Another bird flu strain found in U.S. dairy cattle, USDA says

CBC

06-02-2025

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.
The D1.1 version in the Nevada cattle has been found in wild birds. It was also found in a teenager who was infected in B.C. who was treated for two months before being discharged and in a deadly case in Louisiana in January.
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."
Wild birds likely transmitted the second strain to cattle in Nevada, said J.J. Goicoechea, Nevada's agriculture director. Farmers need to ramp up safety and security measures to protect their animals, he said.
"We obviously aren't doing everything we can and everything we should or the virus wouldn't be getting in," he said.
The Nevada Department of Agriculture said on January 31 that herds in two counties had been placed under quarantine due to bird flu detections.
It is important for the USDA to contain the outbreak in the state quickly, so the strain does not spread to dairy cattle elsewhere, said Gail Hansen, a veterinary and public health consultant.
Last year, bird flu spread across the country as infected cattle were shipped from Texas after the virus first leapt to cows from wild birds.
"We didn't get a hold on it before," Hansen said. "We want to avoid that same scenario from happening in Nevada."
Dairy herds that were formerly infected may be at risk again from the second strain, experts said.
"Now it looks like we have new strains of virus that may escape some of the immunity associated with the other strains of viruses that could exacerbate the epidemics among animals and wildlife," said Gregory Gray, a University of Texas Medical Branch professor studying cattle diseases.