General Overview

• What is West Nile virus?
• How do people get infected with West Nile virus?
• Who is more likely to get sick after being infected with West Nile virus?
• When is the risk greatest?
• What can people do to reduce their risk of West Nile virus infection?
• If a person is infected with West Nile virus, does the person develop life long immunity to future infection by the virus?
• What types of birds carry West Nile virus?
• How do I report dead birds?
• What types of mosquitoes spread West Nile virus?

Also see It's Your Health - West Nile Virus

What is West Nile virus?

West Nile virus belongs to a family of viruses called Flaviviridae. It is spread by mosquitoes that have fed on the blood of infected birds. West Nile virus is closely related to the viruses that cause Dengue fever, Yellow fever and St. Louis encephalitis.

How do people get infected with West Nile virus?

The evidence shows that most people infected with West Nile virus got it from the bite of an infected mosquito. A mosquito becomes infected when it feeds on the blood of a bird that is infected with the virus. The mosquito then becomes capable of passing the virus to people and animals by biting them. There is evidence that some species of female mosquitoes may be able to transfer West Nile virus to their offspring.

In 2002, scientists discovered that people could be infected with West Nile virus in other ways, including blood transfusions and organ/tissue transplants. The risk of getting West Nile virus this way is considered to be quite low. There is also evidence that pregnant women can pass the virus to their unborn babies and that the virus may be passed through breast milk. In addition, laboratory workers who handle infected specimens can get West Nile virus through needle punctures or cuts.

For more information about the transmission of West Nile virus, please refer to the fact sheets: West Nile Virus: Transmission through Blood and West Nile Virus: Pregnancy and Breastfeeding.

There is no evidence to suggest that people can get West Nile virus by touching or kissing someone who is infected, or from being around a health care worker who has treated an infected person. Likewise, there is no evidence to date that the virus can pass from infected animals (i.e., horses, birds, pets, etc.) to people.

Who is more likely to get sick after being infected with West Nile virus?

Evidence shows that many people infected with West Nile virus have mild symptoms, or no symptoms at all. People with weaker immune systems and people with chronic diseases are at greater risk for serious health effects. While the overall risk of serious health effects increases with age, persons of any age or health status can be at risk of serious health effects associated with West Nile virus infection. That is why it is so important to avoid mosquito bites, especially if the virus is active in your neighbourhood, town or region.

For detailed information about the symptoms and health effects of West Nile virus, see the fact sheet: West Nile Virus: Symptoms, Diagnosis and Treatment.

When is the risk greatest?

The risk of becoming infected is greatest during mosquito season. In Canada, this can start as early as mid-April and last until the first hard frost in late September or October.

What can people do to reduce their risk of West Nile virus infection?

Because most people infected with West Nile virus get it from the bite of an infected mosquito, you can protect yourself and your family by minimizing your exposure to mosquitoes and by eliminating mosquito breeding grounds around your home and vacation property.

Wear protective clothing when outdoors, and use insect repellents that contain DEET or other approved ingredients. Insect screens on doors and windows will reduce the chance of mosquitoes entering your home; make sure screens have no holes and fit tightly.

Also, take steps to reduce mosquito populations around your home and vacation property by eliminating places where standing water can collect: dispose of old car tires, empty bird baths twice a week, and empty pool covers regularly. Standing water is prime mosquito breeding ground, and it takes only four days for an egg to grow into a flying - and biting - adult mosquito.

For most Canadians, the risk of West Nile virus infection is low, and the risk of serious health effects from the virus is even lower. However, anyone exposed to mosquitoes in an area where West Nile virus is active is at risk for infection.

For details on ways to reduce risks, please refer to the fact sheet: West Nile Virus: How to Protect Yourself and Your Family.

If a person is infected with West Nile virus, does the person develop life long immunity to future infection by the virus?

We believe most Canadians who have been bitten by an infected mosquito have developed some antibodies to West Nile virus. However, their ability to fight off the virus depends on their overall health, and the ability to fight off the disease does decrease with age.

What types of birds carry West Nile virus?

The virus has been found in more than 150 bird species in North America. Some species may have no obvious signs of illness when infected. Others, such as crows, blue jays, magpies and ravens, get sick more often and can die.

How do I report dead birds?

Reporting dead birds is the first thing you can do to help identify West Nile virus in your community. For information on how to report a dead bird in your area, visit the Canadian Cooperative Wildlife Health Centre's web site.

What types of mosquitoes spread West Nile virus?

There are 74 known species of mosquitoes in Canada. West Nile virus infection has been found in 10 of these, but it is more common in species that feed on birds such as Culex pipiens, Culex restuans and Culex tarsalis. Some species of mosquitoes infected with West Nile virus rarely feed on humans.

Not all of the 74 different species in Canada are found in all parts of the country. The actual numbers of mosquitoes in different areas will vary according to the time of year. Temperature and rainfall can also have a dramatic effect on mosquito populations.

It is estimated that fewer than 1% of mosquitoes in any given area are infected with West Nile virus and overall risk of being bitten by an infected mosquito is low. Although it is possible to be bitten by infected mosquitoes whenever West Nile virus is active, to date most human infections with West Nile virus have occurred during July and August, at a time when infection rates in mosquitoes have reached their peak.

The best way to reduce your risk of exposure to West Nile virus is to avoid being bitten by mosquitoes.

For more information on approved insect repellants and ways to reduce mosquito populations, visit the Pest Management Regulatory Agency Web site and click on Mosquito Control, or call 1-800-267-6315 (toll-free in Canada).

Check with your local public health unit if you have questions about the types of mosquitoes in your area.