Over the last four years West Nile virus has quickly spread west across North America, and Yukon officials are now preparing for its possible appearance in the territory.

The Yukon's chief medical health officer, Dr. Bryce Larke, will be one of the main people helping to watch for any signs of West Nile, which could be carried here by infected birds migrating north to their breeding grounds.

Larke helped identify a new virus while working in a virology lab early in his medical career, and monitoring the spread of West Nile is more than another routine professional duty for him.

For more than 40 years he has been tracking the course of various viral outbreaks, and -- based on this experience -- he is not automatically assuming that any Yukoners will ever suffer from the sometimes deadly effects of West Nile.

"One of the lessons that we have learned from things like the influenza virus is that the only thing that is predictable about viruses is their unpredictability," he says.

West Nile is by far the largest outbreak of mosquito-borne virus infection in the western hemisphere, and experts have been surprised by its rapid spread across this continent. To date more than 200 North Americans have died because of the virus and dozens of different animal species have been infected with it.

With West Nile constantly in the headlines, there is understandably concern about its arrival north of 60. But Larke points to the example of the snowshoe hare virus, a strain of the California encephalitis virus. It was identified in the Yukon in the 1970s, but no Yukoners have ever been reported sick with the disease.

"The fact that a virus exists in nature does not necessarily mean that it will spill over into the human population," he says.

A Vancouver virologist, Dr. Donald McLean, studied the snowshoe hare virus in the Yukon for several summers. He also trained Dr. Larke in virology back in the 1960s, when they were both working at the Hospital for Sick Children in Toronto.

While Larke was doing his residency there, he cared for a child with paralytic polio, one of the last cases of this disease to afflict a patient in Ontario. After McLean identified the polio virus in the hospital's virology lab, Larke asked to watch how the procedure was done.

"I wanted to see how you grow viruses. I had never taken any virology in medical school," he said.

This display of interest sparked McLean to ask whether Larke might want to spend time in the virology lab, working towards a doctorate in clinical science.

While working on his degree, Larke had the chance to help hunt down a new virus which had led to a child's death a few years earlier. McLean thought the unknown virus was probably carried by ticks, so he and Larke went looking for possible carriers.

While hunting for infected small mammals in the child's neighbourhood in northern Ontario, Larke made the crucial shot which led to final identification of the Powassan virus. "I was the boy that had the gun that shot the squirrel that had the tick that had the virus," says Larke.

Larke says that conditions in virology labs were much less sophisticated in those days than they are now, and they had to work very carefully while studying these potentially deadly viruses.

The West Nile virus was one of the many viruses that he had the chance to culture and look at under a microscope. "West Nile forms lovely plaques," he says.

There have been outbreaks of West Nile in several different African and European countries since the virus was first identified in Uganda in 1937. But the North American outbreak has been the most severe of them all, with more species of animals infected and more types of mosquitoes spreading the virus.

"To date, a total of 36 species of mosquitoes are capable of being infected, and we do have some of them here," says Larke.

But Larke says that even if West Nile virus does make its way this far north, many different factors would have to line up just right before a human would be infected.

"Clearly we have a huge bird population that migrates here. But you have to have weather favorable to breeding large numbers of these mosquitoes at the right time and the birds have to be here at the right time."

"You have to have the right temperatures and the right mosquitoes and birds in the right numbers. And the right number of people in contact with those mosquitoes," he says.

Also, a healthy person could be bitten by an infected mosquito and never know it, as only a small percentage of the people infected with the virus become seriously ill. In the United States, the median age of the people who died from the disease in 2002 was 78 years.

When birds start migrating north in a few months, Yukon wildlife officials will be watching for any evidence of infected birds. In the meantime, Larke suggests that Yukoners use common sense when outdoors during mosquito season, using mosquito repellant and covering up so that little bare skin is exposed.

"You have to keep it in the perspective of other human health issues. Where is the risk relative to other health problems that we face in an ongoing way?"

More information on West Nile can be found through Health Canada at www.hc-sc.gc.ca/english/iyh/diseases/west_nile.html or through the Centers for Disease Control at www.cdc.gov/ncidod/dvbid/westnile.