Researchers still aren’t sure how H5N1 influenza spreads between cows and from farm to farm. After 2 years, the U.S. outbreak of the H5N1 influenza virus in cattle appears to be waning, easing fears that the virus could cause long-lasting damage to the dairy industry or mutate into a form that could cause a human pandemic. The last new detection of an affected herd occurred on 13 December 2025 at a Wisconsin farm, according to the website of the U.S. Department of Agriculture (USDA). Data from a USDA program that tests milk for the virus suggest 16 of the 19 affected states have gotten rid of the virus, which originated in wild birds.
But it could still bounce back, and efforts to eliminate it entirely face formidable challenges. How it spreads between cattle remains unclear. And although candidate vaccines look promising in early tests, farmers and the government may be reluctant to embrace them.
H5N1 continues to circulate on farms in California and Idaho, those states’ agriculture departments told Science. Texas remains “affected” in USDA’s latest update, even though it has not had a detection since May 2025, because it has not complied with the National Milk Testing Strategy requirement that it sample all silos at milk-processing plants. In other states the virus may simply have escaped detection.