Mosquito-borne Diseases
Learn about local risk factors for West-Nile virus and other mosquito-borne diseases.
Mosquitoes are common in Ontario and are most active during the warmer months. Some species can spread disease, but the risk of becoming seriously ill from a mosquito-borne disease in Wellington-Dufferin-Guelph is very low.
Public Health monitors mosquito activity to identify potential risks early and reduce the chance of disease spreading to people and animals. Targeted, environmentally friendly larvicide is used in areas such as stormwater ponds and catch basins to help reduce mosquito populations.
Avoiding mosquito bites is the best way to protect yourself.
Mosquito surveillance in our region
Mosquito surveillance takes place during the spring and summer months. Monitoring helps identify which mosquito species are present and whether they are carrying viruses such as West Nile virus. Visit our Zoonotic Vector-Borne Diseases Dashboard (ZVBD) for more information.
Mosquito-borne diseases detected in Ontario
The below mosquito-borne diseases have been detected in Ontario. In Wellington-Dufferin-Guelph, the risk of becoming seriously ill remains low, but avoiding mosquito bites is still important.
Wellington-Dufferin-Guelph Public Health conducts mosquito surveillance for West Nile virus and Eastern Equine Encephalitis virus (EEEV).
Risk in our region: Low
West Nile virus (WNV) is a viral infection spread through the bite of an infected mosquito. It is present in Ontario and across much of Canada, with risk highest in the summer and early fall when mosquitoes are most active.
Most people infected with West Nile virus do not develop symptoms. When symptoms do occur, they are usually mild and flu-like.
Symptoms usually begin within 2 to 14 days after a mosquito bite and may include:
- Fever
- Headache
- Body aches
- Fatigue
- Nausea or vomiting
In rare cases, West Nile virus can cause serious illness, including inflammation of the brain or spinal cord. Adults over the age of 50 are at higher risk of severe illness.
There is no specific treatment for West Nile virus. Contact your health care provider if you develop symptoms after a mosquito bite.
Risk in our region: Very low
Eastern Equine Encephalitis virus (EEEV) is a rare mosquito-borne virus that can infect humans and horses. It is spread through the bite of an infected mosquito. There is no evidence of human-to-human transmission.
EEEV is not a reportable disease in Ontario, and human cases in Ontario are very rare; one case of EEEV was reported in a horse in Wellington County in 2025.
Symptoms usually begin several days to two weeks after a mosquito bite and may include:
- Fever
- Headache
- Nausea and vomiting
- Weakness or confusion
- Neck stiffness
If encephalitis develops, illness can be severe and life-threatening.
There is no specific treatment for EEEV. Contact your health care provider if you develop symptoms after a mosquito bite.
Emerging mosquito-borne diseases (not established in Ontario)
The following mosquito-borne diseases are not currently established in Ontario and have not been locally acquired in Wellington-Dufferin-Guelph. Most cases are linked to travel outside Canada.
Risk in Ontario: Very low
Chikungunya is a viral infection spread through the bite of an infected mosquito.
Chikungunya is not established in Canada. The risk in Ontario is very low, and cases are associated with travel to regions where the virus is present.
Symptoms usually begin within 3 to 7 days after a mosquito bite and may include:
Fever
Joint pain
Muscle pain
Headache
Fatigue
Rash
Joint pain can be severe and may last for weeks or longer in some people.
There is no specific treatment for Chikungunya. If you develop symptoms after travelling to an area where Chikungunya is present, contact your health care provider.
Risk in Ontario: Very low
Dengue fever is a viral infection spread through the bite of an infected mosquito.
Dengue is not established in Canada. The risk in Ontario is very low, and cases are associated with travel to tropical and subtropical regions where the virus is present.
Symptoms usually begin within 4 to 10 days after a mosquito bite and may include:
Fever
Headache
Pain behind the eyes
Muscle and joint pain
Nausea or vomiting
Rash
In rare cases, dengue can cause severe illness and may require hospitalization.
There is no specific treatment for dengue fever. If you develop symptoms after travelling to an area where dengue is present, contact your health care provider.
Risk in Ontario: Very low
Zika virus is a viral infection most commonly spread through the bite of an infected mosquito. It can also be spread through sexual contact.
Zika virus is not established in Ontario. No mosquitoes in Ontario have tested positive for Zika virus, and no locally acquired cases have been reported. Most cases identified in Canada are related to travel.
Most people infected with Zika virus do not develop symptoms. When symptoms do occur, they are usually mild.
Symptoms may include:
- Fever
- Rash
- Joint pain
- Red eyes (conjunctivitis)
Zika infection during pregnancy can cause serious birth defects.
There is no specific treatment for Zika virus. If you are pregnant or planning to become pregnant and have travelled to an area where Zika is present, contact your health care provider for advice.
Protect yourself from mosquito bites
Avoiding mosquito bites helps reduce the risk of all mosquito-borne diseases.
- Use insect repellent with DEET or Icaridin
- Wear long sleeves and pants when possible
- Avoid being outdoors at dawn and dusk
- Reduce standing water around your home
For practical steps, visit the Mosquitos page.
