West Nile virus and surveillance

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.

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