Mosquito-borne disease surveillance: Annual reports: Vector-borne disease surveillance in Canada

Human cases overview

• The number of cases was lower compared to the average number of cases reported annually in the previous five years. The average number of cases over the previous five years (2018-2022) was 131, ranging from 36 (2019) to 337 (2018).
• Among the 98 cases acquired in Canada, 46 (47%) were neurological, 32 were non-neurological, 13 were clinically unspecified, and 7 were asymptomatic².
• In addition to the 98 WNV cases acquired in Canada^{Footnote 5}, an additional 6 reported travelling outside of Canada.
• Overall, incidence rate for reported WNV clinical cases (n=91) acquired within Canada was 0.23 per 100,000 population. The rate in females and males were 0.20 and 0.28 per 100, 000 population, respectively.

Demographic characteristics

• More than half of people were over the age of 60 (n=51, 52%).
• There were two deaths associated with WNV infection and both deaths were in people over 70 years of age.

Seasonality

• In 2023, the earliest onset date for a human WNV case acquired within Canada during the mosquito season was June 20 (epidemiological week 25).
• Most (75%) of the reported human cases occurred with episode dates^{Footnote 1} between epidemiological weeks 33 and 38 corresponding to August 13 to September 23, 2023.

Geographic distribution

• The provinces with the highest incidence for 2023 were Ontario and Manitoba at rates of 0.37 and 0.35 per 100,000 population, respectively.

One Health surveillance

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

West Nile virus

• WNV was detected in 224 dead wild birds:
  • In 2023, the number of WNV positive wild birds was lower than in 2022 (n=360) and similar to the average number in the previous five years (2018 to 2022; average number of cases = 222).
  • Birds found dead with WNV³ were found from late April until November in the following provinces:
    • SK (n=37)
    • ON (n=72)
    • QC (n=62)
    • MB (n=53)
• In 2023, WNV was detected in 48 horses:
  • The number of horses positive for WNV in 2023 was slightly higher than the average number of horses positive for WNV in the previous five years (2018 to 2022; average number of cases = 33).
  • WNV positive horses³ were reported in the same provinces as the human cases:
    • AB (n=25)
    • SK (n=10)
    • ON (n=6)
    • QC (n=6)
    • MB (n=1)
• In 2023, 357 mosquito pools were tested for WNV in MB (n=52) and ON (n=305)⁴:
  • Overall, 2.24% of the pools tested positive for WNV.

Eastern equine encephalitis virus (EEEV)

• In 2023, EEEV was detected in 22 horses:
  • The number of horses positive for EEEV in 2023 was higher than the average number of horses positive for EEEV in the previous five years (2018 to 2022; average number of cases = 9).
  • EEEV positive horses were reported in the following provinces:
    • ON (n=18)
    • NB (n=2)
    • QC (n=2)
• In 2023, 115 mosquito pools were tested for EEEV in Ontario⁴:
  • No mosquito pools tested positive for EEEV.