After a new study showed that bird flu has spread to dairy farm workers more often than previously known, the US Centers for Disease Control and Prevention is expanding its recommendations for testing and treatment of workers on farms where animals have tested positive.

Rising H5N1 bird flu cases in the U.S. prompted the CDC to study the virus in ferrets, revealing it may have the potential to spread and cause severe symptoms in other mammals.

Following a CDC study showing bird flu exposure among dairy workers, health officials recommend increased testing and preventative measures for farmworkers, even if they show no symptoms, to prevent virus spread and mutation.