West Nile Virus


West Nile Virus is spread by the bite of a mosquito that has bitten an infected bird. The virus can infect people, horses, many types of birds, and some other animals. Most people who become infected with West Nile Virus don't experience any symptoms.

  • Approximately 20% of infected people will experience mild to severe flu-like symptoms.
  • Approximately one in 150 people infected with West Nile Virus experiences severe encephalitis, which can sometimes be fatal.

Mosquito fast facts

  • Mosquitoes are most active at dusk and dawn.
  • Over 50 species of mosquitoes live in B.C., two of which (Culex pipiens and Culex tarsalis) will likely be the most important carriers for West Nile Virus.
  • In areas known to have West Nile Virus, up to one in 100 mosquitoes (depending on the species) carries the virus.
  • Repellents containing DEET or other approved repellents can help prevent mosquito bites.
  • Mosquitoes have four distinct life stages.
  • Only female mosquitoes bite.
  • Mosquitoes can lay eggs in a very small amount of standing water.

Mosquito lifecycle

The mosquito lifecycle has four distinct stages: egg, larva, pupa and adult. Each stage has its own unique needs and functions. Three of the four life stages take place in water. A mosquito can mature from egg to adult in as little as a week, which is why getting rid of stagnant water around your home is important.

  • Egg stage: Females lay their eggs in water, and most will hatch within 48 hours. Each female can lay up to 300 eggs.
  • Larval stage: These are commonly referred to as wrigglers. Wrigglers usually can be found hanging upside down near the surface of the water. Larvae spend most of their time and energy feeding on microorganisms and growing. After a series of molts they become pupae. Most mosquito control efforts focus on this stage.
  • Pupa stage: This lifestage also lives in water but does not feed as in the larval stage. Often referred to as tumblers, they have two breathing tubes on their back, float near the surface and tumble through the water when they sense danger. The pupae spend their energy in metamorphosis, eventually emerging as adult mosquitoes.
  • Adult: As adults, mosquitoes get around by flying or being carried on strong winds such as storm fronts. Adult mosquitoes have a range of 1-5 kilometres from their breeding site. Both males and females feed on plant nectar until it is time to produce eggs. At this time the female needs a blood meal to supply protein to the eggs, drawing blood for each batch she produces. Certain factors affect biting: carbon dioxide, temperature, moisture, smell, colour and movement. Most biting occurs between dusk and dawn.

Protecting you and your family

Reduce your risk. Remember the four "D"s:

  1. DRAIN your property of standing water
  2. DUSK/DAWN: Take extra care
  3. DRESS appropriately
  4. DEFEND against mosquito bites by using mosquito repellents wisely

Drain: It doesn't take much time, or water, for mosquitoes to develop from eggs into adults. Therefore, anything that can hold water is a potential development site. To destroy common mosquito breeding sites around your home, consistent yard maintenance is the best defense.

  • Empty saucers under flower pots.
  • Unclog rain gutters.
  • Remove used tires and other debris where rainwater may collect.
  • Make sure your lawn is mowed to decrease the area where mosquitoes can rest during the hot part of the day.
  • Change water in bird baths and pet water/ food dishes regularly (1-2 times a week during periods of warm weather).
  • Empty wading pools when not in use; drain tarps, pool covers, and trampolines that fill with rainwater.
  • Aerate or keep fish in ornamental ponds.
  • Install screens on your doors and windows to keep mosquitoes out.
  • Use fine mesh to cover rain barrels and containers that cannot be dumped.
  • If you are aware of a standing water problem in your neighbourhood call your local city by-law office.

Dusk/Down: Mosquitoes are most active at dusk and dawn.

  • Minimize outdoor activities at dusk/dawn.
  • Take appropriate steps to protect yourself when outdoors during dusk and dawn.
  • Avoid or protect yourself, in or near woodland, forests, salt marshes and coastal rock pools and any other places where mosquitoes may bite during the day.


  • Wear baggy, long-sleeved shirts and pants, shoes and socks when outdoors, especially at dawn and dusk.
  • Wear light-coloured clothing since dark colours attract mosquitoes.


