In December 2025, highly pathogenic avian influenza (HPAI) H5N1 clade 2.3.4.4b genotype DI.2.1 virus was detected in a cat living on a dairy cattle farm. Milk and serum samples from the dairy cattle were tested for avian influenza virus. No viral RNA was detected; however, H5N1-specific antibodies were identified in serum samples from 34 (47.2%) of 72 lactating dairy cows and 24 (63.2%) of 38 youngstock. These demonstrate expansion of the mammalian host range of HPAI H5N1 in Europe.