Mosquito-borne disease surveillance: Technical notes: Vector-borne disease surveillance in Canada

Vectors, such as mosquitoes, spread diseases between humans or from animals to humans, usually by biting. Diseases spread by vectors are called vector-borne diseases. Find out how we monitor mosquito-borne diseases in Canada.

  • Last updated: 2025-03-19

On this page, you will learn about mosquito-borne disease (MBD) surveillance in Canada and how to use the interactive data visualization tools to explore over 20 years of data on this topic.

About MBD surveillance

About mosquito-borne disease (MBD) surveillance

The Public Health Agency of Canada (PHAC) works with partners to monitor key mosquito-borne diseases in Canada. On this page, you will learn more about how PHAC uses a One Health approach, which includes monitoring of mosquito and animal populations, to complement human case monitoring.

For a historical summary of the WNV surveillance system, refer to the Overview of the national West Nile virus surveillance system in Canada: A One Health approach.

On this page

Definitions: Zoonotic diseases, vectors and vector-borne diseases

Zoonotic diseases (also known as zoonoses): Infectious diseases that are spread between animals and humans in various ways such as direct contact, indirect contact or through vectors.

Vectors: Living organisms, most commonly invertebrates such as mosquitoes, ticks and flies, that can carry and spread infectious diseases between animals and humans.

Vector-borne diseases: Infectious diseases caused by pathogens such as parasites, viruses or bacteria, which are spread between animals and humans by infected vectors, usually by biting.

Human surveillance

West Nile virus (WNV) is the most commonly reported mosquito-borne disease in Canada. Surveillance for the virus in Canada began in 2000, following the incursion of WNV into North America, in and around New York City. In 2002, the first human case of West Nile virus (WNV) was detected in Ontario, with many more cases reported in both Ontario and Quebec that same year.

West Nile virus was named a nationally notifiable mosquito-borne disease in 2003. Provinces and territories voluntarily report cases that meet the national case definition to PHAC.

Some other mosquito-borne diseases that can be found in Canada, that are not nationally notifiable but of public health importance, include:

  • Eastern equine encephalitis (EEE) virus
  • Western equine encephalitis (WEE) virus
  • Some California serogroup (CSG) viruses such as:
    • Jamestown Canyon virus
    • snowshoe hare virus

West Nile virus surveillance system

The West Nile virus surveillance system monitors and reports trends across Canada.

  • Data collected through this system are analyzed and shared each year during the transmission season (spring, summer, fall) to provide timely information on virus activity and inform disease prevention activities.
  • The system also reports annual numbers and measures the burden of disease over time. The annual number of WNV cases can fluctuate widely, due to occasional epidemics.
  • In addition to collecting data on WNV throughout the year, PHAC meets with its Federal, Provincial/Territorial and non-government partners regularly to share information and discuss outbreaks and emerging issues.
Data sources: West Nile virus human data
  • Local or provincial/territorial public health authorities follow up on WNV cases, and send data to PHAC every two weeks throughout the transmission season, as well as confirm final annual numbers.
    Data collected from each case includes details about:
    • Laboratory results
    • Geographical location
    • Clinical and demographic features (e.g. signs and symptoms, age, sex)
  • Blood donations are routinely screened by Canadian Blood Services and Héma-Quebec in the spring, summer and fall, to prevent transmission through blood products. Blood donors who screen positive are reported to provincial or territorial health authorities. Provinces and territories then follow up with these individuals and report cases to PHAC.
  • The WNV surveillance system is implemented with its provincial and territorial public health partners through the Multi-Lateral Information Sharing Agreement.
  • Many steps are taken to protect privacy and handle data according to the Privacy Act and federal privacy policies.
    • Only data necessary to monitor West Nile virus cases are collected.
    • No identifying personal information, such as names or provincial/territorial health numbers, is collected.
    • Data is sent and stored securely, with only authorized employees having access.

Other mosquito-borne disease surveillance in humans

For mosquito-borne diseases that aren’t nationally notifiable and don’t have a national case definition, PHAC monitors trends more informally.

  • Although EEE virus and CSG viruses are not nationally notifiable, some provinces and territories have made these diseases reportable within their jurisdictions.
    • They may conduct testing at their own provincial labs and classify cases according to their own case definitions.
    • Provinces and territories with less testing capacity may send samples to a reference lab in a larger jurisdiction or to the National Microbiology Laboratory (NML).

We make these counts available where possible.

Non-human surveillance

We use a One Health approach, which recognizes the interdependence of humans, animals, and the environment.

In addition to data on human cases, the surveillance system collects data from various sources about:

  • mosquitoes
  • equine (horse) cases
  • wild birds that test positive

Testing animals and mosquitoes helps to provide an early warning of disease activity and areas of risk.

Mosquito surveillance

Mosquito surveillance programs are sometimes carried out by government, non-government agencies or academic institutions.

