Viral Mastitis Associated with Influenza A in Dairy Cattle

In early 2024, farmers in the southwestern United States described a mysterious disease outbreak causing acute illness and milk drop. Diagnostic investigations determined this syndrome to result from H5N1 influenza A virus infection. Further investigations and research indicate the virus is present in high concentrations in milk from lactating cows, but had to find in other sample types. Receptors that allow for viral attachment are expressed on the mammary epithelium, creating susceptibility of lactating cows. The disease impacts a small percentage of the herd, but the degree of milk drop and increase in somatic cells in milk substantially impact dairy profitability.