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Additional Information

Program Highlights

♦ A total of 41 wetlands and 6 stormwater management ponds were monitored in 2015.

♦ In total, 13 mosquito species including 8 WNV vector species (i.e. those capable of transmitting the virus) and 5 non-vector species were identified. The two key WNV vectors, Culex pipiens and Culex restuans, were found at 21 (45%) and 10 (21%) of the sampled sites respectively.

♦ In total, 8 sites were identified as hotspots, those that show a potential West Nile Virus risk. With the assistance from our regional health partners, control measures were taken to reduce the presence of larvae at these sites.

More Information

West Nile Virus Vector Larval Mosquito Report, 2015 

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Regional Public Health

City of Toronto

Durham Region

Peel Region

York Region



Jessica Fang

Invertebrate Technologist

Environmental Monitoring & Data Management

West Nile Virus Monitoring

WNV Sampling

What Are We Monitoring And Why?

West Nile Virus (WNV) primarily exists between birds and bird-biting mosquitoes. Humans contract the virus through the bite of an infected mosquito that had fed on infected birds. Humans are considered dead-end hosts, meaning humans can be infected with the virus but do not spread it. For people who become infected, the majority will have no symptoms or only mild flu-like symptoms. Severe cases of WNV, including the development of meningitis and encephalitis, are extremely rare but can be fatal.

In 2003, TRCA initiated the WNV Surveillance and Monitoring Program as a measure of due diligence and in cooperation with our Regional Public Health partners in DurhamPeelYork and the City of Toronto. Mosquito populations in natural wetlands and stormwater management ponds (SWMPs) on TRCA properties are monitored throughout the summer months. The data collected are used to identify sites of potential concern or vector mosquito "hot spots" and then follow up with appropriate management actions. This program complements WNV vector source reduction activities carried out by TRCA's Regional Health partners on municipal properties. 

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.

What Are The Data Telling Us?

There are 57 mosquito species occurring in Ontario, of which only 13 species are WNV vectors. Most other mosquito species do not pose serious WNV threats and their larvae are important food sources for fish and other predatory aquatic organisms.

TRCA has monitored mosquito larvae in wetlands and SWMPs since 2003.   Results have consistently shown that natural wetlands do not pose a serious WNV threat. The majority of the mosquito larvae collected in wetlands have been non-vector species, those not capable of transmitting the virus. On the other hand, the vector species Culex pipiens was the most common mosquito species found in SWMPs.

The following figure shows Culex pipiens (the most important vector WNV species) abundance, the numbers of WNV positive mosquito pools, and human WNV cases in 2012 and 2013. This figure shows that larvae surveillance is not only used for the timely detection of WNV vector species and their abundance, but is also vital in predicting adult mosquito emergence and the potential of humans contacting the virus. 

WNV Positive Pools and Human Cases 2013

Vectors:A vector is an organism that transmits a disease or parasite from one animal to another. West Nile Virus vector mosquitoes are the mosquito species which are capable of carrying and transmitting the virus.

Positive Mosquito Pools:mosquito pool is a collection of mosquitoes (usually about 50) of any particular species that are likely to carry a virus. A WNV positive mosquito pool hence is a pool that has been tested positive for the WNV in the lab.

WNV Larvae

Photo: Mosquito Larvae.

Vector:A vector is an organism that transmits a disease or parasite from one animal to another. West Nile Virus vector mosquitoes are the mosquito species which are capable of carrying and transmitting the virus.