In the news: the West Nile virus

In 2002, Canada had its first confirmed human cases of West Nile virus in parts of Quebec and Ontario. That year, 426 Canadians became ill after being infected with the virus. By 2003, West Nile virus had spread to 7 Canadian provinces: Nova Scotia, New Brunswick, Quebec, Ontario, Manitoba, Saskatchewan, and Alberta. 10 deaths were linked to West Nile virus, and more than 1300 Canadians became ill after being infected. In 2004, there were no deaths linked to West Nile virus in Canada, with 25 human clinical cases. Among those 25, there were 6 confirmed cases of West Nile fever and 9 confirmed cases of West Nile virus neurological syndromes. In 2013, there were 115 reported clinical cases.

Experts are not sure how the virus is able to spread so quickly, but its continuing spread indicates that it is probably here to stay. However, Health Canada is working with others to help reduce the risk of exposure to the West Nile virus through education, surveillance, prevention, and response.

Most people infected with West Nile virus show mild flu-like symptoms or no symptoms at all. Less than 1% of those infected with the virus develop severe illness, but for some, especially those who are ill or have a weak immune system, it can result in neurological (brain and nerve) effects, paralysis, or 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.

Of the 74 known species of mosquitoes in Canada, West Nile virus has been found in 10 species. Not all species are found in all parts of the country. In a given area, it is estimated that less than 1% of mosquitoes carry the West Nile virus.

Scientists don't know for sure which mosquito species actually transmit West Nile to people through their bites. Nor is anyone certain how the virus gets through the winter - it could be carried inside birds or sit waiting in hibernating mosquitoes. The uncertainty means that no one can tell health officials which mosquito larvae to kill in order to avoid a repeat of the previous summer.

Mosquitoes spreading disease

How did the West Nile virus spread so quickly during the 2002 Ontario epidemic? One view is that the virus had a spell of "good luck" - strong genes, bird hosts with no natural defenses against a foreign invader, and the favourable environmental conditions of the long and dry summers.

How it spreads so efficiently to humans is also still unclear. Experts suspect that the virus spreads as follows:

  1. In the spring, the West Nile virus lives at low levels among birds like crows, blue jays, or house sparrows. Bird-biting mosquito species take in the virus by ingesting blood from an infected bird, and then spread it by biting other birds.
  2. Other virus-carrying mosquito species, alone or in various combinations, then pass the virus from infected birds to people.
  3. For a significant human outbreak of West Nile, the virus has to first spread widely among birds to some unknown level high enough to be transmitted to people through the mosquitoes at step 2.

Health Canada has put together a surveillance, education, prevention, and response strategy for West Nile virus. Provincial, national, and local authorities will keep track of the virus as it appears in birds, horses, mosquitoes, and humans. In some locations, pesticides may be used to control mosquito populations and to reduce the risk of spreading the virus to people.

There are two types of pesticides used to control mosquitoes: larvicides and adulticides. Larvicides are added to standing water, where mosquitoes breed. They kill the mosquito larvae before they become blood-feeding adults. Adulticides are sprayed into the air or applied to vegetation where mosquitoes rest. They work by killing adult mosquitoes. Larvicides are preferred to adulticides because they target the mosquitoes right in their breeding areas and often reduce overall pesticide use.

The risk of becoming infected with the West Nile virus is greatest during mosquito season. In Canada, this can start as early as mid-April and last until late September or October. To help reduce the risk of West Nile virus in affected areas, early detection of the virus in mosquitoes and animals is important, pesticide use should be exercised when needed, and avoiding mosquito bites is key.

Here are some tips on avoiding mosquito bites:

  • Avoid spending too much time outdoors when mosquito activity is greatest (i.e., at dawn and dusk).
  • Wear light-coloured clothing - mosquitoes are attracted to dark colours.
  • If possible, cover as much of your exposed skin as possible - wear long sleeves, closed shoes, and a hat, and tuck your pants into your socks. You could also consider putting elastics around the wrists of your shirt to prevent bugs from travelling upwards.
  • Use insect repellents - ask your doctor or pharmacist to recommend a safe and effective product.
  • Keep mosquitoes out of your home by sealing any leaks in windows, screens, and doors.

How can I tell if it's West Nile?

The West Nile virus affects the central nervous system. About 80% of those who contract the virus may not show symptoms at all. 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. If symptoms develop, they generally occur within 2 to 15 days after infection.

