The emergence of highly pathogenic avian influenza A(H5N1) in US dairy cattle represents an unprecedented zoonotic event with critical pandemic implications. As of June 2025, over 1,070 dairy farms across 17 states have been affected, with documented spillover to humans resulting in 70 confirmed cases. The mechanistic basis for mammary tropism and high viral loads in milk remains unexplained, hampering control efforts.

A recent study led by Prof. Hualan Chen in Harbin Veterinary Research Institute, China elucidates the pathogenesis of H5N1 mammary gland infection through experimental cattle studies. Prof. Chen’s team demonstrate that mammary invasion occurs via "mouth-to-teat" transmission during nursing rather than systemic dissemination following respiratory infection. Crucially, both inactivated and DNA vaccines provided sterilizing immunity against multiple H5N1 strains. These findings define the transmission pathway underlying this zoonotic outbreak and establish vaccination as a viable intervention strategy.