Risk posed by the HPAI virus H5N1, Eurasian lineage goose/Guangdong clade 2.3.4.4b. genotype B3.13, currently circulating in the US.

The emergence of highly pathogenic avian influenza (HPAI) A(H5N1), clade 2.3.4.4b, genotype B3.13 in U.S. dairy cattle marks a significant shift in the virus' host range and epidemiological profile. Infected cattle typically exhibit mild clinical signs, such as reduced milk production, mastitis and fever, with morbidity generally below 20% and mortality averaging 2%. Transmission within farms is primarily driven by contaminated milk and milking procedures, while farm-to-farm spread is mainly linked to cattle movement and shared equipment. The virus demonstrates high replication in mammary glands, with infected cows shedding large quantities of virus in milk for up to 3 weeks, even in the absence of clinical signs. Shedding through other routes appears limited. Infected cattle develop virus-specific antibodies within 7-10 days, offering short-term protection, though the duration and robustness of immunity remain unclear. Between March 2024 and May 2025, the virus was confirmed in 981 dairy herds across 16 U.S. states, with California particularly affected. Risk factors identified for between-farm spread include cattle movement, shared equipment and contact with external personnel, while biosecurity measures, including waste management and wildlife deterrence, may reduce the risk of virus introduction. In response to the outbreaks, U.S. authorities implemented strict movement controls, mandatory testing and enhanced biosecurity protocols. Potential pathways of introduction of HPAI B3.13 virus into EU via trade from US could be the import of lactating cows and bovine meat, although strict trade regulations, absence of animal import and limited virus detection in meat, especially in muscle tissue, do not support this occurrence. Import of products containing raw milk could also be potential pathways for virus introduction. Migratory birds - particularly waterfowl - pose potential pathways for introduction during seasonal migrations. The detection of mammalian-adaptive mutations and zoonotic cases underscores the virus' public health relevance and the need for research, surveillance and cross-sectoral preparedness.