U.S. health and agriculture officials are ramping up testing and tracking of bird flu in dairy cows in an urgent effort to understand — and stop — the growing outbreak.

So far, the risk to humans remains low, officials said, but scientists are wary that the virus could change to spread more easily among people.

The virus, known as Type A H5N1, has been detected in nearly three dozen dairy herds in eight states. Inactive viral remnants have been found in grocery store milk. Tests also show the virus is spreading between cows, including those that don’t show symptoms, and between cows and birds, according to the U.S. Department of Agriculture.

Starting Monday, hundreds of thousands of lactating dairy cows in the U.S. will have to be tested — with negative results — before they can be moved between states, under terms of a new federal order.