Susceptibility of calves fed unpasteurized milk from cows infected with highly pathogenic avian influenza H5N1

Abstract Highly pathogenic avian influenza (HPAI) H5N1 was confirmed in a dairy cows in Texas March 2024 and milk was identified as a critical source of virus spread within and between dairy herds. This study demonstrates that calves fed infected unpasteurized milk from infected cows became infected, showed clinical signs including nasal discharge and increased respiratory effort, and seroconverted. Live virus was persistently detected in nasal swabs, even after calves were transitioned to milk from non-infected cows. This study also demonstrates the presence of neutralizing antibodies in milk provides from protection from infection as an additional group of calves fed milk from infected cows also shedding neutralizing antibodies did not become infected, show clinical signs, or seroconvert. These data demonstrate that milk diverted from the human food supply in H5N1 positive dairy herds or suspect cows should not be fed to calves without pasteurization, as H5N1 may further adapt to the bovine respiratory tract and the risk of calves as a source of farm to farm transmission has not yet been evaluated. As characterization of HPAI in the dairy community continues the determination of routes of transmission is an essential step to inform subsequent research on intervention and vaccination strategies.