February 12, 2026

Significant presence of H5N1 viral RNA in US retail milk during spring 2024-25 bird flu outbreak, report found

Influenza A(H5N1) viral RNA was widely present in US retail milk during the spring 2024–25 outbreak among dairy cattle in the United States, according to a report published recently in Emerging Infectious Diseases.

In milk samples collected from April 13 to May 3, 2024, researchers detected influenza A viral RNA in 36% of samples from 13 US states, including in five states (Arkansas, Indiana, Minnesota, Missouri, and Oklahoma) that had no reported outbreaks at the time. Across the country, only 29 infected herds had been reported as of April 12, a total that was inconsistent with the number of positive samples.

"Our study revealed that early in the outbreak, the influenza A(H5N1) virus was more widespread than reported," the researchers write.

The team led by scientists at The Ohio State University College of Veterinary Medicine compared the findings with samples collected from December 27, 2024 to January 29, 2025. During the later period, only 6.9% of retail samples were positive. All the infected milk samples had been processed in California.

Federal directives in April and December 2024 mandated increased testing to help identify infected herds and slow the spread of A(H5N1). Reported cases rose to more than 1,000 herds (which is expected when surveillance efforts broaden). The increased surveillance efforts revealed infection patterns more closely aligned with official detection numbers, suggesting that enhanced testing improved outbreak monitoring.

"Taken together, our findings suggest that early in the outbreak, cases in US dairy herds were widespread and went undetected, but federal regulations have since improved detection and worked to control the spread of H5N1 virus in dairy herds," the researchers say.

- CIDRAP