A new strain of the H5N1 bird flu virus has been identified in dairy cattle in Nevada, marking a distinct variation from the version that has been circulating in the United States since last year, according to U.S. officials.
The discovery suggests that the virus has jumped from wild birds to cattle at least twice, raising concerns about its spread and the challenges of containing infections in both animals and humans.
“I always thought one bird-to-cow transmission was a very rare event. Seems that may not be the case,” said Richard Webby, an influenza expert at St. Jude Children’s Research Hospital.
The previously identified version of the virus, B3.13, was first detected in cattle in late 2023 and has since infected more than 950 herds across 16 states. The newly discovered strain, D1.1, was confirmed in Nevada cattle last week through a milk surveillance program initiated in December, according to the U.S. Department of Agriculture.
“Now we know why it’s really important to test and continue testing,” said Angela Rasmussen, a virologist at the University of Saskatchewan, who was involved in identifying the initial spillover.
The D1.1 strain has been linked to the first U.S. fatality associated with bird flu—a person in Louisiana who died in January after developing severe respiratory symptoms following exposure to wild and backyard birds. It was also connected to a severe case in Canada, where a teenage girl was hospitalized for months with an infection traced to poultry.
According to the U.S. Centers for Disease Control and Prevention (CDC), at least 67 people in the U.S. have contracted bird flu, primarily those working closely with dairy cattle. However, no human-to-human transmission has been reported.
Agriculture officials plan to release genetic sequencing data and additional information about the new strain later this week, which researchers say will be crucial in determining whether the virus has recently emerged or has been circulating undetected for a longer period.
