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

On this page

• Human surveillance
• Non-human surveillance
• West Nile virus: Data definitions and notes
• California serogroup viruses: Data definitions and notes
• Eastern equine encephalitis virus: Data definitions and notes
• Data limitations
• Acronyms
• Acknowledgements and suggested citation
• Related links

When we think of mosquito-borne diseases, we often think of malaria or yellow fever, which are more common in warm, humid climates. However, there are mosquito-borne diseases in Canada, and the Public Health Agency of Canada (PHAC) works with partners to monitor the risk of these diseases here.

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

Case-based surveillance versus lab-based only surveillance

Mosquito-borne diseases that are significant to public health are ‘nationally notifiable’ in Canada. A national case definition is developed for each nationally notifiable disease. In this case-based system, provinces and territories voluntarily report cases that meet national case definitions for these diseases.

In 2002, the first human case of West Nile virus (WNV) was detected in Ontario, Canada and many more cases were reported in both Ontario and Quebec that same year. West Nile virus has been a nationally notifiable mosquito-borne disease since 2003.

Some mosquito-borne diseases in Canada are not nationally notifiable and do not have a national case definition. We monitor these diseases through other methods, such as lab-based only surveillance. This method relies on lab data for diseases that are usually identified through the testing of sick people. Some mosquito-borne diseases in Canada that are not nationally notifiable include:

• Some California serogroup (CSG) viruses such as:
  • Jamestown Canyon virus
  • snowshoe hare virus
• Eastern Equine Encephalitis (EEE) virus
• Western Equine Encephalitis virus

West Nile virus surveillance

• West Nile virus data are from a case-based surveillance system.
• Human cases are classified according to the national WNV case definition.
• The annual number of WNV cases can fluctuate widely, due to occasional epidemics.

The WNV surveillance system complies with the regulations of the federal/provincial/territorial Multi-Lateral Information Sharing Agreement. For a summary history of the WNV surveillance system, refer to the Overview of the national West Nile virus surveillance system in Canada: A One Health approach.

Data sources: West Nile virus human data

• Local or provincial public health authorities follow up on WNV cases, and send data to PHAC throughout the transmission season, as well as a final annual dataset.
• Blood donations are routinely screened by Canadian Blood Services and Hema-Quebec in the spring, summer and fall, as individuals who have no symptoms may not realize they are infected. 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.

Other mosquito-borne disease surveillance

• For mosquito-borne diseases that aren’t nationally notifiable and don’t have a national case definition, PHAC monitors trends using lab-based only surveillance. The agency derives infection counts from lab-based criteria only, as clinical data (symptoms) are not collected.
• Some provinces and territories have made diseases that aren't nationally notifiable, such as CSG viruses and EEE virus, reportable within their jurisdictions. They may conduct testing at their own provincial labs and classify cases according to their own case definitions. We make these counts available where possible.
• Provinces and territories with less testing capacity for CSG viruses and EEE virus may send samples to a reference lab in a larger jurisdiction or to the National Microbiology Laboratory (NML).

Data sources: California serogroup (CSG) viruses human data

Human samples are submitted to the NML to test for CSG virus or for further subtyping. Counts are a subset of confirmed lab-based only surveillance cases. They may not represent the true number of cases in Canada. The NML provides us with an extract of this data upon request.

Data sources: Eastern equine encephalitis (EEE) virus human data

Human samples are submitted to the NML to test for EEE virus. Counts include confirmed and probable lab-based only surveillance cases. They may not represent the true number of cases in Canada. The NML provides us with an extract of this data upon request.

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 mosquito-borne disease surveillance system maintains data from various sources about:

• veterinary cases
• mosquitoes
• dead wild birds
• other sentinel animals

Mosquito surveillance

• Mosquito identification programs are sometimes carried out by government, non-government agencies or academic institutions. These programs monitor for current mosquito vectors and can help identify when new mosquito species become established in Canada, particularly if they’re known to be vectors of disease.
• Mosquito surveillance is carried out at the local or provincial level, by trapping and testing adult mosquitoes. Some factors that are considered when setting mosquito traps include:
  • location
    • proximity to humans
    • preferred habitat of the mosquito species
  • time of year
  • weather conditions

For more information about mosquito surveillance, 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 for WNV, EEE virus or CSG virus share data with us as it becomes available. This is generally every other week throughout the mosquito season.