  • Use mosquito repellent. If living or traveling in an area known or suspected to have WNv, use an insect repellent to help protect against mosquito bites. Use only insect repellents that contain DEET or other approved ingredients.


These "floodwater" mosquitoes are typically nuisance related but are not good at picking up and transmitting West Nile Virus, therefore Fraser Health does not recommend management of these populations for human health protection. Some communities in British Columbia do have nuisance mosquito control programs in place, mostly to reduce the annoyance to area residents.

The mosquitoes most capable of transmitting WNV, Culex pipiens and Culex tarsalis are generally called the "vector" mosquitoes. These mosquitoes like to lay their eggs in smaller bodies of stagnant water. Should flooding contribute to the creation of pools of water, and these pools remain for extended periods, there is potential for vector mosquitoes to find and use these pools for breeding. Testing of mosquito pools since 2003 have yet to find the presence of WNV. At this time, Fraser Health does not recommend local governments apply pesticides to specifically control WNV vector mosquitoes. Some local governments have chosen to continue mosquito nuisance control programs.

Protect yourself from mosquito bites that can transmit West Nile Virus by using mosquito repellents, wearing proper clothing, and preventing the entry of mosquitos into your home by installing pest screens at all windows and doors.

In accordance to Integrated Pest Management techniques, the removal of mosquito breeding grounds is a good way to reduce mosquito populations. Bacteria-based larvicides such as Bti (Aqua Bac) are available for sale and give homeowners another way to reduce mosquitoes on their property. This may be a good option for concerned homeowners to address isolated pools of water on their fields left by flooding, especially if the receding water remains longer than a week or two.

Insect repellent tips 

  • Follow instructions on label and apply sparingly.
  • Repeat applications as necessary.
  • Do not apply to broken or irritated skin.
  • Wash with soap and water when you return indoors, or protection is no longer required.
  • Avoid the eyes, rinse thoroughly if contact occurs.
  • Avoid inhaling the spray.
  • Do not use in enclosed areas or near food.
  • Check for sensitivity on a small patch of skin 24 hours before protection is required.
  • Keep insect repellent containers out of reach of children.
  • Wipe repellent on to children, don't spray.
  • There is no indication of a hazard to the unborn or nursing child associated with the use of DEET by pregnant or lactating women.

Using mosquito repellent 

Take extra care when outside at dusk or dawn. If living or travelling in an area known or suspected to have WNV or any other mosquito-borne disease, use an insect repellent to help protect against mosquito bites. Use only insect repellents that contain DEET or other approved ingredients:

  • A product containing 23.8% DEET provides an average of five hours of protection from mosquito bites.
  • A product containing 20% DEET provides almost four hours of protection.
  • A product with 6.65% DEET provides almost two hours of protection.
  • A product with 4.75% DEET provides roughly one and a half hours of protection.

When used as directed, DEET is a safe and very effective ingredient for repelling mosquitoes and preventing bites. There are alternative products available, but most provide minimal protection and should be used with caution. View Health Canada's statement paper on travel health.

Alternative repellents containing:

  • P-mentane-3,8-diol
    • A derivative of oil of Lemon Eucalyptus can provide protection up to two hours against mosquitoes and five hours against black flies.
    • Not for usage on children under three years of age.
    • Maximum application of up to two times a day.
  • Soybean Oil at 2%
    • Can offer about one to four hours of protection against mosquitoes and up to eight hours against black flies.
    • No age restrictions or limitations on frequency of application.
  • Citronella
    • Can offer about 30 minutes to two hours of protection against mosquitoes.
    • Lacks long term safety data.
    • Not for children.
    • Recommend to not use this product when possible.

DEET and sunscreen 

In general, the recommendations for sunscreen and repellents are opposite. Apply sparingly for repellents and liberally and often for sunscreen. DEET and sunscreen combination products are not recommended, and in fact, the effectiveness of DEET is reduced 34% when the two are used together. If the two are needed together apply the sunscreen first and allow 20 minutes to soak in then apply DEET.

