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West Nile Virus Resources

What is West Nile Virus?

West Nile Virus (WNV) is a virus that causes a mild febrile illness (with fever, headache, body aches) in most persons who acquire the virus. Some individuals, however, particularly the elderly, may develop permanent neurological damage due to encephalitis (inflammation of the brain), possibly resulting in death.

West Nile virus was first isolated in the West Nile District of Uganda in 1937. Since then it has spread through Africa, southern Europe, the Middle East, Asia, Oceania and, most recently, North America. It propagates in temperate zones of the world, and new evidence suggests that it may over-winter in northern climates in birds and animals. The disease is fatal in domestic and wild birds. WNV was introduced into North America in 1999. In late summer of 1999 an outbreak of WNV occurred in New York City resulting in 62 cases of encephalitis, including 7 deaths. It has since spread inexorably west and south through the U.S. and into Canada. During 2002, more than 4,000 people in North America became ill after becoming infected with WNV. This is the largest outbreak of West Nile virus infection ever recorded.

  • Public Health Agency of Canada surveillance Case Definitions for West Nile Virus

 Current Case Count

[Ref: PHAC]

The first human cases of WNV in Canada occurred in August, 2002. Since then, human test results in Canada have confirmed WNV in all provinces except PEI and Newfoundland. The number of cases has fluctuated:

  • 2019: 43 cases
  • 2018: 437 cases
  • 2017: 202 cases
  • 2016: 162 cases
  • 2015: 84 cases
  • 2014: 23 cases
  • 2013: 114 cases
  • 2012: 454 cases
  • 2011: 124 cases
  • 2010: 5 cases
  • 2009: 14 cases
  • 2008: 37 cases
  • 2007: 2401 cases
  • 2006: 151 cases
  • 2005: 238 cases
  • 2004: 32 cases
  • 2003: 1488 cases
  • 2002: 414 cases

Etiologic Agent

West Nile virus belongs to a family of viruses called Flaviviridae. It is spread by mosquitoes that have fed on the blood of infected birds. West Nile virus is closely related to the viruses that cause Dengue fever, Yellow fever and St. Louis encephalitis.

West Nile virus primarily infects birds (particularly crows, ravens, blue jays and magpies), although it has been found in other mammals such as horses and bears. Human cases are still very rare.

Transmission of WNV

Most people get West Nile virus from the bite of an infected mosquito. Mosquitoes become infected from feeding on the blood of an infected bird. Two weeks later the mosquito is capable of passing the virus to animals and humans by biting them. Other blood-biting insects do not appear to be able to spread West Nile virus.

In 2002 it became apparent that West Nile virus could be spread in other ways, such as blood transfusions and organ transplants. There is also evidence that pregnant women can pass on the virus to their unborn babies and that the virus can be passed through breast milk. Healthcare workers can get West Nile virus via needlesticks or cuts. West Nile virus is not spread by casual contact or by the respiratory route. It is not airborne. There is no person-to-person spread, or animal-to-person spread. There is no evidence of anyone getting WNV from handling infected carcasses, however it is always recommended that gloves be worn to handle any dead bird or animal remains, to prevent acquisition of other types of infection. Special precautions are not required for patients who have WNV encephalitis.

Symptoms of WNV

Most infections are mild and symptoms include the sudden onset of fever, headache and body aches, nausea and/or vomiting, occasionally a skin rash on the trunk of the body and swollen lymph glands. These symptoms generally last only 3 to 6 days. More severe infection may be marked by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis and, rarely, death. The incubation period of West Nile virus encephalitis is usually 3 to 14 days.

 Preventing WNV Infection

The best way to protect against WNV is to avoid mosquitoes:

  • Consider staying indoors at dawn, dusk and in the early evening
  • Install or repair window and door screens so that mosquitoes cannot get indoors
  • Place mosquito netting over infant carriers when you are outdoors with infants
  • When possible, wear long-sleeved shirts and long pants whenever you are outdoors
  • Apply insect repellent sparingly to exposed skin. Choose a repellent that provides protection for the amount of time that you will be outdoors
  • To avoid helping mosquitoes breed in your environment, drain standing water. Routinely empty water from flower pots, pet bowls, clogged rain gutters, swimming pool covers, discarded tires, buckets, barrels, cans and other items that collect water in which mosquitoes can lay eggs.

Protecting the Blood System

Health Canada began testing blood donations for West Nile Virus on June 17, 2003, using a test developed by the Canadian Blood Services (ref. Canadian Blood Services). On July 22, 2003 a unit of blood donated in Saskatchewan became the first blood sample to test positive for the virus. The blood was immediately withdrawn from circulation and the donor was notified (ref. Canadian Blood Services). Canadian Blood Services and Héma-Québec have had contingency plans in place to ensure that Canadians have enough safe blood available throughout the West Nile Virus season.

 Reporting and Submitting Dead Birds

The Canadian Cooperative Wildlife Health Centre has a wealth of information regarding birds that carry WNV and how to submit suspicious dead birds for testing:

WNV Links

Alberta British Columbia Manitoba New Brunswick Nova Scotia Ontario Quebec Saskatchewan


  • MMWR October 4, 2002 Investigations of West Nile Virus Infections in Recipients of Organ Transplantation and Blood Transfusion - Michigan, 2002
  • MMWR November 1, 2002 Public Health Dispatch: Investigations of West Nile Virus Infections in Recipients of Blood Transfusions
  • MMWR December 20, 2002 Laboratory-Acquired West Nile Virus Infections - United States 2002
  • MMWR December 20, 2002 Intrauterine West Nile Virus Infection - United States 2002