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Infectious Diseases

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West Nile virus

When spending time outdoors from spring to fall, you may come in contact with mosquitoes. Some mosquitoes can carry diseases like West Nile virus (WNv). To reduce your exposure to mosquitoes, we encourage you to reduce mosquito breeding sites around your home or cottage and take personal precautions to prevent mosquito bites. Our role is to reduce public risk to vector borne diseases, like WNv, by providing education and monitoring for the presence of mosquitoes (vectors) of public health concern.

WNv is a mosquito-borne virus that can cause illness in people, in rare cases, encephalitis (inflammation of the brain). WNv can be spread to people by the bite of an infected mosquito. Mosquitoes become infected with the virus after they bite an infected bird.

Human infections from the virus are rare. Most people who are bitten by an infected mosquito do not develop symptoms. Those who become ill usually experience mild symptoms such as fever, headaches, body aches, rash, nausea, or vomiting. These flu-like symptoms often develop about 3 to 14 days after a bite from an infected mosquito and may last for several days.

In extremely rare cases, WNv can cause severe illness involving the central nervous system. Inflammation of the brain (encephalitis) can occur in a small number of cases. Those at greatest risk of severe illness are people over the age of 50 and people with chronic diseases or weakened immune systems.

There is no vaccine or antiviral treatment available for WNv. In most cases, over the counter pain relievers can be used to help reduce fever and other symptoms that you may be experiencing. In rare cases, hospitalization is necessary where supportive treatment will be provided. If you are experiencing symptoms and are concerned, speak with your health care provider.
You can reduce your risk of being bitten by mosquitoes by staying indoors when mosquitoes are most active (from dusk until dawn) and removing standing water sites on your property. When spending time outside, it is important to wear light-coloured clothing, long-sleeved shirts and long pants, and use an insect repellent. When using an insect repellent, be sure to follow instructions on the label.

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 the product's label carefully before using;
  • use small amounts on any exposed skin (do not apply underneath clothes), be careful when using a repellent around your face and if you get repellent in your eyes, rinse with water immediately;
  • use in well ventilated areas;
  • check to make sure you are not sensitive to a product by applying the repellent to a small area on your arm and waiting 24 hours; and
  • if the repellent is causing you or your family to have a reaction, stop using the product immediately, wash the area, and seek medical attention.

Repellents that have P-menthane 3,8-diol and soybean oil can also protect people from 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, visit Government of Canada's website.

A mosquito breeding site is any container or area that contains stagnant water. When stagnant water sits for more than seven days (time required for mosquito eggs to develop into adult mosquitoes), mosquitoes can breed. Examples of containers where mosquitoes can breed are: unscreened rain barrels, artificial ponds, bird baths, pool and boat covers, old tires, and lawn ornaments. By preventing and removing breeding sites on your property, your risk of being bitten by a mosquito is reduced.

To prevent and remove 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 and Forestry before stocking any water body;
  • keep swimming pools, spas, and hot tubs chlorinated and covered tightly when not in use;
  • change the bird bath water weekly;
  • empty water from tires and tire swings;
  • empty buckets, wheel barrows, and pots;
  • store small boats and canoes upside down;
  • be sure boat covers do not have water;
  • drain or fill tree holes;
  • screen or eliminate water in sump pumps;
  • keep eavestroughs and rain gutters clean; and
  • drain plastic covers on lawn chairs and other outdoor equipment weekly.

We monitor mosquito populations by setting mosquito traps and looking for larvae in selected locations, investigate reports of humans who have been diagnosed by a health care provider with WNv, and inform local residents and visitors on how to reduce exposure to insects like mosquitoes.

Mosquito traps are set weekly and mosquitoes within the traps are sent to a laboratory for identification and tested for WNv. This information informs us of the types of mosquitoes present in Simcoe and Muskoka and if they are carrying WNv.

Larval dipping surveillance provides information about the number and type of mosquito larvae present in an area. Larval dipping is the collection of mosquito larvae from sites such as storm water management ponds, catch basins, and natural sites such as ditches. We do this to monitor the types of mosquitoes breeding in an area. Larval dipping is also conducted weekly.

As of 2009, dead birds are no longer collected by the health unit for WNv surveillance information. If you are concerned about a dead bird you have found on your property, the Canadian Wildlife Health Cooperative (CWHC) may be interested. For more information, visit their website or contact them toll-free at 1-866-673-4781.

To determine the need for mosquito control in local municipalities, we review historical and recent data to make decisions about the need for the application of larvicide in municipal catch basins. Larviciding reduces the type of mosquitoes that spread the virus. We work with municipalities to determine if larviciding is needed in their area for WNv control.

Larvicide is preferred over the use of adulticides (adult mosquito control) because larvicides target just mosquito larvae and are more environmentally friendly. The Ministry of the Environment, Conservation and Parks (MECP) has approved methoprene for use in catch basins and sewage lagoons. Methoprene, when used as per the label's instructions, is not harmful to people.

For more information about larviciding, see the MECP website Key Facts About Applying Larvicides to Catch Basins and Stagnant Water Located on Private Property.

Other Mosquito Borne Diseases

In addition to West Nile virus (WNv), Eastern Equine Encephalities virus (EEEv), and Zika virus are other diseases that can be spread through the bite of an infected mosquito.

Eastern Equine Encephalitis

Eastern Equine Encephalitis virus (EEEv) is a vector borne disease spread through the bite of an infected mosquito. Although EEEv is considered rare in humans, it has been identified in horses and mosquitoes. Only one human case has been reported in Ontario to date. For additional information on EEEv, refer to the Centers of Disease Control and Prevention.

Zika Virus

Zika virus is a vector borne disease transmitted by infected mosquitoes. Travelling to countries where Zika virus is present puts you at a greater risk of exposure to mosquitoes that could be infected with the virus. The types of mosquitoes that transmit the virus to humans are predominantly found in tropical climates and the risk of exposure to these mosquitoes in Ontario is low. For more information about the Zika virus, refer to the Government of Canada.

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