By Leah Douglas and Julie Steenhuysen

(Reuters) – The U.S. Food and Drug Administration has begun testing more dairy products for evidence of the bird flu virus as outbreaks spread among dairy herds across the country.

More than 120 dairy herds in 12 states have tested positive for bird flu since March, according to the U.S. Department of Agriculture. Federal officials have warned that further spread among dairy cows could heighten the risk of human infections.

The focus of additional testing is to ensure that pasteurization inactivates the virus, said Don Prater, acting director of the FDA’s Center for Food Safety and Applied Nutrition, on a call with reporters.

Prior FDA testing of 297 retail dairy samples came back negative for evidence of the virus.

The agency continues to strongly advise against consumption of raw milk products, Prater said.

More than 690 people who were exposed to infected or suspected infected animals have been monitored for flu symptoms, and 51 people who developed flu-like symptoms have been tested, Demetre Daskalakis, director of the U.S. Centers for Disease Control and Prevention’s National Center for Immunization and Respiratory Diseases, said on the press call.

Three dairy farm workers have tested positive for the virus with mild respiratory or conjunctivitis symptoms, and all have recovered.

The risk to the general public from bird flu remains low, federal officials have said, though it is higher for workers on dairy farms, who should wear personal protective equipment to reduce the risk of infection.

(Reporting by Leah Douglas; Editing by Bill Berkrot)