Avian Influenza H5N1 Reaches Dairy Cattle in the U.S. and Raises Alerts in Europe for Potential Spread

The emergence of the highly pathogenic avian influenza (HPAI) virus H5N1 clade 2.3.4.4b, genotype B3.13, in dairy cattle in the U.S. represents a significant change in the epidemiological profile and host range of the virus. Until May 2025, outbreaks have been confirmed in 981 herds across 16 states, with California being one of the most affected.

Infected animals often show mild clinical signs such as fever, mastitis, and reduced milk yield. Although the average mortality rate is 2%, the virus has a high replication capacity in the mammary glands, allowing for the excretion of large amounts of virus in the milk for three weeks, even in cows without visible symptoms. Transmission between animals within a farm mainly occurs through contaminated milk and milking procedures. Spread between farms is related to the movement of cattle, shared equipment use, and contact with external personnel.

In response, U.S. authorities have implemented strict movement controls, mandatory testing, and improvements in biosecurity protocols.

In Europe, the EFSA (European Food Safety Authority) has issued a report analyzing the risks of the virus entering European territory. Although the importation of dairy cows and beef from the U.S. is limited and regulated by strict standards, entry through products with raw milk cannot be completely ruled out.

Additionally, the migratory routes of wild birds represent another possible entry channel, especially in high concentration areas such as Iceland, Great Britain, Ireland, Western Scandinavia, or the Wadden Sea wetlands.

The EFSA will publish an assessment of the potential impact of this genotype in Europe at the end of the year and will offer recommendations to curb its spread, in a context of growing concern over the detection of mutations adapted to mammals and sporadic cases in humans.

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You can consult the references of the article: 

EFSA (2025).
Avian influenza: update on HPAI A(H5N1) clade 2.3.4.4b in dairy cattle and potential entry pathways into the EU.
Published in June 2025.
Available at: https://www.efsa.europa.eu