In March 2024, the U.S. Department of Agriculture (USDA) confirmed the first case of highly pathogenic avian influenza (HPAI) spreading between dairy cattle herds in the United States. This marked a significant shift in the virus’s behavior, as HPAI had predominantly affected avian species prior to this event. The initial detection occurred after dairy producers reported unusual illnesses in their lactating cows over the preceding months. Subsequent investigations revealed that the virus had spilled over from wild birds into dairy cattle, leading to transmission between herds.
The adaptation of HPAI to mammalian hosts, particularly livestock, raises concerns about the virus’s evolving capabilities. In early 2025, a new genotype of the H5N1 virus, identified as D1.1, was detected in dairy cows in Nevada. This strain differs from the previously identified B3.13 form and has been associated with severe human infections. The detection of HPAI in dairy cattle suggests that the virus is expanding its host range, potentially increasing the risk of transmission to humans.
The presence of HPAI in dairy cattle has significant implications for human health. While the risk to the general public remains low, there have been instances of human infections linked to exposure to infected livestock. In April and May 2024, three human cases of HPAI A(H5N1) virus infection were reported among dairy farm workers in the United States. These cases were attributed to direct contact with infected cattle, underscoring the importance of monitoring the virus’s spread and implementing biosecurity measures to prevent further transmission.
To mitigate the risk of HPAI transmission to humans, it is crucial to enhance surveillance and biosecurity protocols within the agricultural sector. Farmers and workers should be educated on the importance of personal protective equipment and hygiene practices when handling livestock. Additionally, monitoring wildlife populations that may serve as reservoirs for the virus can help in early detection and prevention efforts. Public health agencies must collaborate with agricultural departments to develop comprehensive strategies aimed at controlling the spread of HPAI and safeguarding both animal and human health.