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West Nile Virus (WNV)

West Nile Virus (WNV)  is considered to be the most widely distributed vector-borne disease in North America.

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 have any symptoms, the disease can be serious for one person out of about every 150 who are infected. Fortunately, you can lower your risk of infection by avoiding mosquito bites.

West Nile virus brochure [PDF, 1087KB]

Information for Health Professionals

West Nile Virus (WNV)is most commonly transmitted to humans by mosquitoes. Mosquitoes pick up the virus by biting an infected bird, and humans can become infected through the bite of an infected mosquito.

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 pass on the virus when they bite another bird or a mammal, like a human or horse. The virus is transmitted through mosquito saliva.
  • Birds 
    Birds are the main reservoir host for WNV. Many avian 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). When disposing of a dead bird, please follow safe handling practices
  • Horses 
    Horses are incidental hosts for WNV (similar to humans), meaning that they cannot pass the virus on to other animals. Many infections in horses are asymptomatic, however WNV can cause encephalitis and should be taken very seriously. Symptoms 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. There is a vaccine that can prevent WNV infection in horses. Horse owners should consult their veterinarian for information about available vaccines and recommendations.
  • Humans 
    Humans are incidental hosts for WNV, as the virus is not transmitted from person to person. Most infected humans will not develop symptoms but in those who do, WNV can cause a wide range of symptoms.

Most infected people will never know they have had the disease. Research indicates that only about one out of every five people who are infected with WNV develop symptoms.

West Nile non-neurological syndrome 

Most people who 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. 

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 

In less than one per cent of infected individuals (about one in 150), WNV can cause severe illness resulting in hospitalization. This is called West Nile neurological syndrome and includes swelling of the brain (encephalitis), inflammation of the lining of the brain (meningitis) or paralysis. For people who develop West Nile neurological syndrome, about 10 per cent die.

West Nile neurological syndrome includes some or all of the following symptoms:

  • severe headache
  • stiff neck
  • seizures
  • confusion
  • loss of consciousness
  • 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-neuorological 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.

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 are the only treatments available.

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. Check Health Canada for the most up to date recommendations.

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 does not 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.
  • Clear leaves and drains: don't let things pile up.
    • Clear leaves and twigs from eavestroughs, 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.

Where is WNV found?

  • In BC, WNV has been detected in the eastern Fraser Valley, the Okanagan and the southern Kootenays. The first cases in B.C. occurred in 2009. 
  • In 1999, the first North American cases were detected in New York City. Since then, it has established itself in the continental United States and provinces of Canada. 
  • WNV is also found in South America, Africa, Europe, Asia and Australia.
How likely am I to get sick with WNV from one mosquito bite?

  • In areas where mosquitoes 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 cannot. Therefore, it is important to prevent any mosquito bite.
Is there a vaccine against WNV?

  • No, a human vaccine for WNV doesn't exist to protect people. A vaccine exists for horses and has been used experimentally in other animals.
SOURCE: West Nile Virus (WNV) ( )
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