The U.S. Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) has recently confirmed a new spillover of H5N1 avian influenza in dairy cattle, marking the first detection of the D1.1 genotype in dairy herds. This particular genotype has been circulating widely in wild birds and has been linked to human infections, including a fatal case in Louisiana involving contact with sick backyard poultry.
Up until this point, all reported cases of H5N1 in dairy cattle involved the B3.13 genotype, which was believed to have originated from a single spillover event from wild birds in late 2023 or early 2024. This strain primarily caused mild infections in dairy workers and poultry cullers, mainly presenting as conjunctivitis. However, the emergence of the D1.1 genotype signals a shift in the outbreak dynamics.
The D1.1 genotype has been the dominant strain in North American migratory flyways over the past fall and winter, affecting wild birds, mammals, and domestic poultry. Its detection in dairy cattle raises new concerns about how the virus is spreading and evolving.
The discovery of the new genotype was the result of an investigation into H5N1 found in milk samples from Nevada dairy cows. APHIS recently reported four new cases of H5N1 in Nevada dairy herds, the first detections in the state since December. In response, Nevada officials have been working to control large populations of European starlings—an invasive species known to outcompete native birds and pose disease risks to livestock.
Despite this new detection, APHIS has stated that its overall eradication strategy remains unchanged. The finding highlights the effectiveness of the National Milk Testing Strategy, which has enrolled 36 states and currently covers milk from two-thirds of the nation’s dairy herds. In collaboration with Nevada agriculture officials, the USDA is working to further investigate the detection and mitigate additional spread. A technical brief on the findings will be released soon, along with the genetic sequence published on GenBank for further study.
This recent development underscores the importance of ongoing surveillance and monitoring of infectious diseases in livestock. By detecting and responding to outbreaks early, authorities can implement measures to prevent further spread and protect both animal and human health. Continued research into the evolution and transmission of influenza strains is essential for developing effective control strategies and mitigating the impact of these diseases on agricultural industries.