Health Canada and the US Centers for Disease Control and Prevention estimate that fewer than 1% of those infected develop severe symptoms, and fewer than that experience life-threatening complications. People over 50, those 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.

About 1 out of every 150 infected people will develop severe illness. Severe symptoms include high fever, severe headache, muscle weakness, neck stiffness, confusion, tremors (shaking), numbness, sudden sensitivity to light, and convulsions (seizures). Infection with West Nile virus can result in encephalitis (an inflammation of the brain) or meningitis (an inflammation of the layers 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.

There is no cure for West Nile virus, and there are no medications or treatments that are specific to this condition. Although one may be available within the next few years, there is presently no human vaccine for the West Nile virus.

According to Dr. Andrew Simor, head of the department of microbiology at Sunnybrook and Women's College Hospital in Toronto, other infectious diseases (including influenza) pose a much greater risk to Canadians. For example, the flu is responsible for about 4,000 deaths in Canada each year.

While positive tests for mosquitoes carrying the virus are reported, most experts are urging Canadians to minimize their risk of mosquito bites. (See "Mosquitoes spreading disease" for tips.)

You should consult your doctor if you have the following symptoms:

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

How to protect yourself against West Nile Virus

Are you heading to the cottage this summer or planning on camping? Worried about being exposed to the West Nile virus? The risk of becoming infected with the West Nile virus is greatest during mosquito season. In Canada, this can start as early as mid-April and last until late September or October. Here are suggestions to help you avoid mosquitoes:

  • Apply a bug repellent that contains no more than 30% DEET (chemical name N,N-diethyl-meta-toluamide) to your clothes:
    • Adults can also apply 30% (or less) DEET-containing repellent to any exposed skin, but children should use the least concentrated DEET products (less than 10%).
    • Children should not have DEET-containing repellent on their face or hands.
    • Children under 2 years of age should usually not have DEET applied to clothes or skin. In situations where there is a high risk of complications from insect bites, repellent containing 10% or less DEET may be applied once daily for children aged 6 months to 2 years. Infants under 6 months of age should not have DEET applied to their skin or clothes.
    • Children between the ages of 2 and 12 years can use insect repellent containing less than 10% of DEET - it can be applied up to 3 times per day.
    • Products with a lower concentration of DEET are just as effective as higher-concentration products, but they remain effective for a shorter period of time.
    • Apply DEET sparing to exposed skin or on top of clothing (not under clothing).
    • Do not apply DEET to open wounds or to skin that is irritated or sunburned.
    • Avoid getting DEET in the eyes (for this reason, it is recommended not to put DEET on children's hands) - flush the eyes immediately with water if this happens.
    • 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).
  • Wear light-coloured clothes, including long-sleeved shirts and pants. Mosquitoes tend to be attracted to dark-coloured clothes.
  • Minimize your time outside from dusk to dawn, which are peak periods when mosquitoes are most active. Enjoy the summer weather inside a screened-in patio or enclosure during these hours.
  • Make sure there is no stagnant water (including bird baths) or standing water on your lawn. As part of your spring and summer lawn cleanup, regularly (twice a week) drain rain barrels, bird baths, swimming pool covers, eavestroughs, flowerpots, and planters. Keep wheelbarrows and wading pools overturned when not in use. The most common places for mosquitos to nest are bird baths, old tires, unused containers, flower-pot saucers, swimming pool covers, wading pools, clogged gutters and eavestroughs, clogged drainage ditches, and unused children's toys.
  • 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.
  • Purchase an aerator or ornamental pond that keeps the surface water moving. This environment will be less attractive for mosquito larvae.
  • Check all your window and door screens for holes.
  • Doing some barbecuing this summer? There are products available at some camping stores that repel mosquitoes by emitting sounds that imitate the sounds of the mosquito's natural enemies.
  • Environmentally friendly ways to reduce mosquitoes include installing bat- and birdhouses and encouraging species such as dragonflies, frogs, and beetles.
  • Natural oil-based repellents using plant-based ingredients are available. Oil of lemon eucalyptus has been found to be as effective as DEET when used at similar strengths. Other plant-based repellents, such as citronella or soybean oil, may be used. Keep in mind that they are not as effective as chemical-based repellents, that products containing essential oils need to be tested for skin sensitivity, and that they need to be applied frequently. In general, they are more appropriate if you are spending less than 30 minutes outside. Not all products may be safe for young children. Check with your doctor or pharmacist for more information.

For more information on mosquito-proofing your home, visit Health Canada at