H5N1 bird flu is branching out. New data from the Centers for Disease Control and Prevention indicates that a second type of H5N1 has likely spread from dairy cows to humans.
The CDC and health officials in Nevada detailed the case on Monday—the first human case of highly pathogenic avian influenza H5N1 recorded in the state. The afflicted individual is a farm worker who was presumably infected by cows carrying the D1.1 genotype of H5N1, which differs from the type of virus most commonly seen in cows since 2024. The worker reportedly only experienced conjunctivitis, and no further human cases connected to the worker have been identified to date.
Following years of circulation in wild birds, H5N1 jumped the species barrier from birds to dairy cows in early 2024. The virus has caused outbreaks among cows in 16 states since last year, and it’s occasionally spilled over into other mammals such as cats and people. It’s also continued to cause mass die-offs of wild birds and poultry.
The genotype of H5N1 that originally jumped into cows from birds is called B3.13. This type has been associated with relatively mild illness in cows and humans. But earlier this month, federal and local officials confirmed that the D1.1 genotype was spotted among cows at a dairy farm in Churchill County, Nevada. In the recent case, the farm worker had been exposed to sick cows prior to his illness, and his infection was confirmed to be D.1.1, suggesting that he caught it from the cows. It’s the 68th total case of H5N1 in humans found in the U.S. since 2024.
D.1.1 has quickly become the prevalent type of H5N1 currently circulating in wild birds, and there have been occasional human infections tied to birds in recent months. In January, Louisiana health officials reported a case of D.1.1 in a resident who was exposed to a combination of infected backyard chickens and wild birds. The person ultimately died from the infection—the first human death tied to H5N1 in the U.S. during this current outbreak. Last November, Canada reported its first human case of H5N1, which involved a teenage girl who likely caught D.1.1 from wild birds. The teenager was hospitalized in critical condition as a result but appears to have survived.
These newest cases in cows and a human are additionally concerning for several reasons. For one, it indicates that H5N1 has spilled over from birds to cows at least twice lately, and that D.1.1 can then spread from cows to humans. USDA officials have also identified one mutation in the cow-derived strains of D.1.1 that is associated with mammalian adaptation—in other words, a change that should make it easier for H5N1 to infect and potentially sicken mammals.
The good news, for now, is that neither D.1.1 nor any other type of H5N1 appears to have spread from person to person. But we’re finding ourselves in a precarious situation. The longer these flu viruses are able to freely circulate between birds, cows, cats, humans, and other mammals, the greater the risk that a novel strain well adapted to humans will emerge. From there, the next major epidemic or pandemic may not be too far off.
