A true public health success would be preventing the virus from further adapting to humans. The outbreak of highly pathogenic avian influenza A (HPAI) (H5N1) in dairy herds across the U.S. appears to be expanding, with over two dozen farms being affected in eight states. Although only one mild human case in a dairy farmer in Texas has been reported during the current outbreak, the situation is raising concerns for several reasons: it’s happening against a steady, worrisome background of viral evolution and it sets up the potential for increased exposure among humans as well as a possible impact on our food sources.
A Worrying Trend of H5N1 in Mammals
This outbreak marks the first documented instance of H5N1 infection in dairy cowso, highlighting a worrying trend that has been occurring over the last 2 years where the virus has moved from infecting birds to a growing range of mammals. More than 40 new mammalian species have been affected, and more than 26 countries have reported infections in mammals during the current zoonotic wave (2021-2024) compared to 10 countries prior to 2019. This represents an increase in both the geographic and host range of the virus.
The individual outbreaks have also been deadly, affecting large numbers of birds and animals. For example, last spring the virus killed over 5,000 sea lions on the coast of Peru, along with more than 100,000 birds. The year prior to that, a smaller outbreak hit New England Harbor seals, where seal-to-seal transmission was suspected. The CDC reports that nearly 86 million wild aquatic birds, commercial poultry, and backyard or hobbyist flocks in 48 states have been infected in the U.S. beginning in January 2022. The increased infections in birds have created more opportunities for spillover events — instances where the virus jumps from a bird host to a mammal.