The U.S. Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) has recently confirmed a new spillover of H5N1 avian influenza to dairy cattle, marking the first detection of the D1.1 genotype in dairy herds. This 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.
Historically, all reported cases of H5N1 in dairy cattle have involved the B3.13 genotype, believed to have originated from a single spillover event from wild birds in late 2023 or early 2024. While this strain primarily caused mild infections in dairy workers and poultry cullers—mainly presenting as conjunctivitis—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 resulted from an investigation into H5N1 found in milk samples from Nevada dairy cows. Recently, APHIS 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 the 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 taking steps 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.
In conclusion, the confirmation of the D1.1 genotype in dairy cattle marks a significant development in the outbreak of H5N1 avian influenza. By staying vigilant and continuing to implement effective testing and control measures, authorities aim to prevent further spread of the virus and protect both animal and human populations. Ongoing collaboration between federal and state agencies is crucial in managing and responding to emerging infectious diseases in livestock.
