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U.S. Department of Agriculture detects second bird flu strain in dairy cattle

U.S. dairy cattle tested positive for a strain of bird flu that previously had not been seen in cows, the U.S. Department of Agriculture said on Wednesday, ramping up concerns about the persistent spread of the virus.

The H5N1 virus has reduced milk output in cattle, pushed up egg prices by wiping out millions of hens, and infected nearly 70 people since April as it has spread across the country.

Genome sequencing of milk from Nevada identified the different strain, known as the D1.1 genotype, in dairy cows for the first time, the USDA said. Previously, all 957 bird flu infections among dairy herds reported since last March had been caused by another strain, the B3.13 genotype, according to the agency.

Reuters reported news of the detection of the second strain on Wednesday ahead of USDA’s announcement.

The second strain was the predominant genotype among wild birds this past fall and winter and has also been found in poultry, the USDA said. It was identified in dairy cattle through an agency program that began testing milk for bird flu in December.

“We’re seeing the H5N1 virus itself be smarter than all of us,” said Beth Thompson, South Dakota’s state veterinarian.

“It’s modifying itself so it’s not just staying in the poultry and the wild waterfowl. It’s picking up a home in the mammals.”

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2026 T.K. Cheung Lecture in Animal Science - Dan Weary

Video: 2026 T.K. Cheung Lecture in Animal Science - Dan Weary

T.K. Cheung Lecture in Animal Science: "Using science to assess and improve the welfare of dairy cattle"

Dan Weary is a Professor at the University of British Columbia. Dan did his BSc and MSc at McGill and Doctorate at Oxford before co-founding UBC’s Animal Welfare Program where he now co-directs this active research group. His research focuses on understanding the perspectives of animals and applying these insights to develop methods of assessing animal welfare and improving the lives of animals. His work has helped drive changes in practices (including the adoption of higher milk rations for calves and pain management for disbudding) and housing methods (including the adoption of social housing for pre-weaned calves). He also studies cow comfort and lameness, social interactions among cows, and interactions between cows, human handlers and technologies like automated millking systems that are increasingly used on farms. His presentation will outline key questions in cattle welfare, highlight recent UBC research addressing them, and showcase innovative methods for improving the lives of cattle and their caretakers.