West Nile Virus Monitoring
The TRCA West Nile Virus (WNV) Monitoring Program was established in 2003 to conduct vector larval surveillance for the presence of two key vector mosquito species, Culex pipiens and Culex restuans, on TRCA properties. The work complements WNV vector source reduction activities carried out by TRCA's Regional Health partners in Durham, Peel, York and the City of Toronto. In addition to monitoring, the WNV program includes public education and collaboration with Regional Health Units. Concerns raised by citizens or staff are addressed through TRCA's Standing Water Complaint Procedure, and TRCA participates on Regional WNV advisory committees, and in information sharing/notification about vector hot spots.
Surveillance comprises an ongoing seasonal assessment of selected TRCA natural wetlands and Storm Water Management Ponds (SWMPs) to determine the presence of WNV vector larvae, the abundance of both vector and non-vector larvae, and to identify vector "hot spots".
For more information on West Nile Virus see our Fact Sheet.
Selected monitoring sites are visited four times per season from May through September. Using a standard dipper, 10 dips are collected, the larvae counted and pooled into a replicate sample. Four replicate samples are taken per visit and subsequently identified to species in the lab.
Risk ranking for each site is assessed for each vector species based on the average number of vector larvae found in 4 replications. A site is ranked as:
nil/no risk site if no vector larvae are present
low risk site if the average number of vector larvae collected is below 2 per sample
moderate risk site if the average number of vector larvae collected is between 2-30 per sample
high risk site if the average number of vector larvae 10 dips is greater than 31 per sample
Measurements of water temperature, pH, electrical conductivity, total dissolved solids and dissolved oxygen are also taken to quantify the relationships between mosquito species presence/abundance and water quality parameters. Qualitative Information about water clarity, the type of predators present at the time of site visit, marginal and total vegetation is also recorded.
Results and Highlights
TRCA has monitored mosquito larvae in 36 wetlands and 9 storm water management ponds over the last 5 years. Results have consistently shown differences between natural wetlands and storm water ponds in terms of the species and abundance of mosquito larvae found.
The majority of the mosquito larvae collected in natural wetlands over the past 5 years have been non-vector species. This suggests that naturally functioning wetlands do not generally support high numbers of vector species. Wetlands were also found to have a highly diverse mosquito community, with the non-vector Culex territans being the most common species found. Overall, natural wetlands had a higher total abundance of mosquito larvae and showed increases in years with higher precipitation.
Storm Water Management Ponds (SWMP)
Storm water management ponds were found to have an overall lower total abundance of mosquito larvae than wetlands, and a lower species diversity, but a higher proportion of vector species. The vector Culex pipiens was the most common mosquito species found in storm water management ponds.
Wetlands have high species richness and abundance of mosquitoes, with most being non-vectors for WNV
Storm water management ponds have a lower species richness and abundance with a higher proportion and abundance of vectors of WNV
Since both wetlands and SWMPs contain occasional isolated pockets of high densities of larvae, continuous monitoring is warranted
Overall, the number of both wetlands and SWMPs ranked as high risk sites is low
All mosquitoes are carriers of WNV
Presence of large number of mosquitoes is always a serious concern for WNV
Wetlands pose significantly higher risks than SWMPs
West Nile Virus Monitoring Reports:
2013 WNV Annual Report - 1,419 KB New!
West Nile Virus Fact Sheet - 337 KB
Wetlands and West Nile Virus - Fact and Fiction. 2010 GTA Conservation Authorities Watershed Monitoring Forum
Ontario Ministry of Health and Long-term Care - http://www.health.gov.on.ca/en/
Peel Public Health - http://www.peelregion.ca/health/
Toronto Public Health - http://www.toronto.ca/health/
York Region Public Health and Safety - http://www.york.ca/Services/Public+Health+and+Safety/default+Public+Health+Services.htm
West Nile Virus:
West Nile Virus (WNV) is a mosquito borne virus that normally cycles between birds and adult mosquitoes. On rare occasions, the virus is spread to humans through the bite of an infected mosquito. This is likely to happen when the virus amplifies in the bird population through a cycle of bird-mosquito-bird transmission. The cycle is driven predominantly by mosquitoes that feed exclusively on birds (enzootic vectors). As greater numbers of birds become infected, there is an increased chance that the virus will spill over into the human population through mosquitoes that feed on both birds and mammals (bridging vectors).