PREPARED BY: Environmental Health 

DATE: 2025-11-20

The Windsor-Essex County Health Unit’s (WECHU) vector-borne disease surveillance program involves identifying disease trends in humans, determining the distribution of the mosquito and tick populations throughout the Windsor-Essex County (WEC), and taking a proactive approach to inform the community about prevention strategies to reduce the risk of vector-borne diseases through the comprehensive public awareness campaign “Fight the Bite!". 

The mosquito surveillance program consists of the following components: identifying mosquito larval populations, larvicide treatments of municipal catch basins, and adult mosquito trapping for species identification and testing for West Nile Virus (WNV), Eastern Equine Encephalitis (EEE), and Zika Virus. As Lyme disease is spread by the bite of infected blacklegged ticks, active tick dragging is conducted to assess areas where blacklegged ticks may be established as climate change continues to impact tick population. This report provides a summary of the program outcomes for the 2025 season. 

DISCUSSION

Mosquito Surveillance program: The program ran for a total of 21 weeks from May 20 to October 9, 2025. Once a week, 40 mosquito traps (25 CDC light traps and 15 BGS-2 traps) were set up across WEC to collect mosquitoes for identification and viral testing. The number of mosquitoes trapped increased by approximately 30,000 as compared to last year (see Table 1). However, the number of positive mosquito pools for WNV and human cases remained consistent with the previous year. The surveillance program remains an important tool for tracking both the mosquito population and the spread of WNV and emerging vector borne (mosquito) diseases in our region.

In 2024 and 2025, there were no positive pools or human cases for EEE or Zika identified. These two diseases are spread by the Aedes albopictus mosquito. 

The WECHU also received reports from Public Health Ontario (PHO) of seven dead birds in WEC that tested positive for WNV. Mosquitoes become infected with WNV when they feed on infected birds. The bite by an infected mosquito can then transmit WNV to humans and other animals. Table 2 (see below) indicates that the number of WNV positive in birds in Ontario has increased over the years. However, no cases of WNV positive in birds were reported in WEC between 2021 and 2024. The surveillance of infected birds provides additional information of the prevalence of the WNV in WEC. 

Data Source: Canadian Wildlife Health Cooperative -WNV Testing Results

Mosquito larviciding program: Mosquito larviciding involves applying larvicides to catch basins, standing water sites, and lagoons that hold water where mosquitoes may breed. Larvicides are chemical or biological agents that control mosquito populations by preventing mosquito larvae from developing into adults. To support municipalities, the WECHU contracts a service provider to conduct larvicide treatment between May and September. In 2025, 134,485 roadside catch basins through three rounds of larvicide treatments. As a result of the increase in disease activity in birds and mosquitoes, an additional round of larviciding was applied to key areas in the community (18,233 catch basins). 

Active Tick Surveillance: The WECHU conducts active tick surveillance (tick-dragging) twice per year, in the spring and fall, to track the migration of tick populations throughout our region and identify tick-borne diseases they are carrying. The field locations selected for active tick dragging is based on the PHO’s blacklegged tick risk area map, in areas where blacklegged ticks are not yet established but environmental conditions of the area suggest an increased risk. The PHO map utilizes data from previous years' tick dragging results as well as community data from the eTick.ca platform, which is a free online tool for residents to submit pictures of ticks that they find on themselves or their pets. 

Tick dragging involves the dragging of a white cloth through grassy areas whereby ticks attach themselves to the fabric and can be easily spotted and identified. In 2025, active tick dragging was conducted at four sites across WEC in May and September. No blacklegged ticks were found this year. To date, there have been 14 Lyme disease cases in humans in 2025 (17 cases in 2024). Prior to 2024, the average annual number of  human cases of Lyme diseases was seven. Based on the increasing local risk of Lyme disease, the WECHU continues to advocate for prevention-based public policies. In 2025, the WECHU Board of Health endorsed a resolution with recommendations for comprehensive municipal strategies to reduce tick habitats and/or limit human contact with tick hotspots in our region.