On December 14, 2025, USDA’s Animal and Plant Health Inspection Service (APHIS) announced the first detection of highly pathogenic avian influenza (HPAI) in a dairy herd in Wisconsin. On December 17, the National Veterinary Services Laboratories (NVSL) completed whole genome sequencing and confirmed that the virus is H5N1 clade 2.3.4.4b genotype D1.1. Analysis indicates that this detection is a new spillover event from wildlife into dairy cattle, separate from previous events.

Key Points

Most detections in U.S. dairy herds have resulted from movements linked to the original spillover event that occurred in the Texas Panhandle in late 2023, involving the B3.13 strain.
In early 2025, through the National Milk Testing Strategy, USDA detected two spillover events in Nevada and Arizona dairy herds. Both were identified early, and no further herd infections occurred through animal movements. These events involved the D1.1 strain.
The Wisconsin herd, also detected through the National Milk Testing Strategy, represents a new, separate spillover event and involves the D1.1 strain. At this time, no additional dairy herds have been identified as infected in association with this event.
Public Health and Food Safety

This detection does not pose a risk to consumer health or affect the safety of the commercial milk supply. Pasteurization effectively inactivates HPAI virus, and milk from affected animals is diverted or destroyed to prevent entry into the food supply. The Centers for Disease Control and Prevention (CDC) continues to consider the risk to the public to be low.