Map of Simcoe Muskoka


Mosquito Surveillance

Key Messages
Adult Mosquito Surveillance
Historic West Nile Virus Surveillance
Eastern Equine Encephalitis Virus (EEEV)
Larval Surveillance
Technical Notes

Key Messages

  • There were no pools of mosquitoes collected in 2021 or 2022 that contained mosquitoes that tested positive for West Nile Virus.
  • No traps have tested positive for Eastern Equine Encephalitis Virus since 2013.

Adult Mosquito Surveillance

  • In 2022 there were 33 traps where mosquitoes were collected
  • There were no pools of mosquitoes containing mosquitoes positive for West Nile Virus in 2021 or 2022.
  • From 2005 to 2022, between 0 and 5 traps tested positive for West Nile Virus each year.
  • In previous years, positive pools have been found in various municipalities across the health unit.
  • There were three horses in Ontario that tested positive for West Nile Virus in 2021 and no horses tested positive in 2022. Four horses in Simcoe Muskoka have tested positive for West Nile Virus in 2017, no local horses have tested positive since.


Historic West Nile Virus Surveillance

  • On average, 9% of mosquitoes found in traps are Culex pipiens or Culex restuans, the mosquitoes most commonly associated with transmitting 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, as measured by accumulated degree days (ADD).


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 (EEEV)

  • Fifteen traps were tested for Eastern Equine Encephalitis Virus (EEEV) in 2022.
  • No traps in Simcoe Muskoka have contained mosquitoes testing positive for EEEV since 2013.
  • Between 43% and 80% of mosquitoes trapped in SMDHU are species that can carry EEEV.
  • In 2021, there was one horse that tested positive for EEEV in Ontario, none in Simcoe Muskoka.
  • In previous years, there have been between 0 and 24 horses testing positive for EEEV in Ontario.
  • In Simcoe Muskoka, there were two horses in 2020 and one in 2017 that tested positive for EEEV. Horses are only tested for EEEV in response to symptoms.
  • In 2016, one human case of EEEV was reported in Ontario.

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.

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.

Larval Surveillance

  • Of all the larvae collected by the health unit in 2022 and submitted to the provincial lab for testing, 28% were species that can transmit West Nile Virus between animals or can infect humans.
  • In 2022, 334 samples were collected from open water sources across the health unit.
  • The majority of these samples (93%) had low numbers of mosquito larvae (≤25).

Technical Notes

West Nile Virus (WNV) and Eastern Equine Encephalitis Virus (EEEV) are viruses that causes disease in humans and other animals. WNV is a reportable disease in Ontario. The health unit sees an average of one to two human cases per year. 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.

Since 2003, the Simcoe Muskoka District Health Unit has been conducting mosquito surveillance for 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 2016.

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.

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.

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.

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.

Further Reading

For more information on mosquito-borne diseases and surveillance, see:

  • Simcoe Muskoka HealthSTATS West Nile Virus infection page
  • Simcoe Muskoka West Nile Virus website
  • Public Health Ontario West Nile Virus Surveillance

    Page last modified March 8, 2023