West Nile proves hard to swat

As temperatures start to rise and summer finally seems more of a possibility than a theoretical concept, West Nile watchers are turning their eyes to British Columbia.

As temperatures start to rise and summer finally seems more of a possibility than a theoretical concept, West Nile watchers are turning their eyes to British Columbia.The pesky virus from far-off Africa belted Ontario two years ago, then rampaged cross the Prairies in 2003. This year, many believe, British Columbia will feel its sting.”I think if I had to guess, I would say there’s a really good chance the virus will move into B.C. this year,” says Dr. Murray Fyfe, who heads the West Nile surveillance program at the British Columbia Centre for Disease Control.”It could jump over the mountains, like it has done in the . . . southern U.S.,” Dr. Fyfe says. “Or the other possibility is that it will be brought north along the coast with migrating birds from California.”Less certain is how the famously erratic virus will behave in other parts of the country where it has already taken up residence.”I’m not aware that anybody’s going to be better able to predict what’s going to happen this year, from what we know of last year, other than we can make a good guess that it’s going to go West,” says Dr. Donald Low, chief microbiologist at Toronto’s Mount Sinai Hospital.For British Columbia, Dr. Fyfe hopes the virus will follow what appears to be a pattern that has emerged since its surprise arrival in North America in 1999. In most cases, West Nile has made an initial, limited foray one year, killing birds and maybe causing limited human illness, but only really making a big impact on human health in its second year in a locale.But West Nile’s most predictable feature is its unpredictability. Dr. Fyfe needs only to look at Alberta to see that if the spread of the virus follows a pattern, that pattern hasn’t yet come into focus.”Last year was their first year for having West Nile virus and they had over 200 cases,” Dr. Fyfe admitted.Alberta had 275 to be exact, making it the second-hardest-hit province in 2003. Saskatchewan was far and away the most badly affected, with 935 cases and six deaths.Ten deaths were wholly or partially attributed to West Nile in Canada last year, with a total of around 1,480 human cases of West Nile infection.Newfoundland and P.E.I. had no cases, and the people from Nova Scotia, New Brunswick, B.C. and Yukon who caught the virus were believed to have been infected during travels outside those areas.Experts believe a type of mosquito prevalent in Saskatchewan holds a key to why last year’s outbreak there was so bad. Culex tarsalis readily bites both birds and humans, easily passing the virus from one to another.The chief medical officer of health for Saskatchewan, Ross Findlater, is hoping another possible pattern will hold true for his jurisdiction this summer.In many places, West Nile hasn’t hit the same location hard for two years running. Ontario saw minor West Nile activity last year, as did Michigan and Illinois. All three were badly affected in 2002.Dr. Findlater is hoping West Nile will cut his province a break.”We wish British Columbia the best of luck this summer,” Dr. Findlater says, though he notes Saskatchewan public-health authorities know they have to prepare for the worst-case scenario.”Probably the extent to which we’re going to have problems here depends to a large extent on the weather, which doesn’t seem terribly in our control.”The number of hot days in a summer seems to play a role in the amount of activity an area will see. But scientists still don’t have all the answers.In areas where West Nile has been endemic for decades, the virus causes little illness, probably because people build up immunity to it.Experts here know that the longer the virus is present in Canada, the more immunity will develop within human and bird populations. But they think it’s too soon for that to explain why Ontario got off so lightly last year.”I’m not sure that those mechanisms of immune resident birds and people with antibody kick in so early,” says Dr. Harvey Artsob, Health Canada’s chief of zoonotic diseases.”If we knew the relative importance of all these factors, it would be easier for us to say ‘We think this is going to be a quiet year.’ . . . But we still don’t know all those factors.”(Source: The Globe Health News, April 2004)