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West Nile baffles Prairie health officials Sudden drop in human cases reported makes preparing for next year a challenge

Sudden drop in human cases reported makes preparing for next year a challenge

Canadian Press


ic-health officials on the Prairies are already preparing for next year's West Nile virus season, even though they're perplexed by the way the disease performed this year.

Saskatchewan was one of the busiest spots in the country for West Nile virus in 2003, with 941 human cases and six deaths. This year, the number is down to just 10 human cases.

Manitoba had 142 human cases last year, including two deaths, but only three cases in 2004.

Last week, Alberta recorded its first human case of the year when a young woman from the east central area of the province tested positive. She became ill in mid-September.

A lot is still unknown about how the disease works in North America, said Ross Findlater, Saskatchewan's chief medical health officer.

"We actually did have quite a few mosquitoes in some parts of the province, though a lower percentage of them were positive for West Nile virus. Ten cases is strikingly lower than last year," he said.

According to Dr. Findlater, 20 per cent of the mosquito pools tested last year showed infections of West Nile virus. This year, 5 per cent tested positive.

"We're going to try and create models for predicting the disease risk based on the mosquito data chiefly, I think," he said. "I think it will be quite useful to have such two different years, in a way."

Experts say that besides the cool, wet weather this summer -- which they believe kept the mosquito population down -- birds that carry the illness may also be developing immunity to West Nile. If mosquitoes bite birds that have built up an immunity, there's no virus to spread to humans.

In Manitoba, public-health officials are contemplating changes to their surveillance programs, including looking at migrating birds, said Joel Kettner, the province's chief medical health officer.

"Whether we'll add on any surveillance, such as innovative ways of measuring migratory birds bringing in the virus, that's something I'm interested in as one of the factors," Dr. Kettner said.

Public-health officials say that despite two very different West Nile seasons, the public shouldn't assume the threat is over. "The message has to be that even though there was one case this year, we're not sure what's going to happen in future years," said Gerry Predy, chief medical health officer for the Edmonton area.

"We're advising people to continue to take the precautions."

Dr. Findlater said there has been a big increase in the number of Saskatchewan municipalities applying for the $1-million in funding set aside for mosquito control programs. Almost all that money was used up this year.

"On the basis of one cold summer that's had very few cases, we shouldn't dismantle our mosquito-control activities either," he said. "We'll have to figure out the longer-term pattern before we think of doing that."

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