This spring, pest management crews in Regina treated pools of water with bacteria that attack mosquito larvae. (CBC)

Cooler temperatures east of British Columbia may be thwarting the spread of the West Nile virus this summer.

So far, there are no human cases of the mosquito-borne illness in Canada compared with three reported by this time last year. If there is no major heat wave, health authorities say it's likely this summer could see the fewest West Nile cases in years.

Rain in Manitoba, Ontario and Quebec may have created standing water for mosquitoes to breed in, but without a heat wave, there seems to be fewer of the pests.

"Hotter weather seems to be associated with higher viral loads in mosquitoes," said Dr. David Fisman, a Toronto-based scientist who studied the arrival of West Nile in North America. "It revs them up so that they bite more and they breed quicker."

Heat waves helped mosquitoes to spread the virus across North America and into parts of Canada, he said.

The risk to humans arises from an "amplification cycle" where mosquitoes bite birds and infected birds get bitten by mosquitoes. Once so many mosquitoes and birds become infected, the disease starts to spill over into humans since some mosquitoes can bite both birds and humans, Fisman said.

Huge number of cases not expected

By this time last summer, 42 regions in Ontario, Manitoba and Saskatchewan had reported mosquito pools carrying West Nile. Despite increased surveillance this year, no human cases have been found and no infected mosquitoes have been reported.

"It's not to say we won't get human West Nile cases, in fact we probably will," said Dr. Nicholas Ogden of the Public Health Agency of Canada based in Winnipeg.

"But unless we have some very strange weather from now till the end of the year, we wouldn't expect to see a huge number of cases this year. It is a good news story."

There are other factors besides temperature. Many birds have already been killed by West Nile, so there are fewer to fly it as far afield. But the generally warming climate may also favour mosquitoes and greater spread of the virus in years to come.