In March 2024, highly pathogenic avian influenza (HPAI) H5N1 of the clade 2.3.4.4b was detected in dairy cows in Texas and has since been detected in several other U.S. states. Virus has been detected within cow’s milk, indicating that the mammary epithelium may support viral replication. Virus has also been detected on milking machines, leading to a hypothesis that influenza is spreading through fomites from udder to udder instead of the intranasal route. There have been studies using cows to better understand mammary infections; however, the cow model is costly and limited. We sought to establish a model for intramammary infections of H5N1 and H1N1 influenza virus in mice.
To test the hypothesis that intraductal inoculation is a route of infection, lactating C57Bl/6 mice were inoculated with 0.5× mean 50% lethal dose (mLD50) of A/bovine/Ohio.B24OSU-439/2024 (H5N1) influenza virus into actively lactating nipples (an average of seven lactating nipples per mouse) using a 33G needle. The dams and respective pups were monitored daily to assess clinical signs, weight loss, and survival.