  • These programs monitor current mosquito vectors and can help identify when new mosquito species become established in Canada, particularly if they’re known to be vectors of diseases.
  • These programs may also conduct testing for pathogens.

For more information, please refer to Mosquito surveillance.

Data sources: Mosquitoes
  • The number of provinces and territories participating in mosquito surveillance varies from year to year. Those that conduct mosquito surveillance share data with PHAC every two weeks throughout mosquito season (spring, summer, fall).

Equine (horse) surveillance

The Canadian Food Inspection Agency (CFIA) is responsible for safeguarding food, animals and plants. A key role of the CFIA is to protect farmed and wild animals from the introduction and/or spread of certain animal diseases that are important to public health and/or animal production.

  • Suspected and confirmed cases of WNV and EEE virus in horses are immediately notifiable diseases, meaning that animal health labs must immediately report suspicion or diagnosis of these diseases to CFIA.
  • CFIA uses animal health epidemiology and surveillance case definitions to count cases of immediately notifiable diseases in horses.
  • Among large animals, horses are especially susceptible to WNV and EEE virus infection.

For more information, please refer to Horse surveillance.

Data sources: Equines
  • CFIA monitors WNV and EEE virus in horses across Canada, and shares data with PHAC every two weeks during the mosquito season. These data include location, time of illness onset and testing information.

Dead wild bird surveillance

The Canadian Wildlife Health Cooperative (CWHC) coordinated a large national surveillance project to monitor WNV in wild birds from 2000 until 2012. Since then, monitoring has continued as part of ongoing surveillance efforts, but there’s no longer a dedicated WNV program. Testing for WNV and other viruses like EEEV or CSG viruses occurs at the discretion of the wildlife pathologist, based on postmortem findings or other case information.

For more information, please refer to Bird surveillance.

Data sources: Dead wild birds
  • CWHC maintains a comprehensive database on wildlife, with data about deaths (e.g. location, date), testing information and disease outcome (cause of death). CWHC collects these data for dead wild birds tested in all provinces and territories except Manitoba, and shares mosquito-borne disease related deaths with PHAC during the mosquito season.
  • Manitoba doesn’t routinely test dead wild birds, but they do share any positive test data with PHAC throughout the mosquito season.

West Nile virus: Data definitions and notes

Human data definitions
Case classification
Cases are classified as suspect, probable or confirmed, based on the national WNV case definition. For surveillance reporting purposes, probable and confirmed cases are counted.
Disease classification
Cases are classified using clinical and lab criteria. If a case meets both, it's classified as:
  • West Nile neuroinvasive disease (West Nile neurological syndrome)
  • West Nile non-neuroinvasive disease (West Nile non-neurological syndrome), or
  • West Nile unspecified clinical case, if no clinical classification has been provided
If lab criteria are met but none of the clinical criteria, it's classified as:
  • West Nile asymptomatic infection.
Asymptomatic cases are typically identified through testing of donated blood.
Death
A case of WNV that was reported as deceased. In most cases, WNV was the direct or contributing cause of death. Death is recorded at the local, provincial or territorial level at the time of investigation or public health follow-up.
  • There may be differences in the way death is classified or investigated from one jurisdiction to another.
Episode date
The earliest of the following dates reported to PHAC, used to assign a case to an epidemiologic week:
  • symptom onset date
  • specimen collection date
  • lab tested date, or
  • report date
Travel
The case travelled to an area outside of Canada that was relevant to acquiring West Nile virus. Categorization of travel and public health follow-up on travel-related cases varies by jurisdiction.
Mosquito data definitions
Number of mosquito pools positive for WNV
The number of mosquito pools positive for WNV by time and place is based on the:
  • date the mosquitoes were collected or tested, and
  • location of the trap
Mosquito pool
A mosquito pool is generally a group of 1 to 50 female mosquitoes of the same species found in a trap.
Positive mosquito pool
A positive mosquito pool is a mosquito pool with at least 1 mosquito testing positive.
Equine data definitions
Number of horses positive for WNV
The number of horses positive for WNV by time and place is based on the:
  • date submitted (for example, the date the lab received the sample), and
  • province of residence
Dead wild bird data definitions
Count of dead wild birds positive for WNV
The count of dead wild birds positive for WNV by time and place is based on the:
  • date the bird was found, or
  • date the lab received the sample, and
  • location where the bird was found
Data notes
  • The West Nile virus surveillance system is considered passive, in that it relies on health authorities to voluntarily report cases. Moreover, only about 20% of people with WNV infections have symptoms, therefore many asymptomatic or mild cases aren’t diagnosed or reported. For these reasons, WNV infections in humans are likely under-diagnosed and under-reported. Detection and reporting of WNV neurological syndrome (cases with more severe symptoms and signs) are considered more complete than that of WNV non-neurological syndrome (cases with mild symptoms and signs).
  • Data collection methods, case definitions and reporting vary within Canada and over time, leading to some challenges with interpretation. Of note, starting in 2014, Saskatchewan only reports cases of WNV neuroinvasive disease (WNV neurological syndrome).
  • Classification or investigation of deaths varies from one province or territory to another.
  • Counts of asymptomatic WNV infections before 2020 may represent cross-reactivity with the Japanese encephalitis vaccine, rather than true WNV asymptomatic infections.

