West Nile Virus


The West Nile Virus (WNV) was identified in 1937, when it was first isolated in the blood of a patient from the West Nile province of Uganda. The virus is in the same class as the one that causes Japanese encephalitis. It is spread by mosquito bites and is found across the globe. Several bird species constitute its animal reservoir. The virus has been associated with numerous outbreaks in several African, Asian, Middle-Eastern, and European countries.

In America, the virus was first observed in late summer 1999 in the New York City area. At that time, 62 cases of encephalitis and 7 deaths were reported. In October 2000, the New York City Department of Health conducted a study in people 5 years and older who lived in the neighborhood where most cases of encephalitis had occurred. The study revealed the presence of antibodies (a sign of exposure to the virus) in 2.6 percent of the population. Most of the affected individuals reported no specific symptoms, other than mild flu-like symptoms.

In Canada, the virus was detected for the first time during the summer of 2001. The presence of the virus was later confirmed in birds and mosquitos. Only a few human cases were diagnosed in 2002. Most provinces now have a monitoring system in place.

It is estimated that about 20 percent of those infected will develop mild symptoms. Only 1 percent of those infected will develop a severe form of the illness, and most of them will be elderly. In the New York State outbreak, 60 percent of those infected were 65 years or older.


The West Nile Virus is transmitted to humans and animals by the bite of an infected mosquito. The virus is found in the salivary glands of mosquitoes that feed on infected birds. Several bird species can be infected, including crows, sparrows, and various exotic birds. This is the reason why health authorities in Canada are keeping track of the number of dead birds and are taking blood samples as a means to monitor the virus.

The latest data shows that the virus can also be transmitted through blood, organ transplants, pregnancy and breastfeeding. The virus is not transmitted through person to person contact. It is impossible to catch it by kissing an infected person or caring for a person who has the virus. There is no evidence to show that the virus is transmitted from an infected bird or animal to a person. West Nile encephalitis is a seasonal illness because it depends on the presence of mosquitoes. In Canada, outbreaks are more likely to occur during late summer or fall.


The disease usually has a sudden onset. Patients present with a fever (lasting 3 to 5 days) which is sometimes associated with the following symptoms:

  • Severe headaches
  • Skin rash, usually on the abdomen
  • Eye pain
  • Swollen lymph glands
  • Gastrointestinal upset
  • Muscle weakness
  • Muscle pain

The incubation period usually lasts 5 to 15 days. In most cases, the person has no symptoms or suffers from a benign flu-like syndrome. Rarely, and mostly in the elderly, it may cause encephalitis or meningitis, with or without muscle weakness.


There is no specific treatment or vaccine against this disease. Evidence suggests a person cannot get West Nile disease more than once; the initial infection appears to confer lifelong immunity. Treatment is supportive and based on the patient's condition. A human vaccine is being developed.


Diagnosis is based on a specific blood test. Usually 2 blood samples are drawn 14 to 21 days apart.

For monitoring purposes, the presence of the virus is suspected when there is:

  • Acute febrile illness
  • Signs of viral encephalitis
  • Altered mental state or muscle weakness
  • No other infectious agent yet identified

This definition excludes cases of viral meningitis not associated with encephalitis. Suspected and confirmed cases of encephalitis should be immediately reported to the Public Health Branch of Health Canada.


Here are a few simple steps you can take to reduce your risk of contracting the disease:

  • Wear light-coloured clothing, including long sleeves and pants, as well as a hat and shoes - avoid wearing sandals;
  • Apply repellents that contain DEET or icaridin on exposed skin;
  • Stay in places with air conditioning or that use screens;
  • Avoid exposure by staying indoors from dusk to dawn when mosquitos are most active;
  • Get rid of mosquito breeding sites by emptying standing water (e.g., remove water from pool cover, clean gutters regularly).
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