Transmission among mammals of bovine highly pathogenic avian influenza (HPAI) H5N1 viruses, which have caused outbreaks in US dairy cattle, has been demonstrated in ferrets by our group (4, 5) and the US Centers for Disease Control and Prevention (CDC).These studies showed that these viruses can be transmitted among ferrets via respiratory droplets, albeit with lower efficiency than seasonal human influenza viruses. In contrast, bovine HPAI H5N1 viruses spread easily among ferrets through direct contact (3 of 3 [100%] ferrets) .