Eastern equine encephalitis virus: Data definitions and notes

Human data definitions
Case classification
In the absence of established national case definitions, infection counts are based on reports from provincial and territorial partners, in collaboration with the National Microbiology Laboratory.
Mosquito data definitions
Number of mosquito pools positive for EEEV
The number of mosquito pools positive for EEEV by time and place is based on the:
  • date the mosquitoes were collected or tested, and
  • location of the trap
Mosquito pool
A mosquito pool is generally a group of 1 to 50 female mosquitoes of the same species found in a trap.
Positive mosquito pool
A positive mosquito pool is a mosquito pool with at least 1 mosquito testing positive.
Equine data definitions
Number of horses positive for EEEV
The number of horses positive for EEEV by time and place is based on the:
  • date submitted (for example, the date the lab received the sample), and
  • province of residence
Data notes

Diseases that are not nationally notifiable, such as EEE virus, are underreported and should be interpreted with caution. Comparisons over time may not be reliable. National counts may differ from provincial and territorial counts because of different case definitions or surveillance method used.

Data limitations

  • Surveillance data are reported by province or territory of residence and not the location of exposure.
  • Not all provinces or territories submit data every 2 weeks to PHAC, so updates for some jurisdictions may be delayed.
  • This dashboard is based on the latest data available, but is subject to change as provinces and territories provide updated case information.
  • Data on this dashboard may not match surveillance data on provincial and territorial disease reporting websites. Provinces and territories may update surveillance data on their sites at different frequencies.
  • In less-populated provinces and territories, even slight changes in case numbers may result in substantial changes to incidence rates.
  • The absence of detected or reported disease activity in humans, mosquitoes and animals shouldn’t be interpreted to mean there’s no risk of infection.
  • Indicators such as positive mosquito pools and positive dead wild birds may not be reliable for assessing trends over time or an association with human cases. This is because sampling methods and allocated resources may not have been consistent over time or across jurisdictions.
  • For provinces and territories that conduct mosquito surveillance, they may have to subsample the number of mosquitoes identified from each trap as well as the number of pools tested. This means they may not be able to test all mosquitoes trapped to maintain an efficient and cost-effective program.
  • When interpreting horse and mosquito data trends, keep in mind that prevention measures impact these numbers year-to-year. Examples include vaccination in horses, insecticide application and other mosquito control measures.

Acronyms

PHAC
Public Health Agency of Canada
CFIA
Canadian Food Inspection Agency
CWHC
Canadian Wildlife Health Cooperative
CBS
Canadian Blood Services
HQ
Héma-Quebec
WNV
West Nile virus
CSG
California serogroup viruses
JCV
Jamestown Canyon virus
SSH
Snowshoe hare virus
EEE
Eastern equine encephalitis virus
WEE
Western equine encephalitis virus
NML
National Microbiology Laboratory

Acknowledgements and suggested citation

This dashboard would not be possible without the collaboration and dedication of local, provincial and territorial public health data providers. We would also like to acknowledge the Canadian Wildlife Health Cooperative, Canadian Food Inspection Agency, Canadian Blood Services, and Héma-Quebec for their data contributions to this dashboard.

Suggested citation: Vector-borne disease surveillance in Canada. Ottawa, Canada: Public Health Agency of Canada. https://health-infobase.canada.ca/zoonoses/mosquito/

Government of Canada

Provincial, territorial and international surveillance

About this dashboard

About this dashboard

The Mosquito-borne disease surveillance Health Infobase dashboard includes:

  • Seasonal update: This tab is updated regularly during the transmission season (approximately July to October). It provides a broad overview of West Nile virus (WNV) activity across Canada, including human cases reported, as well as cases in horses, dead birds and virus activity detected in mosquitoes.
  • Annual reports: This tab summarizes the latest annual data available regarding WNV cases and detections in non-human species. It also includes information on other mosquito-borne diseases (Eastern equine encephalitis) reported in Canada. Previous annual reports are also available.
  • Explore the data: This tab includes a set of tools and interactive data visualizations (maps and graphs) to help you navigate and understand the data, spanning back to 2002.

How to interact with the visualizations

Depending on the visualization, there may be different options for interacting, including:

  • Drop-down menus to select which information to display
  • Hover over fields for the individual data points in the maps to display specific information relative to that location
  • Hover over elements of the graphs to display data in the legend
  • An option to export data

The visualizations also include additional details and descriptions of methods and limitations (in text descriptions and notes), to help interpret the data.

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