West Nile virus (WNV) is an arbovirus in the family Flaviviridae. Infected mosquitoes can spread WNV to both people and other animals.

WNV is maintained in nature in a transmission cycle between wild birds and mosquitoes. In most parts of Québec, West Nile virus season is from April to October.

For more details on the risk of infection in humans, visit the West Nile virus (WNV) page.

At-risk animals

Several species of animals, both domesticated and wild, are affected by West Nile virus.

Among domesticated animals, horses are particularly susceptible to West Nile virus. Cases of the disease are sometimes seen in poultry. Old and unvaccinated horses, as well as young geese, are more likely to develop severe symptoms of the disease. Wild animals, crows, jays and birds of prey are also particularly susceptible to the virus. Cases of infection are also occasionally seen in some mammals, such as squirrels. 

Signs of the disease

The vast majority of infected animals do not show any symptoms.

Some horses can die without showing signs of illness prior to death. In some cases, horses may develop any combination of the following symptoms at the onset of disease:

  • General deterioration of health and loss of appetite
  • Weakness
  • Lameness
  • Fever (in 25% of cases)

Other symptoms may appear as the disease progresses:

  • Depression, lethargy, drowsiness
  • Impaired vision
  • Muscle tremors or spasms in the muzzle, head or neck
  • Hypersensitivity to noise, touch or visual stimuli
  • Lack of coordination or abnormal gait
  • Head tilt
  • Difficulty swallowing, which may progress to an inability to swallow
  • Partial or total paralysis of the face, tongue or one or more limbs
  • Colic
  • Lying on its side
  • Convulsions
  • Coma and death

Crows, jays and birds of prey may develop neurological signs such as tremors and weakness and may die as a result of infection.

Transmission and incubation period

West Nile virus is spread by the bite of a mosquito carrying the virus. Mosquitoes that have fed on an infected bird can transmit the virus to their offspring and to other birds, horses, other animals and humans.

Some birds that are predators or scavengers, such as birds of prey and corvids, may become infected after eating an infected carcass. Birds may also contract the virus if they eat mosquitoes.

Only birds appear to develop a high enough level of viremia (virus in their bloodstream) to be infectious. This includes domestic geese.

Viremia levels in horses and humans are too low to transmit the virus to mosquitoes when bitten. Thus, like birds and humans, infected horses cannot infect other horses, other animals or humans directly, except through contact with their blood. Precautions must therefore be taken when taking blood samples or performing necropsies, as the virus can be spread through contact with the blood or tissue of infected horses.

Keep in mind that if an animal is infected with West Nile virus, that can indicate that there are mosquitoes carrying the virus in the immediate environment or in an area it has recently been to.

Time between the virus entering the animal’s body and the appearance of symptoms

In horses, it can take seven to ten days after infection for the first signs of the disease to appear.

Infected horses typically see an improvement in their condition within three to seven days of the onset of symptoms. Of those whose condition improves significantly, 90% will make a full recovery in one to six months, while the rest will continue to exhibit residual neurological signs (such as lameness). Approximately one third of horses with West Nile virus will die or require euthanasia.


There is no treatment for West Nile virus. Supportive therapies are sometimes used to alleviate symptoms in domesticated animals.

Protection and prevention

West Nile virus in animals is prevented through vaccination and mosquito control.


Vaccination of horses annually in the spring, prior to the start of the peak risk season (meaning by early May at the latest), is recommended.

Booster shots may be required for unvaccinated horses, horses with an unknown vaccination history, pregnant mares and foals, so vaccination of these horses should be initiated earlier in the season. Your veterinarian can help you create an effective vaccination protocol to protect your animal.

Mosquito control

Remove any stagnant water from your property, and remove any items where stagnant water can accumulate, such as containers, buckets, old tires and garbage, as these are ideal breeding sites for mosquitoes. Learn more about how to reduce the number of mosquitoes in your surroundings.

Keep animals indoors during peak periods of mosquito activity, at sunrise and at sunset. Stables should be covered in mosquito netting to prevent insects from coming in.

You may choose to use insecticides or insect repellents, provided they are appropriate for your animals. Some approved products for horses seem to provide effective short-term protection, but always follow the manufacturer’s directions.


West Nile virus surveillance is conducted to record cases of infection in animals and humans. Surveillance findings can be found by visiting Résultats annuels de surveillance intégrée du VNO This hyperlink will open in a new window. (in French only) on the Institut national de santé publique du Québec’s website.

Suspected cases of West Nile virus in domesticated animals must be reported to a veterinarian.

Sick or dead wild birds can be reported by calling 1-877-346-6763. This will help our surveillance efforts for a number of diseases, including West Nile virus. Avoid approaching or handling the birds while awaiting instruction.

Last update: February 28, 2024


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