West Nile virus (WNV) is transmitted to humans primarily through the bite of an infected mosquito. WNV can affect anyone – 1 in 5 people who do become infected become ill. Severe neurological illness associated with the virus is rare, but risk increases with age.

The first evidence of WNV in Alberta was confirmed in July 2003.

WNV in Alberta

Each year, the Alberta government conducts passive surveillance for WNV in humans through public health laboratories and Canadian Blood Services. Veterinarians and animal health laboratories report cases of WNV identified in horses.

Until 2009, surveillance of mosquito pools and bird populations was conducted in Alberta to determine when and where the greatest risk of WNV transmission to humans was occurring. This surveillance was stopped when it was determined there was risk of WNV Alberta every year.

WNV in animals

The risk of WNV

There is a risk of WNV in Alberta every year. The period of greatest risk of WNV transmission to humans by mosquitos usually occurs between mid-July and mid-August. This also depends on the presence of consistently warm temperatures.

The majority of WNV activity occurs in southeastern Alberta (mainly grassland area). There have also been cases reported further north during seasons with more activity.

The risk of WNV depends on:

  • the number of Culex mosquitoes that have successfully survived the previous winter
  • the amount of WNV circulating in these same Culex mosquito populations
  • weather conditions in the spring and early summer affecting the spread of WNV from mosquitos to birds and birds to mosquitos
  • how close human populations are to WNV-infected mosquito populations

2020 WNV cases

There was 1 case of asymptomatic WNV reported in 2020 in Alberta.

Summary of cases* 2012 to 2020

Zone/Year North Edmonton Central Calgary South Alberta total
2012 1 0 1 1 6 9
2013 0 1 3 1 16 21
2014 0 0 0 0 0 0
2015 0 0 0 0 0 0
2016 0 0 0 2 3 5
2017 0 0 1 2 4 7
2018 0 0 3 7 40 50
2019 0 0 0 3 1 4
2020 1 0 0 0 0 1

* Includes probable and confirmed cases as per federal reporting requirements.

Surveillance reporting

Surveillance degree day maps

WNV degree day maps help health professionals determine the risk of WNV in Alberta each year.

2019 WNV and Degree Day Maps in Alberta

2018 WNV and Degree Day Maps in Alberta

2017 WNV and Degree Day Maps in Alberta

2016 WNV and Degree Day Maps in Alberta

2015 WNV and Degree Day Maps in Alberta

2014 WNV and Degree Day Maps in Alberta

Past surveillance summary reports

2003–2011 West Nile virus surveillance monitoring reports

West Nile Virus Seroprevalence in Alberta, Estimating the Infection Rate 2009

West Nile virus Infection Rate Study 2005

Surveillance in Canada and United States

Government of Canada




British Columbia

Centers for Disease Control and Prevention, USA

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