Three poultry workers from a farm in northeast Colorado are suspected to have contracted bird flu, state and federal health officials announced Friday.

The three tested presumptively positive for H5N1, also known as avian influenza, while working at a “commercial egg layer operation,” the Colorado Department of Public Health and Environment said in a statement.

They were culling poultry at the farm when they showed mild symptoms, including pink eye and common respiratory infections, the agency said. None required hospitalization.

Specimens have been sent to the U.S. Centers for Disease Control and Prevention for confirmatory testing.

The CDC said that the three worked “at a poultry facility experiencing an outbreak of the H5N1 virus that is circulating in wild birds and has been causing multistate outbreaks in dairy cows and poultry.”

There have been four confirmed human cases of bird flu in the U.S. since March, all in farmworkers, with two in Michigan, one in Texas and one in Colorado, the CDC said.

In the latter case, a Northern Colorado farm worker suffered pink eye after having direct contact with cattle that were infected with avian flu, CDPHE previously reported.

According to the U.S. Department of Agriculture, there have been bird flu outbreaks reported in 152 dairy herds so far this year.

The CDC said the risk to the public from bird flu remains low, noting that “there are no signs of unexpected increases in flu activity otherwise in Colorado, or in other states affected by H5 bird flu outbreaks in cows and poultry.”

However, it added that “human infections with this novel influenza virus (and others) are concerning because of the potential to cause severe disease.”

Federal officials are considering if and when to deploy 4.8 million doses of bird flu vaccine. Finland announced last month it would offer shots to workers who might be exposed to the virus.

— Alexander Tin contributed to this report.