Map of Simcoe Muskoka

Environment

Mosquito Surveillance

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

Key Messages

  • No mosquitoes trapped by the health unit in 2019 tested positive for West Nile virus.
  • Three traps contained mosquitoes positive for Eastern Equine Encephalitis virus

Adult Mosquito Surveillance

  • In 2019 there were 33 traps where mosquitoes were collected
  • No traps contained mosquitoes testing positive for West Nile virus in 2019.
  • Three traps contained mosquitoes testing positive for Eastern Equine Encephalitis virus (EEEv), in Huntsville and Bracebridge.
  • From 2005 to 2019, between 0 and 3 traps tested positive for West Nile virus each year.
  • In previous years, positive pools have been found in various municipalities across the health unit.

2018_Mosq_Trap_Sites

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).

191127MosqperTrap2005to2018

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)

  • Between 43% and 80% of mosquitoes trapped in SMDHU can carry EEEv
  • No humans horses tested positive for EEEv in Simcoe Muskoka or across Ontario in 2019.
  • In 2017, one horse in Simcoe Muskoka tested positive for EEEv. Horses are only tested for EEEv in response to symptoms.
  • In 2019, one horse 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.

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

  • 33% of mosquito larvae collected by the health unit were species that can transmit West Nile virus between animals or can infect humans.
  • In 2019, 600 samples were collected from open water sources across the health unit.
  • The majority of these samples (88%) had low numbers of mosquito larvae (≤25).
  • Of all the larvae submitted to the provincial lab for testing, 33% were species that can transmit WNv between animals or can infect humans.

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 2017.

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: August 18, 2020.