The newer variant of avian influenza that recently infected dairy cattle in Nevada has a genetic change that’s thought to help the virus copy itself in mammals — including humans — more easily, according to a new technical brief from the US Department of Agriculture’s Animal and Plant Health Inspection Service. However, it’s unclear whether these viruses pose a bigger threat to people. The CDC says the risk of H5N1 to the public is still low, although people who work on farms or who have backyard flocks are at higher risk. The USDA report comes as a dairy farm worker in Nevada has screened positive for H5N1, the first human infection identified in the state. The worker’s symptoms include red, inflamed eyes, or conjunctivitis, according to a source familiar with the details who was not authorized to speak to the media. The US Centers for Disease Control and Prevention is working to confirm the initial positive test. Scientists are closely watching the new infections in Nevada because gene sequences from affected cattle have shown that they are a subtype of the H5N1 virus, D1.1, which has been associated with two severe human infections in North America, including one death. The affected herds, in the same valley in Churchill County, are the first known instances of spillover from D1.1 into cattle. The strain had previously been found only in birds and people. Most bird flu infections in dairy cattle in the US have been the B3.13 variant, or what’s become known as the “cattle clade.” Researchers aren’t sure how the D1.1 variant was transmitted to the Nevada cows. Dairy farmers with infected herds reported large die-offs of wild birds near their farms before their cows got sick, according to the USDA. That led investigators to believe that the cows encountered infected birds, or perhaps their droppings and caught the virus that way. Wild birds are hard to control on farms, where a feed trough filled with grain can look like a giant bird feeder. Gene sequencing of these D1.1 viruses has found a mutation that helps the virus copy itself more efficiently into the cells of mammals, including people. This change hasn’t been seen in other D1.1 infections in wild birds or poultry, according to the USDA. It raises the possibility that another animal, perhaps a cat or fox, brought the virus onto these farms. “This mutation … provides the virus with the ability for enhanced replication, which poses a threat to humans that are exposed to these cows,” said Dr. Seema Lakdawala, a microbiologist and immunologist at Emory University, who has been studying how H5N1 has been spreading in dairy herds.