The U.S. issued a federal order on Friday mandating that the national milk supply be tested for bird flu, Agriculture Secretary Tom Vilsack told Reuters, as authorities seek to grapple with rapid spread of the virus among dairy herds.

The move comes amid an outbreak of bird flu that has sickened dozens across seven states and marked the first time the virus was identified in dairy cows. Here’s what to know.

The mandate targeting bulk milk transporters and dairy processors is meant to help identify herds that have caught the virus, which has spread among livestock.