Equine (horse) surveillance

• The Canadian Food Inspection Agency (CFIA) is responsible for safeguarding food, animals and plants.
• CFIA protects us from preventable health risks related to diseases that can be transmitted between animals and humans. CFIA also protects farmed and wild animals from foreign animal diseases and prevents the spread of certain domestic animal diseases.
• Animal diseases are categorized as:
  • reportable diseases:
    • diseases that can impact human or animal health or the Canadian economy
    • animal owners, veterinarians and labs must report suspect cases of these diseases to a CFIA district veterinarian
  • immediately notifiable diseases:
    • diseases that are foreign to Canada for which there are no control or eradication programs
    • some rare diseases that are indigenous to Canada
    • only labs must report to CFIA regarding the suspicion or diagnosis of one of these diseases
  • annual notifiable diseases:
    • diseases that are present in Canada but not classified as reportable or immediately notifiable
    • reported by Canada to the World Organization of Animal Health
• Suspected and confirmed cases of WNV and EEE virus in horses are immediately notifiable diseases.
• CFIA uses animal health epidemiology and surveillance case definitions to count cases of immediately notifiable diseases in horses.
• Among large animals, horses are especially at risk for WNV and EEE virus infection.

For more information about equine surveillance, please refer to Horse surveillance.

Data sources: Equines

CFIA monitors WNV and EEE virus in horses across Canada, and shares data with PHAC every 2 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 massive 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 EEE virus or CSG viruses occurs at the discretion of the wildlife pathologist, based on postmortem findings or other case information.

For more information about bird surveillance, please refer to Bird surveillance.

Data sources: Dead wild birds

CWHC maintains a comprehensive cause-of-death database, with data about:

• deaths, including:
  • environmental circumstances
  • location
  • date
  • species
• testing information
• disease outcome

CWHC collects these data for dead wild birds tested in all provinces and territories except Manitoba, and shares it with PHAC every 2 weeks during the mosquito season.

Manitoba doesn’t routinely test dead wild birds, but they do share any positive test data with PHAC throughout the 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 the clinical and lab criteria in the WNV national case definition. If a case meets both the lab and the clinical criteria, it’s classified as West Nile neuroinvasive disease (West nile neurological syndrome), West Nile non-neuroinvasive disease (West nile non-neurological syndrome), or, if no clinical classification has been provided, West Nile unspecified clinical case. If a case meets the lab criteria but none of the clinical criteria, it’s classified as a 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

This refers to the earliest of the following dates reported to PHAC:

• symptom onset date
• specimen collection date
• lab tested date, or
• report date

Travel

Whether the case involved travel that was relevant to acquiring West Nile virus. Travel cases are categorized as ‘infected in Canada’ or ‘infected outside of Canada’ in the dashboard. Categorization of travel and public health follow-up on travel-related cases vary 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 CWHC lab received the sample, and
• location where the bird was found

Data notes

• The West Nile virus surveillance system is passive, as it relies on health authorities to report cases. Moreover, only about 20% of people with WNV infections have symptoms. Many asymptomatic or mild cases aren’t diagnosed or reported. For these reasons, it’s likely that WNV infections in humans are under-diagnosed. Detection and reporting of WNV neurological syndrome are considered more complete than that of WNV non-neurological syndrome.
• Data collection methods and case definitions vary within Canada, leading to challenges with interpretation. Of note, starting in 2008, Saskatchewan only reports WNV neurological syndrome cases.
• Classification or investigation of deaths varies from one province or territory to another.
• Most analyses exclude travel-related cases of WNV that were probably acquired outside of Canada.
• Counts of asymptomatic WNV infections before 2020 may represent cross-reactivity with the Japanese encephalitis vaccine, rather than true WNV asymptomatic infections.
• The number of dead wild birds positive for WNV in a given year is based on the date the specimen was received by the CWHC, which may not be the date the bird died or was found.

California serogroup viruses: Data definitions and notes

Human data definitions

Case classification

In the absence of clinical data, we derive infection counts from lab-based criteria only. For CSG virus infections, counts are a subset of confirmed lab-based only surveillance cases.

