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West Nile virus: What you need to know

What is West Nile?

West Nile is a virus 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.

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.

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.

How dangerous is it?

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.

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.

How can you 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.

How can I help reduce mosquito breeding sites?

Take action to reduce mosquito breeding sites around your home, local parks and community.

  • 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