The CDC had previously confirmed four cases and a fifth presumptive positive case of bird flu among Colorado farm workers who were killing and disposing of chickens that had contracted the virus.
Four other farm workers have contracted avian flu this year from infected dairy cows in Michigan, Texas and Colorado.
The risk to the general public from bird flu remains low, Nirav Shah, principal deputy director of the CDC, said on a call with reporters.
When workers were killing the infected chickens, it was 104 degrees Fahrenheit (40 degrees Celsius) in Colorado, and strong industrial fans made it hard for workers to keep protective gear on their faces, Shah said.
"PPE use was not optimal, particularly the masks and eye protection," Shah said, referring to personal protective equipment.
There have been about 160 people responding to the poultry farm outbreak, including staff from the U.S. Department of Agriculture and contractors killing and disposing of animals, Julie Gauthier, an official at USDA's Animal and Plant Health Inspection Service, said on the call.
About 60 workers at the farm developed symptoms and were tested, Shah said. Those who tested positive for bird flu had a mix of mild symptoms including conjunctivitis, or pink eye, and mild respiratory symptoms.
Initial analysis of the virus does not show worrisome changes to the virus making it easier to spread among people, and there is no evidence of person-to-person transmission, Shah said. The CDC is not recommending that livestock workers be vaccinated against bird flu, Shah added.
The infected chicken farm is in a county where cows have tested positive for bird flu, Eric Deeble, a U.S. Department of Agriculture official, said on the press call.
More than 150 dairy herds across 13 states have been infected with the virus since March, according to the USDA.
An analysis of DNA from an infected worker indicates that the infections at the chicken farm are "largely the same" as those in some of the dairy herds, suggesting that this outbreak may be related to dairy outbreaks in Colorado and other states, Shah said. A CDC investigation into the origin of the outbreak is ongoing, Shah added.
