Selected zoonotic pathogens and diseases from Ontario identified at the AHL in 2025
Tanya Rossi
Animal Health Laboratory, University of Guelph, Guelph, ON
AHL Newsletter 2025;30(1):9.
The term ‘One Health’, an integrated, unifying approach to balance and optimize the health of people, animals and the environment” is a relatively new one in medicine: however the contribution of veterinarians to public health dates back 100s of years. This contribution has taken many forms, including using comparative physiology and anatomy to further human health, informing policies involving food safety and ecosystem health, and detection of zoonotic pathogens, among others. AHL participates in many of these initiatives, but our primary contribution is in the surveillance and annual reporting of zoonotic pathogens identified at our laboratory (Tables 1, 2, 3).
Case numbers for most zoonotic pathogens isolated or identified by the AHL in 2025 are relatively unchanged from the previous year; however, a few changes were identified (Tables 1, 2). Avian West Nile virus positives have increased again this year, rising from 11 in 2021 to 76 in 2024 and 81 in 2025. Case counts of equine West Nile virus and eastern equine encephalitis stayed stable this year after rising in 2023 and 2024. There was a moderate increase in positive serology submissions for Borrelia burgdorferi in equines from 2023 to 2025. These changes in vector-borne disease may reflect changes in vector populations and distribution that should be investigated further.
After seeing an almost 4-fold increase in isolations of Salmonella enterica in chickens in 2023, case counts have decreased in both 2024 and 2025. Case numbers of equine Parascaris equorum (ascarids) have increased from 5 in 2023 to 16 in 2025.
There was an increase in influenza A PCR positives and H5 PCR positives in wildlife species from 2024 to 2025. Case counts of influenza A in swine and domestic poultry remained stable from the previous year.
The percentage of animals identified as positive for leptospirosis was increased in 2025 in all tested species (Table 3). Submissions in canines decreased slightly from 193 in 2024 to 156 in 2025, however, percent positivity increased from 66% to 99%. Testing numbers in equines and swine were low which limits interpretation of changes in percent positivity in these species. These data are potentially subject to submission biases to the diagnostic laboratory and cannot be regarded as population prevalence estimates. They do not take into account vaccination status, as all except horses may be routinely vaccinated for leptospirosis.
Table 1. Number of cases* for selected zoonotic pathogens in non-food producing species isolated and/or identified at the AHL in 2025.
*Cases may include research submissions
**Other species include wild avian species, and other domesticated and wild species.
Table 2. Number of cases* for selected zoonotic pathogens in food producing species isolated and/or identified at the AHL, 2025.
*Cases may include research submissions
Table 3. Leptospira spp. seropositive, IHC-positive, or PCR-positive cases identified at the AHL, 2025
References
1. One Health High-Level Expert Panel (OHHLEP), Adisasmito WB, Almuhairi S, Behravesh CB, Bilivogui P, Bukachi SA, et al. One Health: A new definition for a sustainable and healthy future. PLoS Pathog. 2022;18(6): e1010537.
2. Levy S. Northern Trek: The Spread of Ixodes scapularis into Canada. Environ Health Perspect 2017;24;125(7):074002.
3. Paz S. Climate change impacts on West Nile virus transmission in a global context. Phil. Trans. R. Soc B. 2015;370:1665.