Triple-digit temps, industrial fans and carts used to kill millions of sick chickens with gas may have caused the largest bird flu outbreak among US workers to date

Federal health officials are revealing more about the largest bird flu outbreak among U.S. workers to date, and the findings raise alarms about how the highly contagious virus is being handled.

Over the weekend, health officials reported that five poultry workers from a commercial egg-laying facility in northeast Colorado contracted H5 bird flu, also known as highly pathogenic avian influenza. The Centers for Disease Control and Prevention said four cases were confirmed positive while the fifth is pending.

The workers were among a group of about 160 who had been tasked with killing 1.8 million chickens at a farm in Weld County after some birds in the flock caught the virus.

The "depopulation" process involved "pulling out chickens, placing several at a time in a cart that would then be filled with carbon dioxide, killing them in less than a minute and a half," Dr. Julie Gauthier, the USDA's executive director for field operations in the Animal and Plant Health Inspection Service, said during a media briefing, per USA Today .

Temperatures inside the barns were in the triple digits at the time, making it uncomfortable for workers to properly wear their PPE -- including light paper protective suits over their clothes, N95 respirators, goggles, boots and gloves -- and leaving them suspectable to infection, USA Today reported.

Additionally, large industrial fans meant to circulate air in the barns also blew around feathers that can carry the virus -- which could have led to infection in the workers.

All workers who tested positive reported conjunctivitis (pink eye) and eye tearing, as well as more typical flu symptoms of fever, chills, coughing and sore throat/runny nose, the CDC said. None were hospitalized.

The CDC has a team of experts on the ground to continue monitoring workers as part of their investigation. Additional cases may be reported and confirmed as monitoring and testing is ongoing.

These five cases bring the total confirmed number of human cases of bird flu in the U.S. since 2022 to nine -- with seven cases this year alone. The four other cases this year were reported in dairy workers, with two in Michigan, one in Texas and one in Colorado, the CDC said.

While the current public health risk is low, the CDC is watching the situation carefully and working with states to monitor people with animal exposures.

People with close or prolonged, unprotected exposures to infected birds or other animals (including livestock), or to environments contaminated by infected birds or other infected animals, are at greater risk of infection and should take precautions. There is no indication of person-to-person spread of bird flu viruses at this time.