Summer is fun, but mosquitoes and West Nile virus…not so much


23 July 2019

Cases of West Nile virus (WNV) human infections in Ontario have fluctuated in the last decade from less than 10 in 2010 to more than 270 in 2012. As of June 16, 2019, we have yet to detect human WNV infections or WNV-positive mosquitoes, keeping in mind that WNV activity in mosquitoes usually doesn’t increase until late July.

Predicting a season’s WNV activity is challenging because of the complex interplay of weather and the ecology of vector mosquitoes, bird reservoirs and humans. In spite of these challenges, we have early warning signs that can help us prepare for WNV. As temperatures increase in July and August, we can anticipate a build-up of WNV in mosquitoes, increasing the risk of human infection. Early detection through surveillance, coupled with increasing public awareness, promotion of personal protection and control of immature mosquitoes, are public health’s main tools in preventing WNV infection. For more information, please refer to the Ministry of Health and Long-Term Care’s West Nile Virus Preparedness and Prevention Plan.

Public Health Ontario’s WNV surveillance webpage is a one-stop resource for monitoring local and regional WNV activity. Our interactive page reports on current and historical WNV human infections and WNV in mosquitoes, with a map highlighting the warmest areas of the province. Weekly summaries report the relative abundance of each species and mosquito species positive for WNV. For the latest on the epidemiology and clinical aspects of WNV in Ontario, check out our recent work in the Canada Communicable Disease Report.

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Updated 23 July 2019