Adult mosquitos in a Strathroy, Ont. laboratory as the 2007 West Nile virus program of the Middlesex-London Health Unit kicked off early due to warm weather in May 2007. (Dave Chidley/Canadian Press)

In Depth

West Nile Virus


Last Updated September 23, 2008

West Nile virus is carried and spread by mosquitoes and can cause fatal inflammation of the brain (encephalitis) or the membranes covering the brain or spinal cord (meningitis) in more than 100 bird species, and nine mammals, including humans, horses and gorillas. It is closely related to St. Louis encephalitis virus, found in North America.

There is currently no vaccine against West Nile virus encephalitis.

West Nile virus was discovered in the West Nile area of Uganda in 1937, then spread to Mediterranean and temperate parts of Europe. In 1960, it was observed in horses in Egypt and France. Between the 1950s and 1999, there were sporadic epidemics in Israel, South Africa, Romania and in Russia.

Scientists in North America had assumed we were facing a strain that exists in Africa. Then birds across Canada and the U.S. started falling from the sky. That pointed to Israel, where the strain can cause more dangerous results than the traditional encephalitis symptoms.

How dangerous is North America's strain of West Nile?

In September 2002, American researchers reported the first polio–like paralysis stemming from West Nile virus. Infectious disease specialists in Ontario began seeing West Nile patients hooked up to ventilators, unable to move or breathe.

"There's more of a tendency to invade the central nervous system and infect neurons and brain stem regions within the individual," said Michael Drebot, a research scientist at the National Microbiology Laboratory in Winnipeg.

In the October 2002 issue of the New England Journal of Medicine, researchers with the U.S. Centers for Disease Control and Prevention said:

  • For every five people infected with West Nile, one has mild illness usually lasting three to six days.
  • Meningitis or encephalitis develops in about one in 50 people infected with West Nile – more commonly in those over age 50.

The researchers suggest severe muscle weakness is a common symptom that may offer doctors a diagnostic clue.

How common is West Nile virus?

  • In September 2008, the Public Health Agency of Canada said the number of human cases to that point in the year totalled 29, with 13 cases reported in Manitoba and 12 in Saskatchewan.
  • There were 2,215 human cases of West Nile virus in 2007. More than half were in Saskatchewan.
  • The Atlanta-based U.S. Centers for Disease Control and Prevention said 122 human cases were reported from January to July of 2007, including three deaths. Health officials said cases have been diagnosed in Alabama, Arizona, Arkansas, California, Colorado, Idaho, Illinois, Iowa, Kansas, Minnesota, Mississippi, Nebraska, Nevada, North Dakota, South Dakota, Texas, Utah, Virginia and Wyoming.
  • In Canada in 2006, there were 151 human cases of the virus, including two deaths.
  • In the U.S. in 2006, there were 4,269 human cases of the virus, including at least 177 deaths.
  • In 2005, there were 236 cases of West Nile in Canada, with 101 of those in Ontario.
  • In 2004, there were only 26 cases of West Nile across the country. Health officials say the cool weather, better blood screening and local larvicide programs led to the lower number of cases.
  • In 2003, there were 1,494 cases of West Nile, with 947 of those cases originating in Saskatchewan, where there were no cases the previous year.
  • In 2003, 14 people died from West Nile-related illnesses in Canada, 10 of them in Saskatchewan. Two deaths occurred in Manitoba and two in Ontario. In some cases, West Nile was listed as a secondary cause of death.
  • In 2002, Ontario had a total of 308 confirmed and 87 probable cases of West Nile; 18 people died. (One case was travel-related; four deaths are still under investigation).
  • In 2002, Quebec had 19 confirmed cases and two deaths from West Nile.
  • In 2002, Alberta had two travel-related cases of West Nile.
  • West Nile caused 284 deaths in the United States in 2002.
  • In 2000, 21 cases were reported in the United States, including two deaths in the New York City area.

What is Canada doing?

Federal and provincial agencies are monitoring birds and mosquitoes:

  • Birds: The Canadian Co-operative Wildlife Health Centre, together with certain provincial veterinary laboratories and Health Canada's National Microbiology Laboratory, is testing for the virus in wild birds found dead from approximately mid-May until hard frost (late September-October).
    In 2003, the first dead bird was found in April, a month earlier than the previous year.
    During 2001, surveillance efforts focused mainly on dead birds that belong to the family known as corvids: crows, ravens and jays. Officials said that experience from the 1999 and 2000 outbreaks has shown that corvids are the most reliable indicators of West Nile virus activity and potential for human cases of infection in an area.

    By 2007, CDC officials said the virus had been found in more than 130 species of birds. U.S. health officials said members of the public should still report dead birds to authorities but cautioned that birds may die from many causes other than West Nile virus.

    Manitoba's provincial program to report dead crows, blue jays, magpies and ravens began on May 5, 2004. Two years later, the province discontinued its dead corvid pickup program. Officials said its mosquito surveillance was a more precise measure of determining West Nile virus human health risks.

    The Canadian Food Inspection Agency, veterinarians and other members of the animal health community across the country will monitor for infection in domestic animals.

    The Manitoba government's program to report dead crows, blue jays, magpies and ravens began on May 5, 2004.

  • Mosquitoes: The focus of mosquito surveillance tries to determine the current species and abundance of adult mosquitoes in the affected area. Mosquitoes may also be tested to determine the role that different species may play in virus transmission to birds, humans or other animals. Factors such as time of the year, predicted weather patterns and proximity of mosquito populations to humans will be used to determine whether spraying or mosquito eradication programs will be implemented.

