauna Hudson said it's hard to know how much higher the numbers will climb.

"It depends on how many physicians repeat testing, it depends on what our public health inspectors and public health nurses find on their case investigations, but I think we'll see it increase."

Most people infected with the virus don't show any symptoms, which means there are likely many unreported cases. 

The severe form of West Nile can cause swelling of the brain and spinal cord, as well as paralysis, or even death.