WINNIPEG - Manitoba is proving to be the West Nile virus capital of Canada this summer.
Eight years after the virus first appeared in North America, scientists are still trying to determine why some summers are almost virus-free, while others see a sharp increase in the number of infected people.
"This is still a disease that is quite early in its history of understanding,'' Dr. Joel Kettner, Manitoba's chief medical officer of health, said Tuesday.
"We're still at the beginning of the learning curve, I think, on this as to how we can expect year-to-year variation.''
So far this year, 42 Manitobans have tested positive for the virus, and one person has died. The human cases are coming more quickly and earlier in the season than any previous year, and the number of mosquitoes testing positive for the virus is about 10 times higher than the average of the last three years.
An elderly man from the southwest part of Manitoba died from the more severe, neurological form of the disease.
The numbers are lower in other provinces -- 17 people have either tested positive or are suspected of having the virus in Saskatchewan, and only one person has tested positive in Alberta.
Health officials say there are three major factors that determine the prevalence of West Nile virus in any given year. A wet spring followed by a summer heat wave creates an ideal breeding ground for mosquitoes. A mild winter, even by Manitoba standards, can allow more mosquitoes to survive until spring, when they can renew their attack.
More tricky is the third factor -- the number and patterns of migrating birds that can carry the disease from one region to another.
"(It's) always a little difficult to measure quantitatively,'' Kettner said.
While most people who contract West Nile don't experience any symptoms, people with neurological cases experience fever, headache and weakness for weeks or months and, in extreme cases, swelling of the brain, paralysis and even death.
With summer halfway over, Manitoba health officials expect the number of human cases to keep climbing.
"I won't be surprised if we still keep getting cases reported right through September,'' Kettner said.
Health officials try to balance the need to warn people about the disease without creating a sense that the disease is widespread.
Kettner and his colleagues in other provinces suggest people cover up, apply bug repellent with DEET and avoid outdoor activities at dusk and dawn.
