California is investigating a possible case of bird flu in a dairy worker who had contact with infected cattle, the state's public health department said Thursday.

The virus' jump to cattle in 14 states and infections of 13 dairy and poultry farmworkers this year have concerned scientists and federal officials about the risks to humans from further spread.

The worker had a "presumptive positive" result to a test for bird flu, and the U.S. Centers for Disease Control and Prevention will do further testing to confirm the finding, the California Department of Public Health said in a statement.

The person, who was not identified, suffered only conjunctivitis, or pink eye, the department said in a statement. The person is being treated with antiviral medication and staying home, it added.

The person works at a Central Valley dairy facility suffering an outbreak in cattle, according to the statement.

Cows at dairy farms in California, the top U.S. milk-producing state, began testing positive for bird flu in late August.

"The risk to the general public remains low, although people who interact with infected animals are at higher risk of getting bird flu," the department said.

Missouri last month confirmed bird flu in a person with underlying medical conditions who had no immediate known animal exposure. Six health care workers who cared for the Missouri patient developed respiratory symptoms, but the virus was not confirmed in any of them.

Scientists are watching closely for signs that the virus has begun to spread more easily in people.

The U.S. Food and Drug Administration on Thursday said it would begin testing raw cow's milk intended for pasteurization at dairy plants to better understand the prevalence of the bird flu virus in milk.

Participation in the study, set to begin October 28, is voluntary, and pasteurized dairy products remain safe to consume, the agency said.

Prior FDA testing of retail dairy samples came back negative, and more such testing is underway.