In 2002, Canada had its first confirmed human cases of West Nile virus in parts of Quebec and Ontario. That year, 426 Canadians became ill after being infected with the virus. By 2003, West Nile virus had spread to 7 Canadian provinces: Nova Scotia, New Brunswick, Quebec, Ontario, Manitoba, Saskatchewan, and Alberta. 10 deaths were linked to West Nile virus, and more than 1300 Canadians became ill after being infected. In 2004, there were no deaths linked to West Nile virus in Canada, with 25 human clinical cases. Among those 25, there were 6 confirmed cases of West Nile fever and 9 confirmed cases of West Nile virus neurological syndromes. In 2013, there were 115 reported clinical cases.
Experts are not sure how the virus is able to spread so quickly, but its continuing spread indicates that it is probably here to stay. However, Health Canada is working with others to help reduce the risk of exposure to the West Nile virus through education, surveillance, prevention, and response.
Most people infected with West Nile virus show mild flu-like symptoms or no symptoms at all. Less than 1% of those infected with the virus develop severe illness, but for some, especially those who are ill or have a weak immune system, it can result in neurological (brain and nerve) effects, paralysis, or death. The neurological effects may be permanent in some people. Scientists do not know why some people recover quickly while others face long-term health problems.
Of the 74 known species of mosquitoes in Canada, West Nile virus has been found in 10 species. Not all species are found in all parts of the country. In a given area, it is estimated that less than 1% of mosquitoes carry the West Nile virus.
Scientists don't know for sure which mosquito species actually transmit West Nile to people through their bites. Nor is anyone certain how the virus gets through the winter - it could be carried inside birds or sit waiting in hibernating mosquitoes. The uncertainty means that no one can tell health officials which mosquito larvae to kill in order to avoid a repeat of the previous summer.
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