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.

Bird flu is sweeping through egg-laying chickens in the United States at an unprecedented rate. So far in 2025, 30 million layers, as they’re known, have been culled, close to the 38 million killed throughout all of last year: Nearly 10 percent of the country’s annual number of egg-layers have been wiped out.