24 May 2026
A breakthrough study led by Prof. Andrew S. Bowman and his team at The Ohio State University reveals that an incredibly low dose of just 10 infectious particles of highly pathogenic avian influenza (H5N1) is sufficient to establish a robust infection in a cow's udder. This finding highlights the extreme susceptibility of bovine mammary tissue while raising critical new questions about how the virus actually spreads on commercial dairy farms. The researchers evaluated several exposure routes in peak-lactation Holstein cows and calves. Infusing individual udder quarters with a remarkably low dose of 10TCID50 successfully established a severe infection. Within three days, all virus-exposed quarters developed clinical mastitis, characterized by altered milk color and consistency, alongside systemic signs. Peak viral loads in the milk surged to over 1011TCID50/mL. When cows were exposed to a higher intramammary dose 107TCID50, the disease progressed rapidly. Both animals exhibited necrotizing mastitis with severe tissue necrosis, profound lethargy, and a complete loss of appetite, necessitating humane euthanasia within 2 to 4 days post-inoculation. In contrast to the udder's extreme sensitivity, exposing cows to a massive dose 108.5EID50 of aerosolized virus intranasally, or bottle-feeding young calves heavily infected, unpasteurized milk, failed to produce overt clinical disease. These findings highlight the heavy economic and animal welfare burden imposed by bovine H5N1, as viral-induced mastitis inflicts direct damage on milk-producing epithelial cells, causing persistent production losses or death. From a public health standpoint, the sustained circulation of H5N1 in cattle is alarming.
Lee et al. (2026) Nat Commun
