Surveillance of West Nile virus

Learn about the different types of surveillance for West Nile virus and the reported number of cases in humans.

View the latest case numbers and seasonal updates

On this page 

• Human surveillance
• Mosquito surveillance
• Bird surveillance
• Horse surveillance
• Surveillance in First Nations communities
• Reports and publications
• References

Human surveillance

During the West Nile virus season (mid-April to November), Canada conducts ongoing human case surveillance across the country.

Monitoring West Nile virus nationally is a joint effort between the Government and its partners, including:

• provincial and territorial ministries of health
• First Nations authorities
• blood supply agencies

The Government relies on the provinces and territories to report the number of West Nile virus cases.

To accurately reflect the annual occurrence of West Nile virus cases in Canada, health professionals need to remain vigilant in:

• diagnosing West Nile virus
• reporting cases to their public health regional authorities

Surveillance information is reported to the Government by its partners.

How many human cases of West Nile virus are reported annually?

The first human case of West Nile virus infection in Canada was reported in Ontario in 2002. The Government currently has data for West Nile virus human cases^{Footnote 1} reported between 2002 and 2022:

• 2022: 47 cases (preliminary)
• 2021: 45 cases
• 2020: 166 cases
• 2019: 45 cases
• 2018: 437 cases
• 2017: 202 cases
• 2016: 162 cases
• 2015: 84 cases
• 2014: 23 cases
• 2013: 114 cases
• 2012: 454 cases
• 2011: 124 cases
• 2010: 5 cases
• 2009: 14 cases
• 2008: 37 cases
• 2007: 2401 cases
• 2006: 151 cases
• 2005: 238 cases
• 2004: 32 cases
• 2003: 1488 cases
• 2002: 414 cases

These numbers may change slightly as provincial or territorial public health organizations can sometimes retroactively identify cases.

Surveillance detects only a portion of the West Nile virus cases in Canada. The true number of West Nile virus infections in Canada is likely greater.

Mosquito surveillance

Mosquito surveillance depends on the level of anticipated or current West Nile virus activity in a particular area.

In areas where West Nile virus has not been found, surveillance of mosquitoes focuses on:

• which kinds of mosquitoes are present
• how abundant the mosquitoes are
• whether West Nile virus has entered the mosquito population

In areas where West Nile virus has already been found, mosquitoes may still be tested for West Nile virus. This information can help to:

• anticipate new outbreaks
• monitor ongoing outbreaks
• identify different kinds of mosquitoes that could spread the virus to:
• birds
• animals
• people
• It can also be used to decide if, when, where and how to act to reduce the risk of infection by applying mosquito control measures

Control of mosquito populations

Provinces and municipalities are responsible for control of mosquito populations.

Bird surveillance

When West Nile virus first came to North America, bird surveillance was used as an early indicator of the virus in animals.

Experience from past outbreaks showed the following birds were highly susceptible to West Nile virus:

• crows
• jays
• magpies
• ravens

Infected dead birds are a good indicator to determine whether people in particular areas are at risk.

Some provinces and territories no longer conduct dead bird surveillance. Instead, they focus efforts on other surveillance activities.

The Canadian Wildlife Health Cooperative tests dead birds for West Nile virus in geographical areas with a sufficiently high risk. They do this in collaboration with:

• provincial laboratories
• the Government's National Microbiology Laboratory

Testing focuses on the detection of West Nile virus in the early season (late April to July).

Report dead birds

Dead birds in your community may be a sign of the presence of West Nile virus. Contact the Canadian Wildlife Health Cooperative to report dead birds that you suspect have West Nile virus.

Horse surveillance

Among large animals, horses are especially at risk for West Nile virus infection and disease. Horses can also be early indicators of West Nile virus in a region. The Canadian Food Agency of Canada monitors West Nile virus in horses with the help of:

• provincial veterinary laboratories
• veterinarians
• other members of the animal health community

Vaccination may be used to prevent infection in horses. Currently, there are three West Nile virus vaccines registered for use in horses in Canada. Local veterinarians should be contacted for guidance on appropriate vaccination products and strategies.

Veterinary laboratories

All veterinary laboratories in Canada are required to report suspicion or diagnosis of West Nile virus in all domestic animal species to the CFIA.

Surveillance in First Nations communities

In First Nations communities, First Nations and Inuit Health regional staff advise:

• Chiefs
• councils
• federal government departments

They are advised on emerging needs for West Nile virus public health control measures.

To get information about specific West Nile activity in their community, First Nations residents can contact their:

• Environmental Health Officer
• Community Health Centre
• Nursing Station

This includes information on surveillance activities and case counts.

References