Bird flu was probably circulating in dairy cows for at least four months before it was confirmed to be the highly pathogenic H5N1 virus, according to a new analysis of genomic data by scientists at the US Department of Agriculture’s Animal Disease Center.

Bird flu likely circulated in U.S. dairy cows on a limited basis for about four months before federal officials confirmed the disease that has now spread to nine states, according to a new federally funded research paper.

Particles of bird flu virus have come up in milk, sour cream and cheese samples. Baby formula has tested negative. See what products are safe.