Health Topics

West Nile Virus

West Nile virus is found in the wild bird population.  A mosquito gets infected by feeding on the blood of an infected bird.  An infected mosquito can then bite a person or animal and give them West Nile virus.  Not all mosquito types carry West Nile virus or bite humans.

How do you get West Nile virus?

West Nile virus is spread to humans mainly when an infected mosquito bites them. In a small number of cases, West Nile virus has been spread through blood transfusions, organ transplants, breastfeeding or in pregnancy from the mother to her baby. The virus is not spread by person to person contact such as drinking from the same cup.

Only about 20% of people with West Nile virus have symptoms. Symptoms usually appear two to 15 days after a mosquito bite from an infected mosquito. Symptoms may last several weeks. Some effects may be severe and permanent, including death for a small number of people. 

Signs and symptoms of West Nile virus

Most people have mild symptoms that include:

  • Fever
  • Headache
  • Nausea
  • Vomiting
  • A rash on the chest, stomach or back
  • Body aches

Some people may have severe symptoms including:

  • A stiff neck
  • Confusion
  • Tremors
  • Muscle weakness
  • High fever
  • Severe headache
  • Numbness
  • Sensitivity to light

Seniors and people with weakened immune systems have the highest risk of developing severe illness from West Nile virus.

Less than 1% of people with West Nile virus develop other serious illnesses such as meningitis and encephalitis.  Meningitis is swelling of the lining of the brain or spinal cord.  Encephalitis is swelling of the brain.  These conditions can cause lasting illness or death.

Read about West Nile virus in French (PDF, 22 KB) |  Arabic (PDF, 106 KB) |  Mandarin (PDF, 304 KB)  | ​ Spanish (PDF, 69 KB)

Talk to your doctor if you have think you symptoms of West Nile virus. West Nile virus in humans is confirmed by a blood test or by testing the fluid around the brain and spinal cord. Test results take one to two weeks. There is no specific treatment or vaccine for West Nile virus.

Here are some things that you can do to prevent West Nile virus:

  • Remove standing water from:
    • Bird baths
    • Old tires
    • Unused containers
    • Flower pot saucers
    • Swimming pool covers
    • Wading pools
    • Clogged gutters and eavestroughs
    • Clogged drainage ditches
    • Small containers
    • Unused children’s toys
    • You should remove standing water at least once every seven days.  Mosquitoes lay their eggs in very shallow standing water.  If you remove the standing water they cannot lay eggs. 
  • Report standing water on public or private property.
  • Avoid mosquito-infested areas. They like weeds, tall grass and bushes.
  • Avoid cologne, perfume and scented body lotion when going outdoors.
  • Wear long-sleeved shirts, long pants, hats, socks and shoes; tuck your pants into socks.
  • Repair damaged doors and window screens so mosquitos cannot get inside.
  • Avoid spending time outdoors from evening until early morning, when mosquitoes are most active.
  • Use an insect repellent that contains DEET for your family members that are older than six months of age.

Mosquitoes can become infected with West Nile virus when they feed on the blood of birds infected with West Nile virus. Because mosquitoes feed on infected birds, it is possible to detect  the virus in mosquitoes through laboratory testing. Mosquitoes are captured  using a trap that is baited with  carbon dioxide, using a black  light and a fan to draw mosquitoes into  the trap. Traps are set up overnight across pre-established sites in the City and are collected the next morning. Mosquitoes collected in the traps are counted; species and sex  identified and tested to determine if the mosquitoes are carrying West Nile virus.

Trapping and testing mosquitoes helps the City determine where infected mosquitoes can likely be found, the different types of mosquitoes and the amount of mosquitoes trapped each week.  We use this data to establish our controls which could be include larvaciding, reducing standing water in the area and activities to increase public awareness.

Mosquitoes breed in standing water and tend to stay close to their breeding site throughout their life. If the number of breeding areas can be reduced, we can reduce the number of mosquitoes and West Nile virus in our community.

Report standing Water

Standing water surveillance starts on April 1 each year and continues until October 31, under Hamilton’s Standing Water By-law No. 03-173  (PDF, 61 kB).  We do not investigate complaints outside of these dates.

If you want to report standing water:

We accept complaints for standing water that is more than 2 ½ cm deep at the deepest point with no movement of the water.  If water has been treated to prevent insects from breeding, it is not considered standing water. Treatment may include continuous movement of the water or appropriate pesticide treatment or other actions approved by the Medical Officer of Health.

Public land complaints

We follow up on public land complaints within five business days. The City looks for mosquito larvae at several hundred surface water sites from June to the fall each year.  We treat sites when necessary to reduce or eliminate the amount of mosquitoes developing in the water.  This helps control the mosquito population in Hamilton.

Private land complaints

We follow up on private land complaints within five business days.  The City investigates and enforces private land complaints that fall within the scope of Hamilton’s Standing Water By-law No. 03-173.

Notice of application of larvicides to City of Hamilton for West Nile Virus control 

The City of Hamilton Public Health Services is conducting ongoing monitoring of mosquito larvae (immature mosquitoes) in standing surface water within the City of Hamilton boundaries on City land. When such monitoring indicates that there is a need for mosquito control, application of larvicide may be required in order to prevent larval development into vectors of West Nile Virus. Application of larvicide may commence as early as June 1, 2021 and end as late as October 31, 2021.

