USDA Confirms Spillover of 2nd H5N1 Avian Flu Genotype Into Dairy Cattle

By Lisa Schnirring

The US Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) today announced a new spillover of H5N1 avian flu to dairy cattle, which involves the D1.1 genotype currently circulating in wild birds and has been implicated in human infections, including the fatal case in a Louisiana resident who had contact with sick backyard birds.

Until now, all dairy herd H5N1 detections have involved the B3.13 genotype, thought to be the result of a single spillover from wild birds in late 2023 or early 2024. The genotype has been linked to mild infections in dairy workers, along with some poultry cullers, with conjunctivitis the main symptom.

"Genotype D1.1 represents the predominant genotype in the North American flyways this past fall and winter and has been identified in wild birds, mammals, and spillovers into domestic poultry," APHIS said in its statement.

Detection came from milk testing in Nevada

The new spillover was detected in an investigation that followed confirmation of H5N1 in milk sampling from Nevada dairy cows. A few days ago, APHIS reported four recent H5N1 detections in Nevada dairy herds, the state's first since December.

Nevada officials said they were removing large nuisance populations of European starlings to curb the spread of the virus. The non-native species typically migrate through Nevada in the winter and outcompete local birds and pose a risk of spreading diseases to livestock.

APHIS said the detection doesn't change its eradication strategy and serves as a testament to its National Milk Testing Strategy, which, as of the middle of January, had enrolled 36 states, covering milk from two thirds of the nation's diary herds.

H5N1 problem is here to stay

University of Minnesota epidemiologist Michael Osterholm, PhD, MPH, said, "We shouldn't be surprised about a new spillover to cattle, given the very significant activity in waterfowl across much of the United States."

He added that the virus is not going away, contrary to those who thought B3.13 would burn itself out. Osterholm is director of the Center for Infectious Disease Research and Policy, which publishes CIDRAP News.

The USDA said it is working with Nevada agriculture officials on the ongoing investigation to better understand the detection and to limit further spread. It added that it will publish a technical brief on the findings and will post the sequence on GenBank in the coming week.