Map of Simcoe Muskoka


Mosquito Surveillance

Mosquitoes are not only nuisances, they have the potential to carry diseases that can be harmful to human health. There are more than 60 species of mosquitoes in Ontario, but only a few of these species can carry pathogens that can cause disease in humans.

West Nile virus (WNv) and Eastern Equine Encephalitis virus (EEEv) are viruses that causes disease in humans and other animals. More information about WNv infection in humans is available on the WNv HealthSTATS page. WNv was first detected in North America in 1999, and in birds and humans in Ontario in 2002. In Ontario, West Nile virus is most commonly carried by mosquitoes from the species Culex pipiens and Culex restuans.

WNv is a reportable disease in Ontario. The health unit sees an average of one to two human cases per year. For more information on human cases of WNv infection, refer to our infectious diseases page.

Since 2003, the Simcoe Muskoka District Health Unit has been conducting mosquito surveillance or WNv in our area by trapping and testing adult mosquitoes to determine whether they are carrying West Nile virus, and by dipping standing water sites for larvae to understand what species are present and breeding in our area. Mosquitoes are also tested for Eastern Equine Encephalitis virus (EEEv). The first human case of EEEv in Ontario was identified in 2017.

Adult mosquito trapping and larval dipping occurs at various sites across the health unit region. This surveillance data helps to guide local decision making and risk assessments, and to identify areas where an increased risk of exposure to virus may exist.

For more information about these activities, see the health unit’s pages on West Nile virus surveillance.

Adult mosquito surveillance

In 2018, there were 35 active trap sites across the health unit region. Traps are set in secure locations, such as homeowner’s backyards, throughout Simcoe County and the District of Muskoka. These trap sites are selected based on where mosquitoes known to transmit WNv are known to breed (urban areas), and in areas populated by humans.

A “pool” is deemed positive when the collection of mosquitoes tests positive for either WNv or EEEv. Positive mosquito pools serve as an indicator of risk of exposure to WNv or EEEv and informs us of current vector and viral activity. Evidence suggests that as the number of mosquitoes carrying the virus increases, risk of transmission increases leading to human case reporting. Mosquito activity and subsequent positive pools are an important indicators for determining disease and exposure risk within a community.

For more information on WNv surveillance activities in Ontario, refer to Public Health Ontario’s West Nile Virus Surveillance page.


It is possible that there are WNv positive mosquitoes in other towns or municipalities where mosquito traps are not currently being set, but the decision of where to set traps is based on risk assessment and understanding of what is happening in our local communities. Adult mosquito trap site locations are usually in the same spot year over year, but sites may move or new sites may be added in response to the previous year’s mosquito surveillance or to human cases of WNv.

Historic West Nile virus surveillance

Since 2005, between zero and three mosquito traps have tested positive for WNv each year. There were three positive traps in 2018 in Barrie, Bradford West Gwillimbury and New Tecumseth. In previous years, positive traps have been also been found in Collingwood, Gravenhurst, Huntsville and Orillia.

On average, around 10% of trapped mosquitoes in Simcoe Muskoka are Culex pipiens and Cx. restuans, the species most associated with transmission of the West Nile virus to humans. The number and proportion of Cx pipiens and Cx. restuans mosquitoes varies from year to year, and is affected in part by outside temperatures.

Accumulated degree days (ADD) measure accumulated heat each year. Mosquitoes move through their life cycles faster, and the WNv multiplies faster in mosquitoes in warmer temperatures. Years with higher ADD are typically associated with a higher presence of WNv carrying mosquitoes, and a higher risk of transmission of WNv to humans.


Eastern Equine Encephalitis virus

Culiseta melanura is the main EEEv vector in Ontario, and is usually found in swamps. Cs. melanura does not typically bite humans. The Aedes vexans and Coquilettidia perturbans species of mosquitoes are common “bridge vectors” for EEEv, meaning they can transmit the virus from an infected animal into a human through feeding. Ae. vexans and Cq. perturbans are also more easily captured by the light traps used in SMDHU’s mosquito surveillance.

The number and proportion of mosquitoes trapped which can carry and transmit EEEv has varied over time, with Ae. vexans and Cq. perturbans representing between 43% and 80% of mosquitoes identified each year and Cs. melanura usually making up less than 5% of mosquitoes identified each year.

In 2017, one horse in Simcoe Muskoka tested positive for EEEv. Horses are only tested for EEEv in response to symptoms. In 2018, 13 horses tested positive for EEEv across Ontario, none in Simcoe Muskoka. In previous years, this number has varied between 0 and 24 horses testing positive for EEEv in Ontario. In 2017, one human case of EEEv was reported provincially.

Larval surveillance

In addition to trapping adult mosquitoes, the health unit collects samples from catch basins, ditches and storm retention ponds to count how many and what kind of mosquito larvae are present. Larvae are the immature form of the insect that have not yet become adult mosquitoes capable of biting and transmitting disease.

In 2018, 765 samples were collected from open water sources across the health unit. The majority of these samples (79%) had low numbers of mosquito larvae (≤25). Of all the larvae submitted to the provincial lab for testing, 21% were species that can transmit WNv between animals or can infect humans.