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West Nile Virus

West Nile Virus (WNV) is a mosquito borne virus that is spread to humans through the bite of an infected mosquito.1 West Nile Virus has been found in many countries in the world, including Canada and the USA.


How does West Nile Virus affect humans?

Humans get West Nile Virus (WNV) from the bite of an infected mosquito.1 The transmission cycle (how the virus is passed) begins when mosquitoes feed on the blood of an infected bird. Once a mosquito bites an infected bird, it becomes infected with WNV and could possibly transmit the virus to humans through a mosquito bite.

Can all mosquitoes transmit West Nile Virus?

A 'vector' is the term used to describe a mosquito species that can transmit (pass) disease through a mosquito bite. Not all mosquito species are vectors. A ‘non-vector’ or ‘nuisance mosquito’ cannot carry a disease, and therefore cannot transmit disease to humans. Vector mosquito species must first bite an infected bird or animal in order to become infected themselves and able to pass the disease on through a mosquito bite. In Ontario, there are 12 different mosquito ‘vectors’ that are able to transmit West Nile Virus.2




Adult Mosquito

A picture of a mosquito biting a person


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Health Unit Finger Character getting a mosquito bite

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What is being done about West Nile Virus in Middlesex-London?

The Middlesex-London Health Unit monitors and controls vector mosquito species in order to reduce the spread of West Nile Virus and other vector-borne diseases such as Eastern Equine Encephalitis. The Health Unit performs surveillance and sampling at about 250 standing water sites located on public property throughout Middlesex-London. Staff monitor standing water sites and collect mosquito larvae samples. If the larvae collected are identified as vector species, then licensed staff will perform a larvicide treatment on that area of standing water. Learn more by reading about mosquito surveillance and control. If you have mosquito or standing water concerns, please contact the Health Unit.

Additional Information

View the Government of Canada's West Nile Virus web page for more information.

For more information on the activities carried out by the Vector-Borne Disease Team, please continue to browse this website, or call:

  • 519-663-5317 ext. 2300 

For more information on Vector-Borne Diseases and human health, please continue to browse this website, or contact the Infectious Disease Control Team by calling:

  • 519-663-5317 ext. 2330


Please note: Where indicated, the source of the information on this web page is the Government of Canada's West Nile Virus web page, which is a copy of the version available at the URL in the references section below, OR the Ministry of Health and Long-Term Care. © Queen’s Printer for Ontario, 2010.

Date of creation: October 1, 2012
Last modified on: May 10, 2018


1Government of Canada. (2015, June 26). Causes of West Nile virus. Retrieved from
2Ontario. Ministry of Health and Long-Term Care, Public Health Division. (2010, June 2). West Nile Virus preparedness and prevention plan. Toronto, ON: Queen’s Printer for Ontario.