Federal testing improves detection of H5N1 avian flu in USA dairy herds – research finds

USA – Influenza A(H5N1) viral RNA had been widely present in USA retail milk during the spring 2024–2025 outbreak among dairy cattle, according to a report published late last week 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 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 figure 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 regulations boost outbreak monitoring

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,” write the researchers.

 USDA confirms highly pathogenic avian influenza in dairy herd

The USDA Animal and Plant Health Inspection Service (APHIS) and the National Veterinary Services Laboratories (NVSL) confirmed a detection of highly pathogenic avian influenza (HPAI) H5N1 clade 2.3.4.4b, genotype B3.13 in a dairy cattle herd in Nebraska.

This confirmation was a result of state tracing and investigation, following an initial detection from pre-movement surveillance milk samples required under the USDA’s April 2024 Federal Order.

This is the first known case of HPAI in cattle in Nebraska. While dairy cattle in a total of 17 states have been infected since the start of the outbreak in March 2024, APHIS has seen cases in only a small number of states this year.

Producers should immediately report any livestock with clinical signs or any unusual sick or dead wildlife to their state veterinarian.

APHIS is working closely with the Nebraska Department of Agriculture to conduct additional on-farm investigation, testing, and gathering of additional epidemiological information to better understand this detection and limit further disease spread.

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