Colorado reported four confirmed infections of H5N1 bird flu virus in poultry workers, and is checking on the status of a fifth suspected case, health authorities said on Sunday.

Human infections with H5N1 avian flu could cause severe disease and even a pandemic if the virus were to change and acquire the ability to spread easily between people, the U.S. Centers for Disease Control and Prevention (CDC) has said.

The CDC has confirmed four of the Colorado cases, the state's public health department said, with a fifth pending confirmation by the agency, which is supporting further investigation of the matter.

"Samples for a fifth worker were presumptive positive at the state laboratory on Saturday and will be sent to CDC for confirmation," it said in a statement.

The workers showed mild symptoms, from conjunctivitis, or pink-eye, to respiratory signs, it said, though none were in hospital.