The Facts

West Nile virus is an illness that spreads from mosquitoes to humans. A mosquito becomes infected when it feeds on the blood of a bird that is carrying West Nile virus. About 2 weeks later, the mosquito is capable of spreading the virus to people and animals while biting for a blood meal. The virus is not spread from person to person, and cannot be spread directly from infected animals, such as birds, horses, or pets to people.

West Nile virus originated in the West Nile region of Uganda in 1937, and for decades it was confined to Africa, the Indian subcontinent, and parts of the Middle East and Europe. In 1999, it was detected in New York City, where 7 people died from it. Canada had its first confirmed infection in a bird in 2001. In September 2002, the first confirmed human cases of West Nile virus were reported in parts of Quebec and Ontario.

By 2003, West Nile virus had spread to 8 Canadian provinces and 1 territory: Nova Scotia, New Brunswick, Quebec, Ontario, Manitoba, Saskatchewan, Alberta, British Columbia, and the Yukon Territory. More than 1,400 Canadians became ill after being infected that year. Since then, there have been West Nile cases every year - the virus's continuing spread indicates that it is probably here to stay. Every year, the Public Health Agency of Canada (PHAC) publishes weekly reports during West Nile virus season on West Nile virus activity in Canada.

West Nile virus affects the central nervous system, and infection usually results in mild flu-like symptoms or no symptoms at all. However, in severe cases, infection with West Nile virus can be fatal. Affected provinces have developed aggressive strategies to tackle the problem, including surveillance programs to track the location and numbers of infected mosquitoes and birds.


Mosquitoes become infected with the virus and spread the disease by biting birds or humans. Sometimes the virus spreads from mosquitoes to horses and other animals. The virus is stored in the mosquito's salivary glands, and infected mosquitoes transmit West Nile virus to humans and animals while biting to take blood. The peak infection rates occur during July and August.

There is evidence that the virus can also be transmitted through blood transfusions and during organ transplants. However, the risk of transmission through these procedures is quite low. Health Canada works closely with Canadian Blood Services and Héma-Québec, the only blood operators in Canada, to screen the blood supply for West Nile and other infectious diseases. Evidence also suggests that West Nile can be transmitted from a pregnant woman to her unborn child as well as through breast milk.

West Nile virus belongs to a group of similar viruses, including others that are less severe, such as the viruses that cause dengue fever and St. Louis encephalitis. This means that several other viruses can cause similar symptoms, and they must be ruled out before a diagnosis of West Nile virus can be confirmed.

Symptoms and Complications

Overall, most doctors believe that the risk of people becoming seriously ill with West Nile is extremely small. About 80% of those who contract the virus may not show symptoms at all. When infection does cause illness, symptoms vary from person to person and will usually appear within 2 days to 15 days.

Up to 20% of those infected may develop West Nile fever, which features mild flu-like symptoms. It is characterized by fever, headache, muscle aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach, and back.

Health Canada and the US Centers for Disease Control and Prevention estimate that fewer than 1% of those infected – about one out of every 150 infected people – will develop severe symptoms, and fewer than that will experience life-threatening complications. People over 50, people with chronic health conditions (such as cancer, diabetes, alcoholism, or heart disease), and those with weakened immune systems are more likely to have serious health effects from West Nile virus.

Severe symptoms include high fever, headache, neck stiffness, confusion, tremors (shaking), and convulsions (seizures). Infection with West Nile Virus can result in encephalitis (an inflammation of the brain) or meningitis (an inflammation of the linings that cover the brain). West Nile virus can cause the brain or spinal cord to swell and block the flow of blood to the brain. This could lead to a coma, paralysis, or even death. The neurological effects may be permanent in some people.

Scientists do not know why some people recover quickly while others face long-term health problems. These problems may include:

  • physical effects such as long-term muscle weakness and paralysis, fatigue, and headache
  • confusion, depression, and problems with concentration and memory
  • difficulties in performing daily tasks such as preparing meals and shopping

Even though the majority of people with mild flu-like symptoms may not need blood tests for West Nile, consult your doctor if you have the following symptoms:

  • confusion
  • convulsions or seizures
  • extreme swelling or infection at the site of a mosquito bite
  • fever
  • muscle weakness
  • severe headache
  • stiff neck
  • sudden sensitivity to light or an inability to perform routine tasks

Making the Diagnosis

Your doctor can take a blood sample and send it to a laboratory to determine if you have been infected with the West Nile virus. Your blood will be tested for antibodies to the virus, a sign that you have been infected. Before the blood test, your doctor will probably ask you questions to determine if you could have been exposed to mosquitoes carrying the virus in the first place. People who live in or travel to areas where the virus has been found are at risk of getting infected.

