Wildlife likely spread H5N1 to Wisconsin dairy herd

The recent detection of highly pathogenic avian influenza (type A H5N1) in a Wisconsin dairy herd has resulted from a new wildlife-to-cattle transmission distinct from earlier outbreaks, according to a genetic analysis by the U.S. Department of Agriculture (USDA).

The virus confirmed as H5N1 clade 2.3.4.4b genotype D1.1 was detected in a herd of approximately 500 cows in Dodge County through the National Milk Testing Strategy (NMTS), according to a December 19 statement from the USDA's National Veterinary Services Laboratories.

The USDA believes there is no reason for concern about the safety of the commercial milk supply or consumer health because products are pasteurized prior to entering the market. Moreover, the Centers for Disease Control and Prevention considers the human health risk for this virus to be low.

H5N1 trends

More than 1,000 infections in dairy cattle in 19 U.S. states have been confirmed, according to the USDA's Animal and Plant Health Inspection Service.

Officials believe the Wisconsin dairy herd was infected during a spillover event caused by wildlife, according to Dr. Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory. He explained that no cattle were moved on or off the affected farm. Also, no other H5N1 cases have been detected through the NMTS, cow movement testing, or other required testing for exhibition or sale within the state. And finally, no clinical signs were noted in the herd.

"Together, these points are very similar to new spillover events that we recorded in Arizona and Nevada in early 2025," Dr. Poulsen said, adding that the Wisconsin outbreak appears to be contained to a single farm.

Dr. Poulsen explained that the B3.13 clade is still the predominant H5N1 strain affecting dairy cattle in Arizona, California, Idaho, Nebraska, and Texas, according to the USDA, and is the result of a single spillover event in the Texas panhandle in late 2023.

The biggest risk factor for interstate movement is movement of lactating cows, he said.

"We have yet to work out the pathophysiology of H5N1 in dairy cattle, likely because we cannot replicate natural infection in experimental settings. B3.13 also causes significant morbidity, especially the first time a farm is affected. We do not see this with D1 clade infections, but there are far fewer instances of D1 infections, so we need more data to be 100% confident," Dr. Poulsen added.

Future considerations

He added that the D1 clade has been the predominant variant in North American flyways for over two years.

"Before the second week of December 2025, we thought that spillover risk was limited to desert areas where migratory birds may share water and feed sources. Midwestern dairies typically do not have waterfowl flying around the free stall barns and there is plenty of food and water in the natural waterways.

"Having a D1 pop up in the upper Midwest, even if it is a one-off event, gives us pause, and we may need to rethink long-term management strategies. As long as HPAI is in global migratory flyways, we are going to have to think about spillover to dairy cows and domestic poultry," he said.

The affected farm in Dodge County, located in southeastern Wisconsin, remains under quarantine while the Wisconsin Department of Agriculture, Trade and Consumer Protection conducts an epidemiological traceback to ensure no further spread has occurred.

Dr. Poulsen said Wisconsin will return to monthly surveillance for the NMTS, a federal order implemented in December 2024 requiring raw milk sampling from silos nationwide.

"The question we all are asking ourselves now is to what end?" he said. "Migration season runs February to June and September to December, but it isn't sustainable to maintain surveillance forever."

Dr. Poulsen strongly supports the biosecurity recommendations from the National Milk Producers Federation because they distill the Secure Milk Supply guidelines to focus on H5N1.

"Biosecurity is not a magic bullet for protection," he said, "but it will reduce the time the herd is affected and in quarantine, increase the time to infection, and have fewer negative health effects for the herd."