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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 measures 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.

On this page, you will find information and resources about WNv and prevention measures to reduce the risk of illness. This page also provides information on how the risk of WNv is impacted by our changing climate.

West Nile virus (WNv) is a mosquito-borne virus spread to people by the bite of an infected mosquito. Mosquitoes become infected with the virus after they bite an infected bird.

WNv can cause illness in people, and in rare cases, meningitis, encephalitis (inflammation of the brain), and acute flaccid paralysis (loss of muscle function). Many people infected with WNv will have mild or no symptoms. 

You can reduce your risk of WNv by adopting protective health behaviours detailed below.

Although WNv is one of the most common mosquito-borne illness in Canada, the risk of illness is relatively low provincially and locally. However, due to our changing climate, the risk of WNv is likely to increase (see the Climate Health Connection section below).

WNv can be transmitted by the bite of an infected mosquito at any time, however, the risk is greatest in summer and early fall. People are more likely to be bitten by mosquitoes at dawn (first light) and dusk (just before dark), when mosquitoes are most active.

Those at greatest risk of severe illness from WNv, including older adults, people with chronic diseases (e.g., heart disease, diabetes), or weakened immune systems (e.g., people who receive chemotherapy). 

We conduct surveillance for WNv to help inform our risk assessments. More details on surveillance can be found in the surveillance expander bar below.

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. Encephalitis (inflammation of the brain), meningitis, or acute flaccid paralysis (loss of muscle function) can occur in a small number of cases. 

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 WNv by taking simple steps to prevent mosquito bites. Make sure you:

  • wear light-coloured clothing (mosquitoes are attracted to dark colours);
  • cover exposed skin. Wear long-sleeved shirts, long pants, socks, and a hat;
  • use insect repellant according to the manufacturer's directions;
  • reduce mosquito breeding near your property. More details are provided in the expander bar below; and
  • use screens in your windows and doors to keep mosquitoes out of your home.

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.

There are a number of insect repellant products approved for use in Canada. For more information on approved products and safe use, please see personal insect repellants from Health Canada.

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:

  • 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 Northern Development, Mines, 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, wheelbarrows, and pots;
  • store small boats and canoes upside down;
  • be sure boat covers do not have water;
  • screen the top of rain barrels; 
  • 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 and investigate reports of humans who have been diagnosed by a health care provider with WNv.

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 (e.g., ditches). We do this to monitor the types of mosquitoes breeding in an area.

To determine the need for mosquito control in local municipalities, we review current and past surveillance information to make decisions about the need for the application or larvicide in municipal catch basins. Larviciding reduces the type of mosquitoes that spread the virus. We work with municipalities to implement larviciding programs where needed. 

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 Encephalitis Virus (EEEV), and Zika Virus are other diseases that can be spread through the bite of an infected mosquito.

Eastern Equine Encephalitis Virus

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.

Climate Health Connection

Our climate is changing and these changes are affecting human health and wellbeing in Simcoe Muskoka, including vector-borne diseases transmitted by mosquitoes.

Rising temperatures and increasing precipitation associated with climate change affects mosquitoes (the vector) and disease transmission by making environments more suitable for mosquitoes and the diseases they can carry. These climactic changes are:

  • Increase reproduction rate (more generations of mosquitoes in each season) leading to larger mosquito populations.
  • Reduce the time it takes for mosquitoes to become infectious to humans and animals (horses). Create longer seasons where mosquitoes are active.
  • Support habitat for invasive mosquito species with the ability to transmit WNv and other vector-borne diseases.
  • Milder winters allow for over-winter survival of infected mosquitoes.

Climate change mitigation and adaptation will combat climate change and reduce the risks of mosquito-borne illnesses including WNv and other climate-health risks.

SMDHU is engaged in a number of strategies to help our community adapt to the changing climate-health risks. To help our community adapt to the increasing risk of mosquito-borne illness, SMDHU is engaged in assessing health risks and vulnerability, monitoring and surveillance, and providing public health advice and risk communication. SMDHU also works with our municipal and community partners and the public to understand climate-health risks and implement climate adaptation and mitigation strategies that expand health and economic co-benefits.

Individuals can adapt to increasing mosquito-borne diseases by understanding the risk for themselves and their family and taking protective actions. You can help to reduce climate health risks by taking individual climate action and encouraging all levels of government and businesses to implement climate adaptation and mitigation efforts.
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