A newly published study confirms that a single transmission of highly pathogenic avian influenza (HPAI) H5N1 from a wild bird to dairy cattle in late 2023 has led to widespread infections across the United States. The virus has since spread from cattle to poultry, other mammals, and peridomestic birds.
Veterinarians first noticed signs of illness in dairy cows in January 2024, including reduced feed intake and milk changes. By March, HPAI H5N1 was confirmed in Texas cattle, and shortly after, in herds across eight states.
Genetic analysis shows all cattle infections stem from one spillover event. The virus appears to have reassorted in wild birds before jumping to cattle, where it adapted and spread silently through the movement of asymptomatic animals. This rapid spread extended to poultry farms and other species.
Researchers found low-frequency genetic changes in the virus that are associated with improved ability to infect mammals. These mutations, while not dominant, suggest the virus is evolving in ways that could increase the risk to humans.
Experts warn that ongoing H5N1 transmission in dairy cattle could allow the virus to further adapt, increasing the chance of human infection. Monitoring and surveillance remain critical to controlling the spread and preventing a potential public health threat.
