Four poultry workers in Colorado have been diagnosed with bird flu, health officials confirmed Sunday.

The new cases bring the U.S. total to nine since the first human case of the current outbreak was detected in 2022, also in a Colorado poultry worker. Eight of the nine were reported this year.

Their illnesses were relatively mild — reddened and irritated eyes and common respiratory infection symptoms like fever, chills, coughing, sore throat and runny nose. None were hospitalized, officials said. The other U.S. cases have also been mild.

A fifth person with symptoms is undergoing testing, but those results are not back yet, officials said. The workers were culling poultry at a farm in northeast Colorado, according to state health officials. All had direct contact with infected birds.

A bird flu virus has been spreading since 2020 among mammals — including dogs, cats, skunks, bears and even seals and porpoises — in scores of countries. Earlier this year the virus, known as H5N1, was detected in U.S. livestock, and is now circulating in cattle in several states.