H5N1 virus was undetected for months in dairy cattle, researchers say

Genetic testing indicates the H5N1 bird flu virus jumped from wild birds to dairy cattle in Texas approximately four months before it was identified in late March, said a nationwide team of researchers. “Continued transmission of H5N1 HPAI [highly pathogenic avian influenza] within dairy cattle increases the risk for infection and subsequent spread of the virus to human populations.”

To date, bird flu has been confirmed in 36 dairy herds, from Idaho to North Carolina. There are 12 infected herds in Texas. The scientists said on the preprint site bioRxiv that interstate shipment of asymptomatic cattle “has likely played a role in the spread of HPAI within the U.S. dairy herd.” Infected cows may shed the HPAI virus for two to three weeks, they said.

Meanwhile, the USDA said it had found no bird flu virus in 30 samples of ground beef purchased in states with infected herds. “These results reaffirm that the meat supply is safe,” said the Food Safety and Inspection Service. The FDA says its tests show that pasteurization kills the H5N1 virus and that the milk supply is safe. The CDC says the risk to human health from the virus is low.

“Our analysis of transmission chains within the cattle B3.13 clade [virus] using a phylogenomic approach suggested unsampled transmission in late 2023 and early 2024,” said the researchers, which would be earlier than previously known. Animal health officials identified bird flu in dairy cattle following reports of a mysterious illness among dairy herds in the Texas Panhandle in February.

The team of researchers said surveillance was essential to make sure the virus does not adapt to increase its transmissibility. There have been “as many as five cattle-to-poultry, one cattle-to-raccoon, two cattle-to-domestic cats, and three cattle-to-wild bird transmission events” on dairy farms, they said.

“Monitoring of cattle for HPAI will inform epidemiological risk and provide an early warning for whether this interspecies transmission event and dissemination of the viruses through the U.S. dairy cattle herd represents a future threat to human health,” they said.

During a teleconference on Wednesday, the USDA’s chief veterinary officer, Dr. Rosemary Sifford, said the department’s focus was on identifying infected herds and eradicating the H5N1 virus on the farm.

“We have not seen any evidence of the virus in the beef herd at this time,” she said.

Texas is both the No. 1 cattle state and a leading dairy state.

Some 27 scientists, many of them USDA animal disease researchers, collaborated on the study appearing on bioRxiv. Researchers from Iowa State University, Cornell, Texas A&M, and Washington State University also contributed. The study has not been certified by peer review.