There are two categories of antibody blood tests for West Nile virus:

Front-line testing: Front-line testing is done to determine if a person has recently been exposed to the West Nile virus. The ELISA IgM test is most commonly used, although there are two types of ELISA tests (IgM and IgG) that can be used. The IgM test does not always require a second test to be performed when the first sample is positive. This can speed up the reporting of positive results. If there are confirmed West Nile cases in an area and antibodies are detected in the first blood sample, doctors may consider this to be a case of West Nile and modify patient treatment accordingly.

Confirmatory testing: These tests are done to confirm the diagnosis of West Nile in people whose positive results may have been due to a cross-reaction with related members of the West Nile family of flaviviruses, such as the St. Louis encephalitis virus or the dengue virus. Confirmatory testing also provides researchers and public health officials with important information about West Nile virus such as:

  • the geographical distribution of the virus as it spreads to new areas
  • new ways the virus can be spread
  • who may be at greater risk for health problems
  • whether the virus is turning up again in an area with previous human cases

Treatment and Prevention

There is presently no vaccine available to protect against West Nile virus. Severe cases of West Nile virus are treated with supportive care in a hospital. This involves helping the body fight illness on its own, rather than treating the cause of illness directly. People infected with West Nile virus may receive intravenous (into a vein) fluids and breathing support (by ventilator). Scientists are working on developing a vaccine for West Nile virus.

In the absence of an available vaccine, experts are advising people to protect themselves against the virus by avoiding bites from mosquitoes. The risk of becoming infected with West Nile is greatest during mosquito season. In Canada, the higher-risk season is from mid-April to late September or October.

The following suggestions can help you avoid mosquitoes:

Use insect repellent

  • Adults: Apply a bug repellent that contains no more than 30% DEET (chemical name N,N-diethyl-meta-toluamide) to clothes and exposed skin. In general, the duration of protection for different strengths
    • 6 hours for 30%
    • 3 hours for 10%
  • Children 2 to 12 years of age: Use DEET in concentrations of 10% or less and do not apply more than 3 times daily. Do not apply repellent to the face and hands. Avoid prolonged use.
  • Children aged 6 months to 2 years: Apply DEET repellent once daily in situations where there is a high risk of complications due to insect bites. In these situations, sparingly use concentrations of less than 10% DEET and do not apply to the face and hands. Avoid prolonged use.
  • Children under 6 months of age: Do not apply DEET to clothes or skin.
  • Pregnant women: There are no data to suggest that DEET is harmful for pregnant or breast-feeding women. However, these women may want to use non-chemical methods (such as protective clothing and avoiding times and places where mosquitoes are likely to be present).

Avoid mosquitoes

  • Minimize time outdoors, or remain indoors from dusk to dawn when mosquitoes are most active.
  • Eliminate stagnant water (including bird baths) or standing water on your lawn. Regularly (twice a week) drain rain barrels and/or cover with screens, drain swimming-pool covers, clean eavestroughs regularly, and drain flowerpots and planters. Keep wheelbarrows and wading pools overturned when not in use.
  • Around your yard and lawn, immediately throw away lawn cuttings, raked leaves, and fruit or berries that fall from trees. Place them in sealed garbage bags. Turn over compost piles regularly, and remove dense shrubbery, where mosquitoes are liable to breed and rest.
  • Ensure that door and window screens are secure and free of holes.

Wear protective clothing

  • Wear light-coloured clothes, including long-sleeved shirts and pants.
  • Use a mosquito net over a baby's crib, stroller or playpen when the child is outdoors if there is a risk of being bitten by mosquitoes. Otherwise, dress the infant in long sleeve shirts and long pants when in an area where mosquitoes are present.

Other prevention strategies

Insecticides that destroy mosquitoes at various stages of their development can also be used to control the mosquito population. Provincial and local health authorities are the only people qualified and responsible for determining whether pesticides should be used to prevent the spread of West Nile virus in a particular area. When such a decision is made, the public is warned in advance so they can take precautions and minimize exposure. The workers who carry out mosquito control programs are licensed by provincial authorities.

Mosquito development consists of 4 stages: eggs, larvae, pupae, and adults. Adult females deposit eggs in or around standing water. The eggs hatch into larvae, which continue to develop in the area they were deposited and eventually develop into adult mosquitoes.

Insecticides can either destroy mosquito larvae (larvicides) or kill adult mosquitoes (adulticides).

Larvicides have greatest effect early in the mosquito season (from May to July in Canada) and less effect after mid-August. They are sprayed on areas where mosquitoes typically deposit their eggs. Adulticides are sprayed on plants where adult mosquitoes are commonly found. In general, larvicides are preferred to adulticides because they have less of an impact on human health, other animals, and the environment. They also disrupt the mosquito life cycle before they become adults and move to other areas. Adulticides are usually used as a last resort in situations where a high risk of human infection with West Nile virus exists. Because adult mosquitoes are mobile, adulticides may need to be sprayed repeatedly to control the mosquito population.

Provincial health authorities take West Nile seriously. They are monitoring the spread of the virus and are developing methods to manage outbreaks effectively.