journals.asm.org
mGem: Transmission and exposure risks of dairy cow H5N1 influenza virus | mBio
Highly pathogenic avian influenza (HPAI) H5N1 virus of the 2.3.4.4b clade has been circulating in North America since 2022 (1–3). On 25 March 2024, a strain in this clade, genotype B3.13, was confirmed to be infecting dairy cows in Texas and Kansas (Fig. 1A). Since then, dairy herds in several other states tested positive for this strain and spillover into dairy workers has followed (4–7). Infections of other domestic and peridomestic animals, such as cats and raccoons, have also occurred on the same premises containing infected cows (7, 8). Notably, the spread of cow H5N1 to poultry flocks has resulted in the culling of millions of birds and the infection of workers involved in culling operations on multiple occasions (9). As of 12 December 2024, over 800 dairy herds in 16 states have had confirmed infections, including three of the five biggest dairy-producing states (Fig. 1B) (10, 11). At least 58 known human cases of H5N1 including those exposed to dairy cows, poultry, or unknown exposure source have been reported by the CDC as of 12 December 2024 (12). Disturbingly, in September, cow H5N1 infections were confirmed in two housemates in Missouri without livestock contact (13, 14). Understanding how cow H5N1 transmits within and between farms is critical for curtailing the outbreak. Below, we discuss knowledge regarding the spread of cow H5N1 between farms, between cows, and the risk to humans.
