Cases of West Nile infection among Canadians during this year's mosquito season plunged dramatically compared with last year, and experts suggest cooler, wet weather is likely the reason.
To date, just 38 human cases have been reported for the season, with the highest numbers of cases in Manitoba (13) and Saskatchewan (19). No deaths have been reported this year due to the mosquito-borne virus, says the Public Health Agency of Canada.
Contrast that with last year, when more than 2,200 Canadians were infected by West Nile. Again, the Prairies were the hardest hit, with Manitoba and Saskatchewan tallying a combined total of about 1,900 cases. Twelve people with the infection died.
"We do feel that factors such as weather do contribute to the amount of West Nile virus that is circulating during the mosquito season," Mike Drebot, the agency's chief of viral zoonoses, said Wednesday from Winnipeg.
Cool spring temperatures may have slowed mosquitoes' ability to breed and expand their population, so the number of insects infected with the virus was likely also much lower, he said.
But the dive in human cases this year does not mean Canada is out of the woods as far as West Nile goes in the future, said Drebot.
"In 2003, we had one of our bigger outbreaks, in which close to 1,500 cases were identified," he recalled. "Then in 2004, it dropped down to 26."
Weather was likely the main influence, he said, noting that both 2003 and 2007 seasons were marked by warm springs and hot, humid summers — ideal conditions for the types of mosquitoes that are the prime carriers and transmitters of the virus.
"In 2004 and this year, 2008, we had a cold spring, the summer took a little while to warm up and the numbers were dampened."
Virus 'here to stay'
Drebot said it's also possible that an increasing proportion of the population is becoming immune to infection with the virus, which was first reported in Canada in 2001. The vast majority of people who get infected from a mosquito bite develop either no symptoms or a mild flu-like illness. Only about one in 150 are made seriously ill with West Nile-related disease, which can include potentially fatal encephalitis.
"I think those of us who work and do research on West Nile virus are actually a little surprised that it could come back as strongly as it did last year. This was one of the biggest outbreaks of any mosquito-borne virus we've seen in North America."
"We didn't expect the huge numbers of cases we had last year."
That means the potential is always there for West Nile to come back stronger and have a major impact on people's health, he said.
"I think it's a virus that's here to stay. I think just as individuals have to be prepared for flu season, West Nile is one of those seasonal pathogens that one has to be concerned about from mid-summer to early fall."
"Whether or not we'll see huge outbreaks involving thousands of cases again, time will tell," he said. "But there will always be some activity and hence the message has to go out to the public that this is one virus one needs to take precautions against when the mosquito season is upon us."