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.

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.

Dairy workers who’ve been exposed to bird flu should be tested for the virus even if they don’t have symptoms and be offered Tamiflu to cut their risk of getting sick, the Centers for Disease Control and Prevention said Thursday. The recommendation ...