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.

Where did West Nile virus come from?

The virus was first isolated in 1937 in the West Nile district of Uganda. Since then, there have been outbreaks in Egypt, Israel, South Africa, and in parts of Europe, Asia and North America. The first recorded outbreak in North America happened in New York City in 1999.

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. About two weeks later, the mosquito becomes capable of passing the virus to people and animals by biting them. There is some evidence that female mosquitoes may be able to transfer WN virus to their offspring, but researchers are still looking into this possibility.

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 WN 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 WN virus through needle punctures or cuts.

How is West Nile virus transmitted?

There is no evidence to suggest that people can get WN 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 (horses, pets, etc.) to people. The disease is transmitted largely by infected mosquitoes.

What are the symptoms of West Nile Virus?

Most people who contract West Nile virus will show no symptoms and will have no type of illness. It is estimated that 20% of the people who become infected will develop some or all of the following flu like symptoms:

  • mild fever
  • headache
  • body aches, occasionally with a skin rash on the trunk of the body
  • swollen lymph glands
It is estimated that 1 in 150 persons infected with the West Nile virus will develop a more severe form of the disease. The symptoms of severe infection (West Nile encephalitis or meningitis) are:
  • headache
  • high fever
  • neck stiffness
  • stupor
  • disorientation
  • coma
  • tremors
  • convulsions
  • muscle weakness
  • paralysis

What is the incubation period in humans?( the time from infection to onset of disease symptoms)

Usually 3 to 14 days.

How long do symptoms last?

Symptoms of mild disease will generally last a few days. Symptoms of severe disease may last several weeks, although neurological effects may be permanent.

Who is at risk for West Nile Virus?

  • People over 40 years of age
  • People with chronic disease: diabetes, cancer, heart disease, people with weakened immune systems
  • anyone who is exposed to mosquitoes in an area where West Nile virus has been detected in birds, mosquitoes or other
  • animals.

Is there a vaccine against West Nile Encephalitis?

No, but several companies are working towards developing a vaccine.

Is DEET safe for pregnant or nursing women?

There is no reported adverse effects following use of repellents containing DEET by pregnant or breast feeding women.

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 WN virus infection?

If WN virus is found in birds, mosquitoes, horses or people in your area, you can take steps to reduce the risk of getting mosquito bites by :

  • Wearing protective clothing when outdoors which include long-sleeved shirts and full length trousers and a hat - two layers of clothing is recommended. Light coloured clothing is best because mosquitoes tend to be more attracted to dark colours.
  • Trying to avoid spending time outdoors at dawn and at dusk when mosquitoes are most active.
  • Making sure that door and window screens fit tightly and have no holes that may allow mosquitoes indoors.
  • Using insect repellents that contain DEET or other approved ingredients. Products containing no more than 30% concentration of DEET will normally provide adults with sufficient protection. Studies have shown that products with lower concentrations of DEET are just as effective as the high concentration products, but they remain effective for shorter periods of time:
  1. 30% DEET will provide 6.5 hours of protection
  2. 15% DEET will provide 5 hours of protection
  3. 10% DEET will provide approximately 3 hours of protection and
  4. 5% DEET will provide 2 hours of protection.

Also, take steps to reduce mosquito populations around your home and property. For example, reduce standing water around your home e.g., dispose of old car tires, empty bird baths twice a week, and empty pool covers regularly.

At the Eastern Ontario Handgun Club, we willing be using BTI in pools of standing water on the range property. Larvacides are pesticides that target insect larvae, or the immature stage of adult mosquitoes. The use of biological larvacides, such as Bti (Bacillus thuringensis israeliensis) is preferred over chemical pesticides (e.g. Methoprene) given the concerns with using chemicals in our environment. Bti has not been shown to produce any ill effects in humans, does not result in chronic toxicity and there are no indications that Bti causes reproductive effects or birth defects in mammals. The bacterium contained in Bti occurs naturally in the soil and is harmless to mammals. It is toxic to mosquito and black fly larvae only and does not affect other insects, honeybees, fish, birds or mammals. Given that we have 40 acres out of the 26,000 in the Larose Forest, don't be shocked if you see a mosquito at the range.

For most Canadians, the risk of WN 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 WN virus has been detected is at risk for infection. Anyone who becomes infected is at risk for serious health effects.

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.

Where was WN virus activity reported in North America during 2002?

There were reports of WN virus in more than 40 states in the United States last year. The so-called hot spots, with the highest numbers of confirmed human cases, were Illinois, Michigan, Ohio, Louisiana and Indiana. In Canada, WN virus has been detected in five provinces (Nova Scotia, Quebec, Ontario, Manitoba, Saskatchewan), including confirmed cases of human infection in Ontario and Quebec.

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

For more details, including case counts and maps, visit the Centers for Disease Control and Prevention at West Nile Virus Website For details, see Health Canada's West Nile Virus Monitor.

This page was updated on 5/3/2007 2