Screening of retail milk in Ontario for the presence of influenza A viral RNA

Bovine infections with the H5N1 highly pathogenic avian influenza (HPAI) virus were first identified in the United States (US) in March 2024, as part of an ongoing panzootic of clade 2.3.4.4b H5Nx, which has infected a range of mammalian species across six continents (1). In late April, multiple studies identified the presence of H5N1 HPAI viral RNA (vRNA) in commercially available pasteurized milk across the US, suggesting that infections in dairy cattle were much more widely disseminated than had been initially believed (2). Genomic analysis suggested that the cattle outbreaks originated from a single interspecies spillover from wild birds, followed by sustained cow-to-cow transmission for several months prior to initial identification (3). 

In April, a human case of H5N1 HPAI was identified in a dairy worker who had presumed exposure to infected cattle, indicating a new risk factor for infection in humans (4).