  • Several provinces have larvicide programs, which seek to control the mosquito population by introducing chemicals that kill mosquito larvae in pools of water.

    Winnipeg implemented a mass spraying program in summer 2002 to control populations of the type of adult mosquitoes that carry West Nile virus. The Government of Alberta has said spraying adult mosquitoes isn't very effective because it only temporarily reduces the number of adult mosquitoes in the air. Most jurisdictions say they will spray for adult mosquitoes only if there is a significant risk to human health.

  • Human Surveillance: Health care providers, including family and emergency room physicians, infectious disease specialists and neurologists have been asked to watch for symptoms of West Nile virus infection in their patients and to promptly report any probable and confirmed cases of viral encephalitis to local/provincial public health authorities.

  • Horse Surveillance: Of all large land mammals, horses are particularly susceptible to West Nile virus. The Canadian Food Inspection Agency, along with provincial veterinary laboratories and veterinarians, are monitoring for the virus in horses.

  • "Sentinel Chickens": Chickens exposed to mosquitoes were used in the past as measures of virus spread. However, this technique has proved less reliable than testing dead birds found in the wild. Sentinel chickens are no longer used in Canada.

How is it spread?

The Culex pipiens or common household mosquito spreads the virus when it feeds on a blood meal from infected birds. Scientists believe that the most likely "reservoir" for the virus in North America is the common sparrow, which can tolerate the infection. Among birds, the virus has had the greatest impact among crows. In 1999, in the New York area, the crow population crashed by about 90 per cent in a few months.

West Nile transmission cycle
Courtesy C.D.C.

Ten days to two weeks after the initial blood meal, the West Nile virus reaches the mosquito's salivary glands and can then be transmitted to birds, animals or humans. Since 1999, the virus has been found in wild birds, humans and horses across the United States and Canada.

There is no evidence that West Nile virus can be spread directly from human to human. Nor is there evidence that a human can contract the West Nile virus by handling infected birds although health authorities caution people should wear gloves when disposing of dead birds, particularly if they have open sores or cuts. Scientists believe the human immune system prevents the virus from multiplying in large numbers. That prevents humans from transmitting the virus to mosquitoes.

What about transmission through the blood supply?

Yes, it's possible to contract West Nile virus through blood transfusions and organ transplants. The U.S. Centers for Disease Control says there have been a very small number of cases of transmission through this route.

It is also possible for a mother to spread the disease to a child during pregnancy and breastfeeding.

Canadian Blood Services and Hema Quebec screen donated blood for the virus. If the virus is confirmed, the donated blood is destroyed and the donor is notified. The donor won't be allowed to give blood again for at least 56 days.

What are the symptoms?

Symptoms are usually mild and include fever, headache, body aches, sometimes skin rash and swollen lymph glands. Severe infection is marked by headache, high fever, neck stiffness, stupor, disorientation, with coma, tremors, convulsions, paralysis and occasionally death.

Anyone with those symptoms should seek medical attention as soon as possible.

There is no documented evidence that a pregnant woman or her fetus is at increased risk due to infection with West Nile virus.

If illness occurs, it usually happens within five to 15 days of being bitten by an infected mosquito.

What can you do to prevent it?

Health Canada advises:

  • Minimize your time outdoors at dawn and dusk when mosquitoes are most active and, whenever possible, wear long-sleeved tops and long pants when spending time outside.
  • Use an insect repellent containing 10 per cent or less DEET (N, N–diethyl–methyl–meta–toluamide) for children and no more than 30 per cent DEET for adults.
    • For children between six months and two years of age, use one application per day of a product containing 10 per cent or less DEET in situations where a high risk of complications due to insect bites exist
    • For children between two years and 12 years of age, up to three applications per day of a product containing 10 per cent or less DEET can be used.
    • Individuals 12 years of age and older can use a DEET products of up to 30% DEET concentration.
    • More information on the safe use of insect repellents can be found on Health Canada's Web site.
  • Make sure door and window screens fit tightly and are free of holes.

Take action to reduce mosquito breeding sites around your home, local parks and community. For more information see the Information Sheet entitled Effective Control of Mosquitoes Around Your Home on the Pest Management Regulatory Agency web site at:

  • Ensure that things in and around the yard like pool covers, saucers under flower pots, children's toys, pet bowls and wading pools are regularly emptied of standing water.
  • Clean eavestroughs of debris regularly so water does not accumulate.
  • Empty and clean bird baths twice weekly.
  • Ensure that openings in rain barrels are covered with mosquito screening or tightly sealed around the downspout.
  • Aerate ornamental ponds and stock with fish that eat mosquito larvae.
  • Old tires are one of the most common mosquito breeding sites. Ensure that your yard is free of debris, such as old tires, that can accumulate rainwater.

Sources: CBC News, Health Canada, Centers for Disease Control, New York State

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External Links

West Nile Virus Surveillance Program
West Nile Virus, Public Health Agency of Canada
World Health Organization
Centres for Disease Control

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Quick Facts

According to the U.S. Centers for Disease Control:

Family: Flaviviridae

Genus: Flavivirus Japanese Encephalitis Antigenic Complex

Complex includes: Alfuy, Cacipacore, Japanese encephalitis, Koutango, Kunjin, Murray Valley encephalitis, St. Louis encephalitis, Rocio, Stratford, Usutu, West Nile, and Yaounde viruses.

Flaviviruses: share a common size (40-60nm), symmetry (enveloped, icosahedral nucleocapsid), nucleic acid (positive�sense, single stranded RNA approximately 10,000-11,000 bases), and appearance in the electron microscope.

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