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Santé Montréal

West Nile Virus (WNV)

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West Nile Virus is an infection that is spread through the bite of an infected mosquito. WNV infections first appear in the summer and can occur until the first fall frost.  

Anyone can get this virus. People who spend a lot of time outdoors—either for work or recreational activities—are more likely to catch it. The risk of getting WNV is high in the city, but you can also catch it in the country. Over the past two years, southwestern Québec, including Montréal, has been particularly hard hit.

Most people who get WNV infection have no symptoms. One in five people who have the virus will experience a few minor symptoms such as fever and mild headache.  

It usually takes 3 to 5 days after getting bitten by an infected mosquito for symptoms to appear (ranges from 2 to 15 days).

Among people who are infected, 1 in 150 (less than 1%) will develop a serious illness such as encephalitis (swelling of the brain) or meningitis (swelling of the lining of the brain). People aged 50 and over and those with weakened immune systems are  at greater risk for more serious illness if they are infected with West Nile Virus.

WNV rarely causes death.

There is no specific treatment or vaccine against WNV. Most people recover from West Nile Virus without treatment. Steps are taken to stabilize and improve the health of people who are seriously ill and hospitalized.

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When to Consult

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Protection and prevention

West Nile Virus spreads through the bites of infected mosquitoes. Therefore, avoid getting bitten, particularly starting in July, when mosquitoes are more likely to be infected with the virus.

Take the following measures to protect yourself:

  • Use insect repellent that contains DEET, picaridin or lemon eucalyptus; follow the manufacturer's directions. Use small amounts of repellent and only on skin that is not covered by clothing. For children, apply insect repellent sparingly; you can put it on hats and caps to prevent mosquitoes from biting children on the head or in the face. It is recommended not to use insect repellent on babies under 6 months old;
  • If you're outdoors at the end of the day, wear light-coloured long-sleeved shirts and long pants when there are a lot of mosquitoes.
  • Install screens on windows in houses, tent, etc.

It is also recommended to take measures to reduce mosquito breeding sites:

  • Eliminate water that accumulates in outdoor objects such as buckets, barrels and old tires.
  • Keep your swimming pool properly maintained and get rid of water that accumulates on the pool cover; when the pool is being used, the filter system keeps the water moving and prevents mosquitoes from nesting there.
  •  Cover garbage cans and put a screen  or mesh on top of rainwater barrels.
  • Put fish that feed on mosquito larvae in decorative ponds.

It is important to note that mosquitoes reproduce in stagnant water, even if there is only a very small amount.

Source: MSSS

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Risk factors

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Help and Resources

Services Québec :   

  • 514-644-4545 (Montréal area)
  • 1-877-644-4545 (toll frees, outside Montreal)
  • For medical questions, call Info-Santé at 8-1-1

If you have severe headaches and high fever, a stiff neck, confusion or muscle weakness, see a doctor as soon as possible.

More information

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People at Risk

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Dernière mise à jour le : 2017.06.21