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West Nile virus & Mosquitoes

To protect yourself and your family from West Nile virus (WNv), you need to prevent mosquito bites. The health unit is encouraging you to reduce mosquito-breeding sites at home and your cottage, and take necessary steps for personal protection.

WNv is a mosquito-borne virus that can cause illness, including in rare cases, encephalitis (inflammation of the brain). WNv can be spread to humans by the bite of an infected mosquito. Mosquitoes become carriers of the virus when they feed on infected birds.

Human infections from the virus are rare and those affected usually experience only mild symptoms such as fever or headaches. The illness can be serious and inflammation of the brain (encephalitis) can occur in a small number of cases.

Flu-like symptoms begin in humans, about 3 to 14 days after a bite from an infected mosquito. Although most people will not become sick if bitten by an infected mosquito, WNv can cause severe illness in some people. Those at greatest risk of severe illness are people over the age of 50 and people with chronic diseases or weakened immune systems

Try to stay indoors when mosquitoes are most active (dusk to dawn). When you do go outside, wear light-coloured, long-sleeved shirts, long pants and use a repellent. When using insect repellent be sure to follow the manufacturer’s recommendations.

What should I consider when purchasing insect repellent?

When choosing a repellent, consider a product that meets your needs, and only use personal repellents that are registered in Canada. Never use a product labelled as an insecticide on your body. Make sure you:

  • read repellent label carefully before using
  • apply the repellent sparingly on any exposed skin (do not apply underneath clothes), avoiding your eyes; if you do get repellent in your eyes, rinse with water immediately
  • spray in well ventilated areas
    • are not sensitive to a product, by applying repellent to a small area on your arm and waiting 24 hours
    • if you suspect you or your child may be having a reaction to the repellent, stop using the product immediately, wash the treated area, and seek medical attention.

Are there alternatives to a DEET repellent?

P-menthane 3,8-diol and soybean oil are active ingredients that can protect people against mosquito bites. Products containing P-menthane 3,8-diol can provide up to two hours of protection against mosquitoes but cannot be used on children under three years of age.

Products containing soybean oil can provide between one to 3.5 hours of protection against mosquitoes, depending on the product. Be sure to read the labels before applying any repellent.

Registered products containing citronella protect people against mosquito bites for 30 minutes to two hours. These products should not be used on infants and toddlers. Certain products containing citronella have a limit on the number of applications allowed a day. Read the product label before using. (In 2004, Health Canada completed a review of citronella-based insect repellents that are applied directly to the skin. But since Health Canada did not identify any health risks, citronella-based personal insect repellents will remain on the market until a final decision is made.)

For more information on using personal insect repellents see Healthy Canadians "Insect Repellents" .

A mosquito-breeding site is any container, depression, or area containing stagnant water for a period longer than seven days in which mosquito eggs may be able to develop to maturity. Some sites include rain barrels, ponds, bird baths, pool and boat covers, and old tires and lawn ornaments.

By preventing and eliminating breeding sites, adult mosquito numbers can be decreased, which can then reduce your risk of being bitten by a mosquito.

To eliminate potential breeding sites:

  • keep grass cut and trim shrubs and bushes;
  • fill ponds with Koi or other mosquito-eating fish(do not stock ponds that have an outflow to natural water sources if stocking with non-native fish) and consult the Ministry of Natural Resources before stocking any water body;
  • keep swimming pools, spas and hot tubs chlorinated and covered tightly when not in use;
  • change the birdbath water weekly;
  • ensure water doesn’t accumulate in tires and tire swings;
  • empty buckets, wheel barrows and pots;
  • store small boats and canoes upside down;
  • be sure boat covers do not accumulate water;
  • drain or fill tree holes;
  • screen or eliminate water in sump pumps;
  • keep eavestroughs and rain gutters clean;
  • drain plastic covers on lawn chairs and other outdoor equipment weekly.





The dead bird surveillance program was discontinued by the province in 2009. Dead birds are no longer required as an early indicator of WNv. When WNv first arrived, we didn’t know very much about it. We implemented a broad surveillance program to collect as much information as we could, using dead bird, mosquito and human surveillance. Now we have a lot of information and research about WNv, research that gives us the data we need to make decisions about the program.

If you are concerned about a dead bird you have found, the Canadian Cooperative Wildlife Health Centre (CCWHC) may be interested. For more information visit their website or you can contact them toll-free at 1-866-673-4781.

How is mosquito surveillance conducted?

The health unit's WNv team will set up and monitor mosquito traps. Mosquitoes caught in the traps are then tested for WNv. The WNv team will also conduct regular larval dipping of specific sites. Larval dipping is the collection of mosquito larvae from sites such as storm water catch basins and natural sites such as ditches to establish if species of concern are breeding in an area.

The health unit actively monitors the surveillance of larvae. Larval dipping determines the number and type of mosquito larvae in various locations. Larval dipping is conducted in sites across Simcoe Muskoka, including catch basins and some natural areas, on a weekly basis. Storm water management ponds are also monitored regularly. The health unit will be reviewing and analyzing the data and should larviciding be needed, the health unit will discuss with the municipality when a larviciding program will begin.

Municipalities are notified if larviciding is needed in their area for WNv control, to reduce the type of mosquitoes that spread the virus among birds.

Larvicides are preferred over the use of adulticides (adult mosquito control) because larvicides selectively target mosquito larvae and are a more environmentally acceptable control method. The Ministry of Environment has approved methoprene for use in catch basins and sewage lagoons. Methoprene, when used according to directions, is not harmful to humans. Monitoring studies conducted by the Ministry of Environment have shown that methoprene is not detected in drinking water or surface water. Methoprene is applied in solid form so there is no exposure through the air.

For more information about larviciding see the following fact sheets:

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