The larvicides VectoBac 200G (granular, PCP#18158), VectoBac 1200L (liquid, PCP#21062) and VectoLex CG (granular, PCP#28008) will be applied to identified standing surface water located on public property.

The larvicides Altosid Pellets (pellet, PCP#21809), Mosquiron 0.12 CRD (PCP#31079, ingot) and VectoLex WSP (pouch, PCP#28009) will be placed into catch basins of storm drains on city streets and on selected publicly owned properties within the City of Hamilton. 

The application of these products will be carried out under permit from the Ontario Ministry of Environment, Conservation and Parks. All control products will be applied by licensed applicators or trained technicians of Pestalto Environmental Health Services Inc. under contract with the City of Hamilton

For further information please contact: 905-546-2424 x 7530


The City of Hamilton is committed to reducing the risk of West Nile virus within our community by using strategies with the least negative impact on human health, nature and the environment.  We do many things to control West Nile virus such as:

  • public education
  • reducing mosquito breeding sites and standing water on City land
  • enforcing Hamilton's Standing Water By-law No. 03-173
  • surveillance for West Nile virus in mosquitoes
  • monitor for West Nile virus in humans 

Adult mosquito control

We have a contingency plan for using an ultra-low volume insecticide called malathion to control the adult mosquito population.  This could be used if the Medical Officer of Health determines that it is needed to decrease the risk of West Nile virus to human health.  Spraying to control adult mosquitoes has not yet been required in Hamilton or any other municipality in Ontario for West Nile virus control. 

If we are going to use malathion, Public Health Services will notify residents in the affected areas a minimum of 48 hours before application. Notification will be through the media such as newspapers, television, radio or door-to-door notices.

Sign up for the Adult Mosquito Control Registry

If you want to receive personal advance notification of plans to apply malathion in your area because of health-related concerns, sign up for the adult mosquito control registry.  Let Public Health Services know your:

  • Name
  • Preferred method of contact - phone, fax or email
  • Contact information – phone, fax and/or email

You can submit this information to Public Health by:

We will use an automated system to electronically contact each person registered on the adult mosquito control registry by their chosen method of phone, email, fax or a combination of these to notify them of any adulticiding plans in their area. 

Personal information collected for the purpose of administering the Adult Mosquito Control Registry is being collected under the authority of the Health Protection and Promotion Act, R.S.O. 1990, c. H.7. If you have any questions regarding this collection,  contact the West Nile Virus program, Public Health Services, City of Hamilton, 110 King Street West, Hamilton, Ontario, L8P 4S6 or by Phone: 905-546-2424 ext. 3576.

West Nile virus statistics in Hamilton

This table provides year-over-year surveillance data for the number of positive dead birds, positive mosquito pools, human cases, human deaths related to West Nile virus and horse cases.

Surveillance type 2016 2017 2018 2019 2020 2021
Positive mosquito pools 23 32 13 4 11  
Human cases 4 6 6 2 2 1
Human deaths related to West Nile virus - - - - -  
Horse cases 0 0 1 1
Eastern Equine
Other 1 Crow* 1 Crow* 0 1 - 1 Bird

*Note: Routine surveillance or routine testing of dead birds is not currently conducted in Hamilton as part of the West Nile Virus program

This table provides the number of positive mosquito pools in an area. Positive mosquito pools indicate a group of female mosquitos of the same species. The number of mosquitoes in a pool ranges from one to 50. There can be several pools in one trap.

Area Positive mosquito pools
Lower East Hamilton 3
Lower Central Hamilton 2
Lower Stoney Creek 3
Flamborough 1
Central Mountain 1
East Mountain 1

Positive mosquito pools indicate a group of female mosquitos of the same species. The number of mosquitos in a pool ranges from one to 50. There can be several pools in one trap.

Number of positive mosquito pools

Date collected

Date reported positive Area
1 September 1, 2020 September 1, 2020 Lower East Hamilton
1 August 25, 2020 August 27, 2020 Lower Stoney Creek
1 Aug 18, 2020 Aug 20, 2020 Lower Stoney Creek
1 Aug 18, 2020 Aug 20, 2020 Lower East Hamilton
1 Aug 11, 2020 Aug 13, 2020 Lower Central Hamilton
1 Aug 11, 2020 Aug 13, 2020 Lower Stoney Creek
1 Aug 11, 2020 Aug 13, 2020 Lower East Hamilton
1 Aug 11, 2020 Aug 13, 2020 Flamborough
1 Aug 11, 2020 Aug 13, 2020 Central Mountain
1 August 5, 2020 August 7, 2020 Lower Central Hamilton
1 July 28, 2020 July 30, 2020 Lower East Hamilton

This table provides the number of larvicide applications year-over-year for the previous 5 years. The catch basin colour shows up as a painted dot on the city street catch basins when they receive a treatment. A 4th round of treatment is applied based on a rise in positive mosquito results.

Application of larvacide 2015 2016 2017 2018 2019 2020 2021 Catch basin colour


18 - 27

June 30
July 12
June 12 -
July 10
June 7 - June 28 June 17 - July 5 June 1 - July 6 June 1 - July 6



9 -19

July 27
August 8 
July 11 - August 4 June 29 - July 22 July 9 - July 26 July 7 - July 23  


White for 2019


July 30 -  August 7

August 24 - September 7 August 8 - August 29 July 23 - August 16 July 29 - August 9 July 24 - August 14  


4 None applied None applied None applied August 16 - September 19
(partial treatment)