Rapid spread of H5N1 bird flu through California dairy herds suggests unknown paths of transmission

In the ongoing outbreak of H5N1 bird flu among the nation’s dairy cattle, federal officials have consistently expressed confidence that they know enough about how the virus is spreading to put a stop to it. But among epidemiologists and other infectious disease experts, there has been skepticism that the U.S. Department of Agriculture’s theory of viral transmission is telling the whole story. And perhaps there is no greater cause for scrutiny than what’s currently happening in California.

Since the first identification of three infected herds there in late August, California authorities have found the virus in 659 of the state’s 984 dairies — about half of them in the last month alone.

On Wednesday, in response to the explosive spread of the virus among the state’s dairy herds, California Gov. Gavin Newsom declared a state of emergency. “This proclamation is a targeted action to ensure government agencies have the resources and flexibility they need to respond quickly to this outbreak,” Newsom said in a statement.

California, which despite being the nation’s largest dairy-producing state had avoided the virus for the first five months of the outbreak, now makes up the vast majority of reported dairy cattle infections.

“While some farmers may have been less strict” in following USDA precautions to prevent the spread of H5N1, “I personally know a fair number of producers that pulled out all the stops, followed every suggestion, came up with novel protections of their own,” Mike Payne, a food animal veterinarian and biosecurity expert with the University of California, Davis’ Western Institute for Food Safety and Security, told STAT in an email. “They still got infected and were enormously disheartened and frustrated.”

Early on in the outbreak, genetic analyses suggested that the virus was expanding its footprint primarily through the movement of cattle across state lines. In April, the USDA moved to limit that through a federal order requiring testing of lactating cattle prior to interstate movement, although farmers didn’t have to test every member of larger herds and could choose which animals to test. As the virus continued to pop up in new places, the USDA conducted surveys of farmers in hotspots like Michigan and Colorado, where state authorities were more aggressively testing for the virus.