Skip to main content

West Nile Virus (WNV)

West Nile Virus (WNV)  is considered to be the most widely distributed vector-borne disease in North America, and has now been detected in B.C.

West Nile virus



West Nile virus is only one of many diseases around the world that are spread by mosquitoes.  Although most people who get infected will not feel any symptoms, the disease can be serious for one person out of about every 150 who are infected. Fortunately, it is fairly easy to lower your risk of infection, by avoiding mosquito bites.

West Nile virus brochure [PDF, 1087KB]

Information for Health Professionals

West Nile Virus (WNV) is an infection of birds spread by mosquitoes which was first identified in Africa in 1937. Mosquitoes pick up the virus by biting an infected bird, and the virus is then transmitted to humans through the bite of an infected mosquito.

Where is WNV found?

  • Until 1999, WNV was commonly found in Africa, Eastern Europe, West Asia and the northern Mediterranean area: there have been outbreaks of WNV in Egypt, Israel, India, France, Romania and the Czech Republic.
  • In 1999, the first North American cases occurred in New York City. It is not known how the virus was introduced into the New York area. 
  • Since then, it has established itself in nearly all of the United States (lower 48 States) and much of Canada.
  • In 2009 the first local human cases of WNV were reported in B.C. In the same year WNV was detected in mosquitoes, and in subsequent years it was also found in Corvids and Horses.

How serious is WNV?

  • Most infected people will never know they’ve had the disease. Research indicates that only about one out of every five people who are bitten by a mosquito and infected with WNV will develop symptoms.
  • Most people who do develop symptoms will experience varying degrees of an illness known as West Nile Non-neurological Syndrome. Even non-neurological forms of the disease may cause lasting health effects, including muscle weakness, memory problems and fatigue which may continue for months. 
  • In less than one per cent of infected individuals (about one in 150), WNV can cause severe illness resulting in hospitalization. This includes swelling of the brain (encephalitis), inflammation of the lining of the brain (meningitis) or polio-like paralysis.
  • The fatality rate is about 10 per cent.
Other species affected by WNV
  • Mosquitoes 
    Mosquitoes are the vector for WNV. In BC the two primary competent vector species are Culex tarsalis, and Culex pipiens. These mosquitoes bite an infected bird, become infective, and then bite a mammal like a human or horse. The virus is transmitted through the mosquitoes saliva.
  • Birds 
    Birds are a reservoir host for WNV. Many species are susceptible to WNV, however Corvids (Crows, Ravens and Jays) most often die from the infection. If you notice clusters of dead birds you may notify the BC Interagency Wild Bird Mortality Investigation by calling 1-866-431-BIRD(2473). Please refer to the disposal of dead birds document for safe handling practices.
  • Horses 
    Horses are incidental hosts for WNV (similar to humans). WNV can cause encephalitis in horses, and should be taken very seriously. Symptoms in sick horses can include listlessness, a change in demeanor becoming less active and isolated, reduced appetite, inability to swallow, drooping lips, muscle twitching, a lack of co-ordination, weakness in the limbs, partial paralysis or an inability to get up. A veterinarian should examine the symptomatic or infected horse (Alberta Health, 2015). The good news is that there is a vaccine that can prevent WNV infection in horses. Horse owners should contact their veterinarian for information about the vaccines available and recommendations.
The symptoms of West Nile non-neurological syndrome include some or all of the following: 
  • fever
  • headache
  • muscle weakness
  • muscle and/or joint aches
  • malaise
  • rash
  • sensitivity to light
West Nile neurological syndrome includes these, and some or all of the following:
  • severe headache
  • stiff neck
  • meningitis
  • encephalitis
  • paralysis

Symptoms begin approximately three to 14 days after being bitten by an infected mosquito. Recovery from symptoms may occur in about a week with West Nile non-neuorolgoical syndrome and may take a year or more for West Nile neurological syndrome.

  • People usually become infected with the virus through a mosquito bite.
  • WNV is not spread through person-to-person contact such as touching, coughing, sneezing or drinking from a shared cup.
  • Although the virus is not known to be transmitted to humans from animals, people should avoid handling dead animals or birds with their bare hands. 
  • Less commonly, WNV can be transmitted through blood transfusion and organ transplants if the donor was recently infected with WNV.
  • WNV can also be transmitted from a mother to her unborn child or through breast milk, but these events are rare. The benefits of breastfeeding outweigh the risk from WNV.

