Overview

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.

West Nile virus 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 consistent warm temperatures.

The majority of WNV activity occurs in southeastern Alberta (mainly grassland area). There has 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 (amplification)
  • the proximity of human populations to WNV-infected mosquito populations

2018 West Nile virus cases

Fifty cases of West Nile virus (WNV) were reported in 2018 in Alberta: 8 neurological, 37 non-neurological and 5 asymptomatic. WNV has been the cause of, or contributed to the deaths of 3 neurological cases.

Summary of cases 2003 to 2018

Zone/Year North Edmonton Central Calgary South Alberta total
2003 4 22 45 37 167 275
2004 1 0 0 0 0 1
2005 0 0 0 2 8 10
2006 0 0 0 4 36 40
2007 4 8 25 38 245 320
2008 0 0 0 0 1 1
2009 0 0 0 1 1 2
2010 0 0 0 1 0 1
2011 0 0 0 0 0 0
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

Surveillance reporting

Surveillance degree day maps

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

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

Ontario

Manitoba

Saskatchewan

British Columbia

Centers for Disease Control and Prevention, USA