Note: This article provides public health information about West Nile Virus risk. Always consult official Ontario public health sources and your healthcare provider for medical advice. Part of our Ultimate Mosquito Control Guide.
West Nile Virus in Ontario: The Facts
West Nile Virus (WNV) is a mosquito-borne illness that has been present in Ontario since 2001. The virus circulates between birds (the primary reservoir host) and mosquitoes, with humans infected incidentally when bitten by an infected mosquito. Culex mosquito species — particularly Culex pipiens — are the primary WNV vectors in the GTA.
Ontario public health agencies, including Toronto Public Health and the Peel Region Health Department, conduct annual mosquito surveillance programs, trapping and testing pools of mosquitoes for WNV. Positive pools are confirmed in the GTA every year, with the highest activity typically in July and August.
Who Is at Risk?
Most people (approximately 80%) infected with West Nile Virus experience no symptoms. About 20% develop West Nile Fever — a flu-like illness with fever, headache, body aches, and sometimes rash. Less than 1% of infected people develop severe neurological illness (encephalitis or meningitis).
However, those at higher risk for severe illness include:
- Adults over 60 years old
- People with compromised immune systems
- People with certain chronic conditions
GTA WNV Risk by City
WNV-positive mosquito pools are detected across the GTA each season. Risk is generally higher in areas with:
- Dense populations of Culex mosquitoes (standing water areas)
- Active bird populations in urban green spaces
- Warm summer temperatures that accelerate mosquito breeding
How Professional Mosquito Control Reduces WNV Risk
Reducing the population of mosquitoes in and around your property directly reduces your exposure risk. Professional barrier spray eliminates adult mosquitoes — including Culex species that carry WNV — from your yard's vegetation, creating a significant reduction in the mosquito density your family is exposed to during outdoor activities.
No mosquito control method eliminates 100% of mosquitoes, but substantially reducing the number of biting mosquitoes in your immediate environment meaningfully reduces the probability of encountering an infected mosquito.