Since it was introduced to cattle in late 2023, a version of the H5N1 bird flu virus known as B3.13 has infected more than 950 herds in 16 states. But now, a new version is spreading, per the Guardian. This one, D1.1—the same genotype that infected the first American to die of bird flu—was detected in milk as part of a surveillance program in December and confirmed in dairy cattle in Nevada on Friday, the US Department of Agriculture said Wednesday. Four herds in Churchill County have tested positive for the genotype, with two others under quarantine pending lab results, per CBS News.
The tests indicate H5N1 has spread from birds into cows at least twice and may continue to do so, per the New York Times. "I always thought one bird-to-cow transmission was a very rare event. Seems that may not be the case," influenza expert Richard Webby tells the Guardian. "We need to now consider the possibility that cows are more broadly susceptible to these viruses than we initially thought," adds fellow expert Louise Moncla, per the Times. The USDA said it would post genetic sequences of the genotype to a public repository later this week. The information will help experts understand whether the second spillover occurred recently, or not.
The spillover is a concern because 40 of 67 confirmed human bird flu cases came from exposure to infected cows, and humans infected with D1.1 show more severe symptoms than those infected with B3.13. Apart from the fatal Louisiana case, tied to exposure to a backyard flock, the genotype was also found in a Canadian teenager who suffered organ failure and was placed on life support before recovering. The source of that infection is unknown. In cows, symptoms of D1.1 are similar to those of B3.13. These include fever, reduced feed consumption, reduced milk production, as well as coughing, sneezing, and runny nose. (More bird flu stories.)
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