Episode date

This refers to the earliest date a case was reported to PHAC, which may be the:

• specimen collection date
• lab tested date, or
• report date

Mosquito data definitions

Number of mosquito pools positive for CSG viruses

The number of mosquito pools positive for CSG viruses 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.

Data notes

• CSG virus infections in humans are likely underreported, as CSG viruses is not a nationally notifiable disease and infection counts are a subset of confirmed lab-based only surveillance cases.
• Diseases that are not nationally notifiable, such as CSG viruses, 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.
• CSG virus infection counts by region are not available, as they may be limited or inaccurate. Province or territory of residence is not always available to PHAC, as provinces and territories with less testing capacity for CSG viruses may send samples to a reference lab in a larger jurisdiction or to the NML.
• The Northwest Territories submits data on mosquito pools that test positive for CSG viruses but submit their data in the year following the given mosquito season. Therefore, updates to positive pools will be delayed.

Eastern equine encephalitis virus: Data definitions and notes

Human data definitions

Case classification

In the absence of clinical data, we derive infection counts from lab-based criteria only. For EEE virus infections, counts are a subset of confirmed lab-based only surveillance cases.

Episode date

This refers to the earliest date reported to PHAC, which may be the:

• specimen collection date
• lab tested date, or
• report date

Mosquito data definitions

Number of mosquito pools positive for EEE virus

The number of mosquito pools positive for EEE virus 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 EEE virus

The number of horses positive for EEE virus by time and place is based on the:

• date submitted (i.e., date the lab received the sample) and
• province of residence

Data notes

• EEE virus infections in humans are likely underreported, as EEE virus is not a nationally notifiable disease and infection counts are a subset of confirmed and probable lab-based only surveillance cases.
• Diseases that are not nationally notifiable, such as EEE virus, 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.
• EEE virus infection counts by region are not available, as they may be limited or inaccurate. Province or territory of residence is not always available to PHAC, as provinces and territories with less testing capacity for EEE viruses may send samples to a reference lab in a larger jurisdiction or to the NML.

Data limitations

• Surveillance data are reported by province or territory of residence and not the location of exposure.
• Not all provinces or territories submit bi-weekly data to PHAC, so updates to surveillance data for these jurisdictions may be delayed.
• This dashboard is based on the latest data provided to us but data reported to PHAC is subject to change as provinces and territories receive 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. These updates may not be reflected in this dashboard, particularly for historical data.
• In less-populated provinces and territories, even slight changes in case numbers may result in substantial changes to 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 specimen sampling, and test and trap methodologies have not 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 public health 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

Hema Quebec

WNV

West Nile virus

CSG

California serogroup viruses

JCV

Jamestown Canyon virus

SSH

Snowshoe hare virus

LCV

La Crosse virus

EEE

Eastern equine encephalitis virus

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 Hema-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/

Related links

Government of Canada

• West Nile virus
• Surveillance of West Nile virus
• West Nile virus national surveillance case definition
• Reports on West Nile virus and other mosquito-borne diseases surveillance in Canada
• An Overview of the national West Nile virus surveillance system in Canada: A One Health approach (CCDR)
• Bunyaviruses in Canada (CCDR)
• Fight the bite: Mosquito borne diseases are on the rise in Canada
• Multi-Lateral Information Sharing Agreement

Provincial, territorial and international surveillance

• British Columbia- West Nile virus
• Alberta - West Nile virus
• Saskatchewan - West Nile virus
• Manitoba - West Nile virus
• Ontario - West Nile virus
• Québec - Virus du Nil occidental (French only)
• Québec – Virus du sérogroupe Californie (VSC) (French only)
• Québec – Encéphalomyélite équine de l’Est (French only)
• Nova Scotia- West Nile virus
• New Brunswick- West Nile and other mosquito-borne illness
• Northwest Territories- Mosquitoes and Mosquito-borne diseases
• Prince Edward Island- West Nile virus
• Newfoundland Labrador- West Nile virus Surveillance
• U.S. Centers for Disease Control - West Nile virus
• U.S. Centers for Disease Control- Eastern Equine Encephalitis Virus
• U.S. Centers for Disease Control- Jamestown Canyon Virus
• European Centre Disease Control - Mosquito-borne disease