Scientists are sounding alarms about a genetic mutation that was recently identified in four dairy cow herds, nearly one year after H5N1 bird flu was first reported in Texas dairy cattle. The change is one that researchers have dreaded finding because it is associated with increased mammal-to-mammal transmission and disease severity.

Avian flu is hitting California’s San Joaquin Valley hard — nearly 20 million chickens, ducks and turkeys have been killed in the region since 2022, according to data from the Centers for Disease Control and Prevention.