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Overview

West Nile belongs to a family of viruses called flaviviridae. It is closely related to the viruses that cause Dengue fever Yellow fever and St. Louis encephalitis and is spread to people by mosquitoes that have fed on the blood of infected birds. The virus was first isolated in 1937 in the West Nile district of Uganda. Since then there have been outbreaks in Egypt Israel South Africa and in parts of Europe Asia and North America. In 2003 1300 Canadians became ill after being infected with West Nile virus.

Cause

The principal transmitter of West Nile Virus is the Northern House Mosquito (Culex pipiens). Mosquitoes first become exposed to the virus when they feed on birds that are infected with WNV. The virus is then stored in the mosquito's salivary glands and infected mosquitoes transmit West Nile virus to humans and animals while biting to take blood.

Many birds can be infected with WNV but crows and blue jays are most likely to die from the infection. Horses are also prone to severe WNV infection.

In 2002 scientists discovered that people could be infected with West Nile virus in other ways including blood transfusions and organ/tissue transplants. However the risk of getting West Nile virus this way is considered to be quite low. There is also evidence that pregnant women can pass the virus to their unborn babies and that the virus may be passed through breast milk. In addition laboratory workers who handle infected specimens can get West Nile virus through needle punctures or cuts.

There is no evidence to suggest that people can get West Nile virus by touching or kissing someone who is infected or from being around a health care worker who has treated an infected person. Likewise there is no evidence that the virus can pass from infected animals (i.e. horses birds pets etc.) to people.

Symptoms

For most Canadians the risk of West Nile virus infection is low and the risk of serious health effects from the virus lower still. Only one in five people who contract the virus show any symptoms. Most with the virus have symptoms that are mild and flu-like. About 20 per cent of those infected may develop West Nile fever - a milder form of the virus that includes fever headache muscle aches and a rash that clears up in seven to 10 days.

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

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

People with weaker immune systems and those with chronic diseases are at greater risk for serious health effects. While the overall risk increases with age people of any age or health status can develop serious health problems if they are infected.

One out of 150 infected people will develop a serious neurological illness such as encephalitis (an inflammation of 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. However these extreme reactions tend to appear mostly in people who are over 65 or have weakened immune systems. Of those who become seriously ill approximately one in 10 will die.

Prevention

The risk of becoming infected is greatest during mosquito season. In Canada this can start as early as mid-April and last until the first hard frost in late September or October.

Because most people infected with West Nile virus get it from the bite of an infected mosquito you can protect yourself and your family by minimizing your exposure to mosquitoes.

Staying inside from dusk to dawn and is your best bet but if you are going to be exposed to insects wear protective clothing when outdoors and use insect repellents that contain DEET or other approved ingredients.

• Look for repellents with no more than 30 per cent DEET for adults and no more than 10 per cent DEET for children. Children should not have DEET-containing repellent on their hands and children under six months of age should not have DEET applied (it is recommended where there is a high risk of complications to the child from insect bites that DEET may be considered for children aged six months to two years). Pregnant women should apply repellent to their clothes only and not directly on their skin.
• Neck wrists and ankles are important areas to apply the repellent to and care must be taken to avoid the nose and eyes. When applied on the skin the repellent effect may last from 15 minutes to 10 hours depending on climate the product's composition and the type of bug you are trying to repel. When applied on clothes the repellent effect lasts longer.

Insecticide spray cans are effective for an immediate knockdown and killing but have little effect once the air has cleared. They are best used in combination with other deterrents. Mosquito nets are ideal for travelers and campers and some types are available with insecticide in their material. Look for a mesh size of less than 1.5 mm and repair any holes immediately.

Additional preventative measures:

• Make sure there is no stagnant water (including bird baths) or standing water on your lawn. Regularly drain rain barrels swimming-pool covers eavestroughs flowerpots and planters. Keep wheelbarrows and wading pools overturned when not in use.
• Around your yard and lawn throw away lawn cuttings raked leaves and fruit or berries that fall from trees immediately. Place them in sealed garbage bags. Turn over compost piles regularly and remove dense shrubbery where mosquitoes are liable to breed and rest.

• Check all your window and door screens for holes.

It is estimated that less than 1% of mosquitoes in any given area are infected with West Nile virus and overall risk of being bitten by an infected mosquito is low. Although it is possible to be bitten by infected mosquitoes whenever West Nile virus is active to date most human infections with West Nile virus have occurred during July and August at a time when infection rates in mosquitoes have reached their peak.

Additional Resources

West Nile Virus Surveillance Information (Ontario)

Mosquito Bytes