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.

Health officials continue to characterize the threat to the general public as low and the virus has not spread between people. But officials are keeping careful watch, because earlier versions of the same virus have been deadly to people.

The Centers for Disease Control and Prevention has sent a nine-person team to Colorado to help in the investigation, at the state’s request, CDC officials said.

This cases earlier this year were among dairy farm workers in Michigan, Texas, and Colorado.

The virus detected in the four latest cases is least partly identical to the type found in the earlier U.S. cases, but further genetic analysis is underway to make sure it's exactly the same, officials said.

As of Friday, the H5N1 virus has been confirmed in 152 dairy herds in 12 states, according to the U.S. Agriculture Department. Hundreds of commercial poultry flocks in more than 30 states have reported H5N1 or other types of bird flu.