Adult mosquitoes in a Strathroy, Ont. laboratory in 2007 - the worst year on record for infections in Canada. (Dave Chidley/Canadian Press) Canada has just four confirmed human cases of the West Nile virus so far this year — the lowest tally since public health officials started keeping track in 2002 — but epidemiologists warn there is no cause for complacancy.

"If we have a warm wet year next year, and a short winter, we could get a high abundance of mosquitoes like we saw in 2007, especially in the Prairies," said Dr. Nicholas Ogden, a disease ecologist with the Public Health Agency of Canada.

The virus is spread primarily by mosquitoes after they feed on the blood of infected birds.

The risk of human infection is generally greatest during mosquito season which peaks in late August and early September, but cool summer temperatures east of the Rockies this year have hampered their breeding.

"This is not a great year if you're a mosquito," said Ogden.

The four cases to date are down from the same time last year when there were 38 confirmed human cases, including some where people showed no symptoms.

Last year's numbers are in turn a significant drop from 2007 when human cases peaked at 2,401 - the highest number recorded in Canada.

This year's cases — two in British Columbia and one each in Alberta and Saskatchewan — did not materialize until the end of August and the beginning of September.

In the case of B.C., it is the first time someone has contracted the virus while in the province. All previous cases of human infection were the result of out-of-province travel.

This is also the first year scientists have found infected mosquitoes in B.C., which experienced an unusually warm summer this year.

"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."

That's why scientists warn that a generally warming climate may result in greater spread of the virus in years to come.

The West Nile virus can cause fatal inflammation of the brain (encephalitis) or the membranes covering the brain or spinal cord (meningitis) in more than 100 bird species, and nine mammals, including humans, horses and gorillas.

Symptoms include fever, chills, nausea, vomiting, fatigue, a possible skin rash or swollen glands and headache. Its serious symptoms include neck stiffness, a worsening headache, disorientation, tremors or convulsions, muscle weakness, difficulty moving, and paralysis or coma.

Most Canadians who have been bitten by an infected mosquito have developed some antibodies. But their ability to fight off the virus depends on their overall health, and the ability to fight off the disease does decrease with age, warns the Public Health Agency of Canada.