News release

From: Springer Nature

Milking procedures are likely to be the primary routes of H5N1 influenza virus transmission between dairy cows, suggests a paper published in Nature this week.

Highly pathogenic H5N1 avian influenza was detected in cattle in the USA in the spring of 2024 and has affected more than 190 dairy cattle farms in at least 13 states. Milking equipment may be a likely route of transmission, but more detailed evaluation is needed.

Jürgen Richt, Martin Beer and colleagues experimentally infected calves and lactating cows with H5N1 clade 2.3.4.4b, currently circulating in cattle in the USA, to provide insight about likely modes of transmission. They compared susceptibility and transmission in nine calves infected via the oral and nasal cavity, and three lactating cows via inoculation of the mammary glands; an additional three dairy cows were inoculated with a strain of bird flu circulating in Europe (H5N1 euDG).

Infection of the calves resulted in mild clinical presentation, with moderate viral replication but no transmission to co-housed calves. By contrast, the dairy cows displayed severe mammary gland infection, including fever, mastitis and abrupt reduction in milk yield. A drastic increase in the levels of virus in the milk was observed, but no evidence of systemic infection or nasal shedding in the nasal passages were detected. These findings indicate that the milk and milking procedures, rather than respiratory spread, are the likely primary routes of H5N1 transmission between cattle.