Two dairy workers in California were infected with bird flu, the 15th and 16th human cases detected this year in an ongoing outbreak affecting the nation’s dairy cows, health officials said Thursday.

The latest cases were found in workers who had contact with infected cattle in California’s Central Valley, where more than 50 herds have been affected since August. The workers developed eye redness known as conjunctivitis and had mild symptoms.

California health officials said the workers were employed at different farms and there is no known link between the two cases, suggesting that they were infected through animal contact, not by people.

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The U.S. Centers for Disease Control and Prevention on Thursday confirmed the positive test results, the first for California. CDC officials said new cases of bird flu in people exposed to infected animals is “not unexpected.” The risk to the public remains low, they added.

Across the U.S., more than 250 dairy herds have been infected in 14 states since the outbreak of H5N1 avian influenza was confirmed in March. Avian influenza has been spreading in wild and domestic birds in the U.S. for several years but recently was found in dairy cows.

Before this year, one case of bird flu was detected in a person, a Colorado poultry worker who fell ill in 2022. Most cases this year have been detected in workers who had contact with cattle or poultry in Colorado, Michigan and Texas. A person in Missouri was also infected, but that person had no known contact with animals and the source of that illness has not been determined.