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.

The H5 bird flu is widespread in wild birds and is causing outbreaks in U.S. poultry and dairy farms, the CDC says. There have been outbreaks in 443 U.S. dairy herds, and 48 states have reported outbreaks in poultry.

CDC officials said the study suggests asymptomatic workers who engage in jobs that put them at high risk for exposure at poultry and dairy farms might be at heightened risk for infection.