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.

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.

On Thursday, a USDA spokesperson told STAT in an email that all the research to date suggests that transmission of H5N1 between cattle is largely believed to be due to fomites — that is, objects that come into contact with cattle that carry the virus on them, for example milking equipment and people’s clothing. “Transmission between farms is likely related to normal business operations such as people, vehicles and other farm equipment frequently moving between premises,” the spokesperson said. “That’s why strong biosecurity is critically important in stopping the spread of the virus and why USDA is strongly encouraging farmers to heed biosecurity recommendations.”

“Any ‘expert’ who really is following the outbreak and scientific trials here in California will tell you we think we know some of the ways the virus is being transferred from herd to herd, but not all of them,” Payne said. “Honestly, there’s probably more we don’t understand than we actually do know.”

More: https://www.statnews.com/2024/12/20/california-h5n1-bird-flu-emergency-declaration-avian-flu-spread-dairy-cattle/