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West Nile virus (WNV)


The West Nile virus (WNV) was discovered in 1937 in Uganda (Africa), in the West Nile region. Since then, the virus is present in various countries around the world, including the United States and Canada.

In Québec, the virus was identified for the first time in 2002. It is now present in several regions, particularly in southern Québec.

Mosquitoes are the main pathway for introduction of the virus in humans. In Québec, as in the rest of North America, infections caused by WNV are seasonal. They occur in summer and continue in fall, up to the first frosts.

Infected mosquitoes can also transmit WNV to animals, especially birds. For more information, consult the page West Nile virus in animals.


In most cases (80 %), people infected with WNV do not have symptoms.

However, some individuals may develop symptoms. They appear from 2 to 14 days after being bitten by a mosquito carrying the virus. The severity of WNV infections varies. The most frequent benign symptoms are:

  • Headaches
  • Fever
  • Muscle pain
  • Gastrointestinal disorders
  • Skin rashes with pimples
  • Swollen lymph nodes

Less than 1 % of people infected with WNV will develop a severe form of the disease resulting in neurological disorders. People aged 50 and over, those suffering from chronic diseases such as diabetes and cardiovascular diseases and those who have a weakened immune system have a higher risk of developing the severe form of WNV.

Meningitis (infection of the lining of the brain) and encephalitis (brain inflammation) are examples and may be manifested by the following symptoms:

  • Intense or unusual headaches
  • High fever
  • Stiff neck
  • Nausea or vomiting
  • Drowsiness
  • Confusion or disorientation
  • Loss of consciousness and convulsions
  • Coordination disorders
  • Muscle weakness
  • Paralysis

When to seek medical help

If you experience one of the symptoms associated with the severe form of WNV infection, seek medical help promptly.


There is no specific treatment or vaccine for WNV in humans. However, most people infected recover without treatment.

The capacity to fight the virus depends on your health status and age. This capability decreases with age.

A serious West Nile virus infection generally requires hospitalization. Health care aims to stabilize the infection and improve health status.

Complications and aftereffects

Following an infection resulting in neurological impairment, sometimes permanent neurological deficits have been reported.

In rare cases, WNV may cause death.


WNV is transmitted to humans mainly through bites by mosquitoes carrying the virus. Mosquitoes become infected when they feed on infected birds. WVN mostly infects birds, but also humans and other animal species such as horses.

WNV is not transmitted when a person is in contact with an infected bird or animal or with another person.

Although the risk is low, WNV can be transmitted during blood transfusion or organ transplant. To reduce the risk in Québec, Héma-Québec tightly controls the quality of donated blood.

Anyone can contract WNV. People who work outside or participate in outdoor activities are most at rick of infection, especially if they spend a lot of time outdoors without taking any measures of protection against mosquito bites. The risk of WNV infection is as high in the city as in the countryside.

Protection and prevention

Avoiding mosquito bites is the best way to prevent WNV infection. Simple measures can be taken. For more information, consult the pages Protecting yourself from mosquito and tick bites and Reducing the number of mosquitoes in your surroundings

Avoid approaching and handling dead or sick wild birds that may carry diseases. Report them by calling 1-877-346-6763 to help with monitoring several diseases.

Special conditions

In Québec, WNV infection is a mandatory reportable disease. Laboratory managers and doctors who identify or diagnose a case of WNV infection must notify public health authorities.

Last update: July 5, 2023


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