Canada has had its worst summer ever for West Nile virus, with the problem centred on the Prairies.

So far, 1,790 people have been infected with the mosquito-borne illness, compared to the previous record of 1,481 in 2003.

Saskatchewan accounts for 1,054 of this summer's confirmed or suspected cases. Manitoba has reported another 500 cases.

"It comes and goes," Dr. James Brunton, an infectious diseases specialist at the University of Toronto, told CTV Newsnet on Saturday. "If you have the right climatic conditions and the right bird population, you can have an outbreak."

The virus grows first in birds then is picked up by the mosquitoes. The virus then develops in mosquitoes, he said.

Weather-wise, the wet spring on the Prairies followed by a hot summer created ideal conditions for mosquito growth, Brunton said.

"In addition, the hot conditions are ideal for the growth of the virus within the mosquito's body."

Robbin Lindsay, a Winnipeg-based entomologist with the Public Health Agency of Canada, told The Canadian Press that the weather was particularly good for Culex tarsalis mosquitoes -- the type that transmits the virus.

Paradoxically, the weather wasn't so good for mosquitoes that bite but don't transmit the virus, he 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,' " Lindsay said.

"That translated into not very many people thinking there was a big risk."

As a result, they may have stopped wearing bug spray in the evening, which would have helped protect them against West Nile, he said.

West Nile can potentially be serious. The severe form can lead to swelling of the brain and spinal cord. Paralysis can result, even death.

"It only causes nervous system damage in about one in 150 or so people who actually get the virus," Brunton said. "Many people are actually asymptomatic."

Older people are at higher risk of the worst effects, he said.

In Saskatchewan, 30 people have the most severe symptoms. Health officials suspect that number will climb.

With files from The Canadian Press