Wisconsin Dairy H5N1 Case Tied to Wildlife Spillover

By Blake Jackson

A recent case of highly pathogenic avian influenza (HPAI) type A H5N1 detected in a Wisconsin dairy herd appears to stem from a new wildlife-to-cattle transmission, separate from earlier outbreaks, according to genetic analysis conducted by the U.S. Department of Agriculture.

The virus, identified as H5N1 clade 2.3.4.4b genotype D1.1, was found in a Dodge County herd of roughly 500 cows through the National Milk Testing Strategy, the USDA’s National Veterinary Services Laboratories confirmed in a December 19 statement.

Federal officials emphasized there is no threat to the commercial milk supply, as milk is pasteurized before reaching consumers, and the Centers for Disease Control and Prevention considers the human health risk to be low.

Nationwide, more than 1,000 H5N1 infections in dairy cattle have been confirmed across 19 states, according to USDA’s Animal and Plant Health Inspection Service. However, Wisconsin officials believe this case resulted from a single spillover event involving wildlife.

“Together, these points are very similar to new spillover events that we recorded in Arizona and Nevada in early 2025,” said Dr. Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory, noting that no cattle were moved on or off the affected farm and no clinical signs were observed. He added the outbreak appears limited to one operation.

Poulsen explained that the B3.13 clade remains the dominant strain affecting dairy cattle in several western and central states and originated from a single spillover in Texas in late 2023. “The biggest risk factor for interstate movement is movement of lactating cows,” he said.

Discussing future risks, Poulsen noted, “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.”

The Dodge County farm remains under quarantine as state officials conduct epidemiological tracing. Wisconsin will return to monthly NMTS surveillance, though Poulsen questioned the long-term feasibility, asking, “The question we all are asking ourselves now is to what end?”

He emphasized the importance of industry biosecurity guidance, saying, “Biosecurity is not a magic bullet for protection,” but it can reduce impacts and shorten quarantine periods.

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