DEET and children 

Canada's Pest Management Regulatory Agency (PMRA) allows the following:

  • Up to 30% DEET for adults and children over 12 years.
  • Up to 10% DEET for children two to 12 years up to three times per day.
  • Up to 10% DEET for children six months to two years once per day.
  • Do not use personal insect repellents containing DEET on infants less than six months of age. Use a mosquito net when the child is outdoors in a crib, playpen or stroller.

*Note: For children, alternative methods of personal protection should be considered as first lines of defense. Examples of alternatives are mosquito nets, protective clothing, or avoidance.


There is no specific treatment or cure for a WNV infection. People with West Nile Non-Neurological Syndrome (WNNon-NS) usually recover completely without treatment. People with West Nile Neurological Syndrome (WNNS) usually require hospitalization.

How to tell if a mosquito bite is serious

Most mosquito bites cause itching and minor irritation. A small percentage can lead to serious infections, such as West Nile Virus. If you have any of the following symptoms, seek medical help right away:

  • Fever
  • Muscle weakness
  • Stiff neck
  • Confusion
  • Severe headache
  • Sudden sensitivity to light
  • Extreme swelling or infection at the site of the mosquito bite

If you think you may be ill from a WNv Infection, see your doctor or call HealthLink BC at 8-1-1.

How it spreads

West Nile Virus is primarily spread through the bite of an infected mosquito. Mosquitoes become infected when they bite infected birds. The virus eventually finds its way into the mosquito's salivary glands and during its next meal the virus can be passed to humans and animals, where it could make a person sick.

There are also some less common ways to become infected with West Nile Virus, although very few cases are spread in the following ways:

  • Organ TransplantWest Nile Virus can be passed through transplanted organs.
  • Blood Donor: All blood donations are tested by Canadian Blood Services for West Nile Virus, which has significantly decreased the risk of West Nile Virus being passed through blood transfusion. If you are having surgery when mosquitoes are active in B.C., and you might need a blood transfusion, discuss the benefits and risks of using blood with your doctor. Visit Canadian Blood Services for more information.
  • Breast FeedingWest Nile Virus has been found in breast milk of infected mothers, but does not seem to cause any significant illness in breastfed infants. If you are breast feeding, protect yourself from mosquito bites.
  • Mother to unborn baby: This has occurred in a few cases and at risk mothers should seek the advice of their doctor. If you are pregnant protect yourself from mosquito bites.

West Nile Virus and birds

  • Infections in birds: Most species of birds can carry West Nile Virus. They become infected from the bite of a mosquito that has previously bitten an infected bird. Many species of birds carry the virus for a few days and recover. Birds of the corvid family (crows, ravens, jays) are particularly likely to get sick and will probably die from a West Nile Virus infection within a few days.
  • Report dead birds: A large number of dead corvids (crows, ravens, and jays) in the same place and time, may be an early indication of the arrival of WNv. While routine collection and testing of dead corvids has been suspended in Fraser Health, the public is still encouraged to report dead corvid sightings. Unusual clusters of dead corvid sightings will be investigated .

To report the sighting of a dead corvid, please notify the BC Interagency Wild Bird Mortality Investigation Protocol by calling 1-866-431-BIRD(2473).

Although many bird species can be infected with West Nile Virus, members of the crow family (crows, ravens, magpies, jays) have a high death rate when infected. For this reason monitoring of number of dead crows for West Nile Virus is can be an effective way of determining when the virus moves into a new area.

Surveillance and dead bird reporting

Report stagnant water problems in your neighborhood that may be a mosquito breeding site to your local by-law office.

Since 2013, we have suspended our program of collection and testing of corvid bird species for West Nile Virus. However, residents are still encouraged to report sightings of dead corvids, or clusters of dead birds by notifying the BC Interagency Wild Bird Mortality Investigation Protocol by calling 1-866-431-BIRD(2473). 

Frequently Asked Questions

I Need To...