Dairy cattle moving between states must be tested for the bird flu virus, U.S. agriculture officials said Wednesday as they try to track and control the growing outbreak.

The federal order was announced a day after health officials said they had detected inactivated remnants of the virus, known as Type A H5N1, in samples taken from milk during processing and after retail sale. They stressed that such remnants pose no known risk to people or the milk supply.

“The risk to humans remains low,” said Dawn O’Connell of the federal Administration for Strategic Preparedness and Response.

The new order requires every lactating cow to be tested and post a negative result before moving to a new state. It will help the agency understand how the virus is spreading, said Michael Watson, an administrator with the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service.