West Nile rates soar in 2007

Erstwhile hopes that the West Nile Virus may have been slowly disappearing in Canada dissipated this season as the country experienced the largest outbreak since the virus' arrival in 2001.

A staggering 2260 human clinical cases were reported to the Public Health Agency of Canada as of Oct. 20, a startling jump from 151 confirmed cases in 2006. That increase was most pronounced in Saskatchewan, where the number of cases rose to 1359 from 19 in 2006, and in Manitoba, where cases rose to 559 from 50 (Table 1).

Manitoba leads the nation in the number of cases (46) of the most severe form of the disease, West Nile Neurological Syndrome, in which permanent neurological damage is caused to the brain. Saskatchewan recorded 40 cases, Alberta 19, British Columbia 8, Quebec 1 and Ontario 1.

City of Winnipeg entomologist Taz Stuart says the outbreak in Manitoba was largely a function of environmental conditions, which “were very suitable for an outbreak of the Culex tarsalas mosquitoes. In May and June, we had a large amount of rain, which produced a habitat for the first generation. The hot weather that followed provided conditions for lots of eggs.”

Stuart also said that increased media attention resulted in a higher number of people being tested, which may have contributed to the increases in Manitoba. But Dr. Michael Drebot, head of Health Canada's Viral Zoonoses Section, says the high incidence of 2007 “stems more from the ecological factors such as climate and the high number of infected Culex tarsalas.”

Stuart and Drebot also indicated that the 2007 increases are not necessarily an indicator of future incidence. While more accurate predictive models are being sought, using accumulated data, none are completed, or foolproof. “We hope to provide these models and possibly, risk indices to be more accurate,” Stuart says. But it's a challenge because “each year is unique in itself.”

“The message is that every season holds the potential for a large epidemic. People need to take precautions even if there are not swarms of mosquitoes around; the few that are around may be infected with the virus,” warns Drebot. “People cannot be lulled into a false sense of security.”