Government Partners Track Spread of West Nile Virus

West Nile virus is closely related to the viruses that cause Dengue fever, Yellow fever and St. Louis encephalitis. It is spread by mosquitoes that have become infected by feeding on the blood of infected birds. The evidence shows that most people infected with West Nile virus get it from the bite of an infected mosquito.

The virus was first isolated in 1937 in the West Nile district of Uganda, but the first recorded outbreak in North America happened in New York City in 1999.

During 2002, more than 4,000 people in North America became ill after being infected with West Nile virus. In 2003, the number grew to 10,000. These are the largest outbreaks of West Nile virus infection ever recorded.

Health Canada's scientists, in collaboration with provincial and territorial colleagues and other partners, monitor for the presence of West Nile virus across Canada. This provides researchers and public health officials with important information about West Nile virus such as:

• Whether the virus is turning up again in an area where human cases were previously recorded;
• Whether the virus has spread to new areas;
• The geographical distribution of the virus as it spreads;
• New ways the virus can be spread, ex.: through blood transfusions;
• Who may be at greater risk for serious health effects.

How is Surveillance Done?

Experience from past outbreaks has shown that certain species of birds - crows, jays, magpies and ravens - suffer high fatality rates due to West Nile virus. This makes them the best indicator for determining whether people in particular areas are at risk.

Previously, dead birds had to be tested centrally at Health Canada's National Microbiology Laboratory in Winnipeg. For the first time in 2003, a new testing system called the VecTest allowed testing to take place locally, through the Canadian Cooperative Wildlife Health Centres and provincial laboratories.

In 2004, the local testing of dead birds using VecTest will continue to be the primary means of monitoring West Nile virus. In cases where humans or horses display symptoms of the virus, they will also be tested. And in areas where there is evidence of West Nile virus, mosquitoes in the region will be tested and monitored.