West Nile Virus Fact Sheet

What is West Nile virus?

West Nile virus (WNV) belongs to a family of viruses called Flaviviridae. The virus circulates among mosquitoes and wild birds, but can infect many other species.

Horses and humans show signs of disease more than other mammals.

Most wild birds show no clinical signs. However, birds such as crows, ravens, blue and grey jays are very susceptible to infection with WNV. They usually die once infected.

In the domestic bird population, chickens and turkeys usually show no signs of infection. Geese often show neurological signs.

Is West Nile virus a risk to human health?

An infected mosquito transmits the virus by biting a human. Most people infected with the virus either have no symptoms or flu-like symptoms. Rarely, people can become very ill resulting in hospitalization and even death.

There is more information about WNV in people on the Health Canada website.

Almost every province of Canada has had human cases of WNV.

What are the clinical signs of West Nile virus infection?

Animals (particularly horses) infected with the virus may show the following clinical signs:

  • ataxia (lack of coordination);
  • depression or lethargy;
  • fever;
  • head pressing;
  • head tilt;
  • impaired vision;
  • inability to swallow;
  • loss of appetite;
  • muscle weakness or twitching;
  • partial paralysis;
  • coma;
  • death

The clinical signs of WNV can be confused with rabies in mammals. It can look like Newcastle Disease and Avian Influenza in domestic birds.

Where is West Nile virus found?

Europe, Africa and Asia have reported cases of WNV. In 1999, the virus was found in the U.S. for the first time. It was found in Canada in 2001. In 2003, some South American countries reported their first WNV cases.

The Canadian Wildlife Health Cooperative and the Public Health Agency of Canada (PHAC) monitor the progression of WNV in Canada. You can find reports of surveillance information on these websites.

How is West Nile virus transmitted and spread?

West Nile virus is usually spread by the bite of a mosquito that has fed on an infected bird. Mosquitoes transmit the virus from wild birds to mammals and domestic poultry.

Very rarely, the virus can be spread through contact with infected animals, their blood, or other tissues.

How is West Nile virus infection diagnosed?

In horses and geese, neurological signs may suggest infection. Only laboratory tests can confirm the diagnosis of WNV.

During the outbreak in 2001, scientists used crows as an indicator of virus spread. Community members reported findings of dead crows. Scientists used this information to track the distribution of the disease in Canada.

How is West Nile virus infection treated?

There is no treatment available to kill the virus. Fluid therapy and anti-inflammatories can reduce the severity of clinical signs.

Horses can be vaccinated to prevent infection. There are currently several WNV vaccines for horses in Canada. The vaccines need to be given every year for continued protection. Contact your veterinarian for more information on WNV vaccines for your horse.

Vaccinated horses may test positive on certain blood tests. This may affect their eligibility for export to countries that require negative blood test results for the virus. Some other countries require that horses be certified as vaccinated against the virus prior to import. For information on specific import/export requirements, contact your CFIA district office.

What is done to protect Canadian livestock from West Nile virus?

WNV is an immediately notifiable disease. This means that all laboratories must notify the CFIA when they suspect or diagnose this disease.

The CFIA considers WNV a domestic disease, meaning that it is commonly found in Canada. Your vet will submit samples to a provincial or other lab for diagnosis. You will have to pay for WNV testing if your vet suspects it.

Additional information

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