California, the biggest US milk-producing state, on Thursday confirmed two human cases of bird flu in people who had contact with dairy cattle infected by the virus, reported Reuters. 

The cases bring total US bird flu infections among people in contact with dairy cows and poultry to 15 this year. Another person in Missouri who had no immediate known contact with animals has also tested positive.

The virus' jump to cattle in 14 states and infections of farm workers have concerned scientists and federal officials about the risks to humans from further spread.

There is no known link or contact between the two human cases in California, suggesting only animal-to-human spread of the virus in the state, the California Department of Public Health said in a statement.

One infected person, identified as a dairy worker, only suffered conjunctivitis, or pink eye, the department said. 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 of bird flu in cattle, according to the statement.

The second person also was described as a Central Valley individual and experienced mild symptoms, including conjunctivitis, the department said. It said neither person reported respiratory symptoms nor was hospitalized.

Cows at dairy farms in California began testing positive for bird flu in late August. Pasteurized milk and dairy products continue to be safe to consume, the department said.

"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 healthcare 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 US 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 Oct. 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.