US confirms new bird flu spillover in Wisconsin dairy herd
H5N1 case traced to wildlife, separate from earlier Texas outbreak
22 December 2025
1 minute read
By: The United States Department of Agriculture (USDA) on Friday confirmed that a case of highly pathogenic avian influenza in a Wisconsin dairy herd marked a new spillover event from wildlife to cattle, separate from previous outbreaks, reported Reuters.
The virus, identified as H5N1 clade 2.3.4.4b genotype D1.1., was confirmed through whole genome sequencing by the National Veterinary Services Laboratories on December 17, USDA’s Animal and Plant Health Inspection Service said in a statement.
It said most detections of highly pathogenic avian influenza in US dairy herds have resulted from movements linked to an original spillover event that occurred in Texas in late 2023, involving the B3.13 strain.
Earlier this year, two isolated spillovers were detected in Nevada and Arizona, involving the D1.1 strain.
The Wisconsin case, detected under USDA's National Milk Testing Strategy, has not led to additional herd infections, APHIS said.
USDA said the findings do not pose a risk to consumer health or the commercial milk supply, as pasteurization kills the virus and milk from affected animals is diverted or destroyed.
It added that the Centers for Disease Control and Prevention continues to consider the risk to the public to be low.
USDA urged dairy producers to maintain strict biosecurity and report any livestock showing clinical signs or unusual wildlife deaths.
A bipartisan group of US senators last week urged the administration of President Donald Trump to finalise a science-based plan for developing a bird flu vaccine for livestock, according to a letter seen by Reuters.
