Dairy industry experts say the virus entered California after local cows were shipped to another state and then returned to California.

However, there is urgent concern over dairy cows. In California’s Central Valley, the carcasses of dead cattle have been piling up, a phenomenon documented in videos taken by a local veterinarian. According to the CDC, there have been confirmed infections in more than 300 herds of cattle in the U.S. in 14 states since March.