Fact Sheet - West Nile Virus

What is West Nile virus?

West Nile virus (WNV) belongs to a family of viruses called Flaviviridae. It is spread by mosquitoes that have fed on the blood of infected birds.

WNV can infect a variety of mammals, including horses, humans and domestic and wild birds (particularly crows, blue and grey jays, ravens and magpies).

Although wild birds are the principle hosts of WNV, the virus has been found to infect many other mammals such as bats, cats, cattle and dogs, among others.

In contrast to birds, infected mammals generally do not carry much virus in their blood.

Research indicates that many domestic poultry species can be infected with the virus at a young age, but usually the level of infection is low and of short duration.

Mosquitoes are less likely to ingest the virus from mammals or domestic poultry than from wild birds.

Is West Nile virus a risk to human health?

Yes. The virus can be transmitted to people through the bite of an infected mosquito. Although most people infected with the virus either have no symptoms or flu-like symptoms, the virus can cause severe illness, resulting in hospitalization and even death.

Human cases of the virus infection have been found in almost every province of Canada.

What are the clinical signs of West Nile virus infection?

Animals (particularly horses) infected with the virus show neurological disturbances. Clinical signs may include:

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

The clinical signs of WNV in mammals can be confused with rabies.

Most infected domestic birds do not show signs of infection, and only domestic geese appear to be particularly susceptible to disease and/or death when infected.

WNV-infected geese will show signs of depression, loss of appetite, inability to stand, weight loss and death. The virus can be difficult to distinguish from Newcastle Disease and Avian Influenza in domestic birds.

Where is West Nile virus found?

WNV has been found in Europe, Africa and Asia. In 1999, the virus was identified in the U.S. for the first time, in 2001 its presence was confirmed in Canada, and in 2003 some South American countries reported their first WNV cases.

The Canadian Cooperative Wildlife Health Centre and the Public Health Agency of Canada (PHAC) monitor the progression of WNV in Canada. PHAC reports the surveillance information on its website.

How is West Nile virus transmitted and spread?

WNV is spread by various species of mosquitoes and ticks. The insects transmit the virus from the principle host (wild bird populations) to mammals and domestic poultry.

How is West Nile virus infection diagnosed?

Infected wild birds rarely display clinical signs. More commonly, they are simply found dead. Therefore, in North America, the mortality rate in wild birds may be an indicator of infection levels.

In horses and domestic poultry, especially geese, neurological signs may be suggestive of the infection. Blood tests or post mortem examination can confirm the presence of WNV.

How is West Nile virus infection treated?

There is no treatment currently available to kill the virus. Supportive therapy is recommended in order to reduce the severity of symptoms during the course of infection.

Vaccination may be used to prevent infection in horses. Currently, there are three WNV vaccines registered for use in horses in Canada. All three vaccines need to be followed by an annual booster for continued protection. Local veterinarians should be contacted for guidance on appropriate vaccination products and strategies.

Vaccinated horses that have developed antibodies to the virus 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 veterinarian.

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

The CFIA's approach to WNV centres on education and prevention.

WNV is an immediately notifiable disease. This means that all laboratories are required to notify the CFIA upon suspicion or diagnosis of this disease.

Since the virus is now considered an indigenous disease in Canada, the responsibility for the virus diagnosis rests with provincial or other laboratories. The virus testing is offered on a fee-for-service basis.

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