A study in Ontario found no traces of influenza A viral ribonucleic acid (RNA) in 117 samples of pasteurized retail milk collected in April and May 2024, according to a report published this month by the Public Health Agency of Canada.
Influenza A is a virus family that includes various strains affecting humans, birds, and mammals. One such strain, highly pathogenic avian influenza (HPAI) H5N1, commonly known as bird flu, has raised global concerns because of its ability to spread between species.
The findings, detailed in the April 2025 issue of the Canada Communicable Disease Report, contrast with U.S. studies from 2024 that detected H5N1 viral RNA in 20 percent to 40 percent of commercial pasteurized milk. Almost 1,000 dairy herds in the United States have tested positive for bird flu, signaling widespread cattle infections south of the Canadian border.
The Ontario study, led by researchers from the Sunnybrook Research Institute and the University of Toronto, was launched after U.S. reports raised concerns about the highly pathogenic avian influenza (HPAI) H5N1. In March 2024, bovine infections with H5N1 were first identified in the U.S., tied to a global outbreak affecting mammals across six continents. By late April 2024, U.S. studies confirmed the virus’s presence in milk, prompting Canada to investigate its own dairy supply.
Surveillance and testing in Ontario
Researchers tested pasteurized cow’s milk from 17 municipalities across Ontario, including Toronto, Ottawa and Greater Sudbury. The samples, collected between April 24 and May 6, 2024, covered 28 brands and included skim, 1 percent, 2 percent and whole milk varieties. Using quantitative polymerase chain reaction (qPCR) on the QuantStudio 3 platform, the team found no evidence of influenza A viral RNA. The assay’s limit of detection was set at 102 plaque-forming units per milliliter, validated through experiments with pasteurized milk spiked with H3N2 virus.
Ontario, home to Canada’s second-largest dairy cattle population, shares borders with some U.S. states where outbreaks have been reported. Regular cross-border cattle movement heightened concerns about undetected infections. However, the study’s results suggest contamination rates in Ontario are significantly lower than in the U.S., with a 95 percent confidence limit indicating fewer than 4 percent of samples would test positive if replicated.
Despite the reassuring findings, researchers acknowledged some limitations. The sample size was not large enough to completely rule out the presence of H5N1. Additionally, milk samples were collected from select areas in Ontario, meaning some regions were not represented in the study. While nearly all milk sold in Ontario comes from local farms and processors, a few samples may have originated from Quebec, introducing a minor variable in the data.
The Canadian Food Inspection Agency (CFIA) responded by establishing ongoing surveillance. As of Oct. 1, 2024, the CFIA had tested 1,211 pasteurized milk samples nationwide — including 307 from Ontario — and 272 raw bulk milk samples, with all negative results. No H5N1 cases in Canadian cattle have been reported as of Oct. 10, 2024. One human case has been reported, with the patient having had severe symptoms.
The researchers emphasized the value of rapid, collaborative responses in assessing zoonotic risks. While pasteurization may not eliminate all H5N1, studies cited in the report confirm it greatly mitigates consumer risk. Ontario continues to monitor human bird flu cases requiring hospitalization and cattle infections, ensuring vigilance amid the U.S. outbreak, which has seen 70 human cases, mostly among dairy workers.
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