How likely am I to get sick with WNV from one mosquito bite? 

In areas where mosquitoes do carry the virus, usually only a small number of mosquitoes will be infected. Most mosquitoes that bite humans are not able to carry WNV, but there is no easy way to tell the difference between ones that can and ones that can’t so it is important to prevent any mosquito bite. Culex tarsalis and Culex pipiens  are the mosquito species most likely to transmit WNV between birds, humans, and other animals in BC (they are competent vectors).

Who is at risk for WNV? 
Everyone who is outside during the summer months gardening, golfing, walking, camping etc. should take precautions to avoid mosquito bites. While anyone can be infected with WNV, the chances of having a severe illness are greater as you get older, even if you are healthy. You may also be at greater risk if you have a weakened immune system.

Presently, there are no specific medications that can cure illness from WNV. Rest, fluids and sometimes care in hospital is the only treatment so far.

Is there a vaccine against WNV?
No, a human vaccine for WNV doesn't exist to protect people. A vaccine is being used for horses and has been used experimentally in other animals.
How can I protect myself and my family when we are outdoors? 

  • Avoid being out from dusk to dawn if you can because mosquitoes that carry WNV tend to be most active at these times.
  • Wear protective clothing, especially outside in the early evening and at dawn. This includes long-sleeve shirts or jackets and long pants that mosquitoes cannot bite through. Tuck your pants into socks for extra protection.
  • Avoid dark coloured clothing as it can attract mosquitoes.
  • Use mosquito netting for babies and toddlers in cribs and strollers.
  • Use mosquito repellent. Putting on personal insect repellents that are federally registered, such as those that contain DEET (N,N-Dethyl-m-toluamide) or Icaridin is an effective way to protect yourself from mosquito bites.
  • There are also many repellents that have been shown NOT to protect against mosquito bites, including bug zappers, devices that give off sound waves and Citrosa plants.
  • Tips when using insect repellent:
    • Apply repellent sparingly on exposed skin or on top of clothing. Do not use under clothing.
    • The repellent doesn’t have to be applied heavily to work- a light coating will do.
    • Use your hands to rub the repellent over your skin after you spray it on. Research shows that mosquitoes will find and bite even very small sections of skin missed by the spray.
    • Do not use repellent on open wounds, or if skin is irritated or sunburned.
    • Do not get repellent in your eyes – if this happens, rinse with water right away.
    • Avoid breathing mist from spray-type repellent. Always apply in a well-ventilated area; never apply spray repellent inside a tent.
    • Do not use near food.
    • Read the manufacturer’s instructions on the label carefully, including restrictions for use on young children and maximum number of applications per day.

How can I help prevent WNV in my neighbourhood? 

Unlike birds or other insects, most mosquitoes do not fly very far and tend to stay close to their breeding sites. You are most likely to be bitten by a mosquito from your own backyard! There are many simple things you can do to reduce the number of mosquitoes in and around your property. See the WNV brochure for more tips. 

  • Clean up areas where mosquitoes like to breed. It doesn’t take much time (generally one week), or water for mosquitoes to develop from eggs into adults.
  • Take a look around your home and get rid of mosquito-friendly places that would make good breeding sites or resting places for mosquitoes.
    • Remove any type of standing water at least once a week.
    • Clean up and empty containers that collect water such as old tires, flower pots, wheelbarrows, barrels, tin cans or even small containers like bottle tops that are outdoors.
    • Drill holes in the bottom of used containers so water can’t collect.
    • Change water in bird baths at least once a week.
    • If you have a swimming pool, immediately remove water that collects on pool covers and make sure the pool’s pump is circulating
    • Turn over wading pools when not in use.
  • Check leaves and drains: don’t let things pile up.
    • Clear leaves and twigs from eaves troughs, storm and roof gutters throughout the summer.
    • Check flat roofs frequently for standing water.
    • Make sure drains and drainage ditches are not clogged.
  • Stop mosquitoes from entering your home
    • Check windows and door screens for holes and make sure they fit snugly into the frames, so mosquitoes cannot get in.

SOURCE: West Nile Virus (WNV) ( )
Page printed: . Unofficial document if printed. Please refer to SOURCE for latest information.

Copyright © BC Centre for Disease Control. All Rights Reserved.

    Copyright © 2016 Provincial Health Services Authority.