West Nile Virus Risk Assessment

In 2003, Toronto and Region Conservation Authority (TRCA) initiated the West Nile Virus Surveillance and Monitoring Program as a measure of due diligence and in cooperation with our Regional Public Health partners in Durham, Peel, York and the City of Toronto.

What is West Nile Virus?

West Nile Virus primarily exists between birds and bird-biting mosquitoes. There are 57 mosquito species occurring in Ontario, of which only 13 are capable of transmitting the virus.

Humans contract West Nile Virus through the bite of an infected mosquito (i.e. vector) that has fed on an infected bird. Humans are considered dead-end hosts, meaning humans can be infected with the virus but do not spread it.

For people who do become infected, the majority will have no symptoms or mild flu-like symptoms. Severe cases, including the development of meningitis and encephalitis, are extremely rare but can be fatal.

More information can be obtained from your Regional Public Health Unit.

What Are We Monitoring?

TRCA monitors mosquito populations in natural wetlands and stormwater management ponds on our properties throughout the summer.

The data collected are used to identify sites that contain a large number of mosquito larvae that are capable of transmitting West Nile Virus as biting adults. These sites are identified as hot spots and followed up with appropriate management actions.

This program complements virus vector source reduction activities carried out by our Regional Public Health partners on municipal properties.

TRCA monitors mosquito populations in natural wetlands and stormwater management ponds on our properties throughout the summer
TRCA monitors mosquito populations in natural wetlands and stormwater management ponds on our properties throughout the summer
TRCA monitors mosquito populations in natural wetlands and stormwater management ponds on our properties throughout the summer

TRCA monitors mosquito populations in wetlands and stormwater management ponds throughout the summer.

In addition to monitoring, the program includes public education and collaboration with Regional Public Health Units. Concerns raised by citizens or staff are addressed through TRCA’s Standing Water Complaint Procedure.

What Are the Data Telling Us?

TRCA has monitored mosquito larvae in wetlands and stormwater management ponds (SWMPs) since 2003. Results have consistently shown that natural wetlands do not pose a serious West Nile threat.

The majority of the mosquito larvae collected in wetlands have been non-vector species: those not capable of transmitting the virus.

However, the most common mosquito species found in SWMPs was Culex pipiens, the most important vector species.

The 2019 figure below shows mosquito species composition in stormwater management ponds. Vector species are in red, non-vector species in green.

pie chart shows that West Nile Virus vector species make up the preponderance of mosquitos found in stormwater management ponds

Larvae surveillance is not only important for the timely detection of vector species and their abundance, but is also vital in predicting adult mosquito emergence and the potential of humans contacting the virus.

More Information

TRCA West Nile Virus Larval Mosquito Surveillance

Regional Public Health Units

Contact

TRCA Watershed Planning and Ecosystem Science: wpes@trca.ca