Most people get West Nile virus after being bitten by a mosquito that has fed on an infected bird. The virus 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.

The virus is closely related to the viruses that cause Dengue fever, Yellow fever and St. Louis encephalitis.

There is currently no vaccine against West Nile virus encephalitis.

The 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 an African strain when it was discovered here in the late 1990s. 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.

The common household mosquito spreads the West Nile virus. How dangerous is North America's strain of West Nile?

In Canada, 42 people have died from the virus since 2002.

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.

How common is West Nile virus?

In 2008, the Public Health Agency of Canada said the number of human cases totalled 38. The number of cases in previous years were:

• 2,401 in 2007.
• 154 in 2006.
• 238 in 2005.
• 26 in 2004.
• 1,495 in 2003.
• 414 in 2002.

What is Canada doing?

Federal and provincial agencies are monitoring birds, mosquitoes and various animals:

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).

During 2001, surveillance efforts focused mainly on dead birds that belong to the family known as corvids: crows, ravens and jays. Officials said experience from the 1999 and 2000 outbreaks showed that corvids are the most reliable indicators of West Nile virus activity and potential for human cases of infection.

By 2007, U.S. Centers for Disease Control 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.

Mosquitoes: The focus of mosquito surveillance is to determine the species and abundance of adult mosquitoes in a given area. Mosquitoes may also be tested to determine the role that different species play in virus transmission to birds, humans or other animals. Factors such as time of 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 to control the mosquito population by introducing chemicals that kill mosquito larvae in pools of water.

Human Surveillance: Health care providers, including family and emergency room physicians, infectious disease specialists and neurologists have been asked to watch for symptoms in their patients. They report any probable and confirmed cases of viral encephalitis to local and 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.

How is it spread?

The Culex pipiens, or common household mosquito, spreads the virus when it feeds on blood from infected birds. Recent research suggests American robins and sparrows are an important reservoir for the virus because they often survive the illness and hence are more likely to spread the virus. Among birds, the virus has had the greatest impact on 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.

West Nile transmission cycle Courtesy C.D.C. 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 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 per cent DEET concentration.
• Make sure door and window screens fit tightly and are free of holes.
• 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, Public Health Agency of Canada, Centers for Disease Control, New York State