The U.S. Department of Agriculture (USDA) has confirmed a new case of highly pathogenic avian influenza (HPAI) in a Wisconsin dairy herd, identifying it as a fresh spillover event from wildlife to cattle, not connected to earlier outbreaks.
According to the USDA’s Animal and Plant Health Inspection Service (APHIS), this case represents a separate introduction of the virus, rather than spread linked to previously infected herds.
The virus was confirmed as H5N1 clade 2.3.4.4b, genotype D1.1, following whole genome sequencing conducted by the National Veterinary Services Laboratories (NVSL) on December 17.
USDA officials emphasized that advanced testing allowed them to quickly determine the virus strain and confirm its origin.
Most detections of bird flu in U.S. dairy cattle to date have been traced back to animal movements linked to an original spillover in Texas in late 2023, which involved the B3.13 strain.
Earlier in 2025, two isolated spillover events involving the D1.1 strain were detected in Nevada and Arizona. The Wisconsin case now adds another independent wildlife-to-cattle transmission to that list.
The Wisconsin case was identified through USDA’s National Milk Testing Strategy, a surveillance program designed to detect infections early.
At this time, no additional herd infections have been found, and APHIS reports that the situation appears contained.
USDA stressed that this detection does not pose a risk to consumers or the commercial milk supply.
Pasteurization kills the virus
Milk from affected animals is diverted or destroyed
Food safety protocols remain unchanged
USDA reiterated that commercial milk remains safe to drink, and ongoing monitoring will continue nationwide.
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