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Middlesex-London Health Unit

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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. Learn about how you can protect yourself from getting West Nile Virus by reading about prevention and personal protection.

What are the signs and symptoms of West Nile Virus?

Most people (70% to 80%) who are bitten by a mosquito infected with West Nile Virus (WNV) will have no symptoms at all.2 The Government of Canada2 indicates that when infection does cause mild illness (e.g. fever, headache, body aches, mild rash, swollen lymph glands), these symptoms will appear within two to 15 days after a person has been bitten by an infected mosquito. Symptoms change from person to person. A few people will experience more severe symptoms of West Nile Virus infection. In serious cases, West Nile Virus infection can affect the brain and spinal cord.

View the Government of Canda's web page for more information about signs and symptoms of West Nile Virus

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 common mosquito ‘vectors’ that are able to transmit West Nile Virus.3 



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 Virus. 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, control and standing water,

Additional Information

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

For more information about West Nile Virus, please contact the Vector-Borne Disease team at:

  • 519-663-5317


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: March 11, 2024


1Government of Canada. (2015, June 26). Causes of West Nile virus. Retrieved from
2Government of Canada. (2015, June 26). Symptoms of West Nile virus. Retrieved from
3Ontario. Ministry of Health. (2023, April). West Nile Virus preparedness and prevention plan. Retrieved from