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 birds and mosquitoes that act as reservoirs for the virus. 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.

Horses, geese and corvids, such as crows and jays, are particularly susceptible to the virus. Old and unvaccinated horses, as well as young geese, are more likely to develop severe symptoms of the disease.

Many wild bird species and some domestic bird species are also at risk.  

Signs of the disease

The vast majority of infected animals do not show any symptoms. Some horses can die without showing signs of illness.

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 twitching in the muzzle, head and/or neck
  • Hypersensitivity to noise, touch or visual stimuli
  • Lack of coordination (ataxia)
  • Circling or 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

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.

Only birds appear to develop a high enough level of viremia (virus in their bloodstream) to be infectious. Among domestic bird species, geese are known to be natural hosts of West Nile virus.

Viremia levels in horses and humans are too low to naturally transmit the virus. Infected horses therefore cannot infect other horses, other animals or humans. However, precautions must 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. Approximately one third of horses with signs of West Nile virus will die or require euthanasia.


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

Protection and prevention

West Nile virus is prevented through vaccination and mosquito control.


Vaccination of horses annually in the spring, prior to the start of mosquito season, is recommended.

Unvaccinated horses, pregnant mares and foals may receive booster shots, so vaccination of these horses should be initiated earlier. 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.

Troughs should also be cleaned and disinfected weekly.

Keep animals indoors during peak periods of mosquito activity, particularly dusk and dawn. Stables should be covered in mosquito netting to prevent insects from coming in.

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


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

Veterinarians and other professionals conduct surveillance for WNV and promote the prevention of risks associated with the virus in animals and humans. Each year, wild birds die from this disease and humans are infected.

Information about the West Nile virus situation in Québec can be found by visiting Résultats annuels de surveillance intégrée du VNO This hyperlink will open in a new window. (in French) on the Institut national de santé publique du Québec’s website.

Last update: February 23, 2023


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