At a press briefing Tuesday, federal officials said temperatures exceeding 104 degrees made it difficult for workers to wear the full-body suits, goggles and N95 masks required to protect them from the virus.
“The barns where the slaughtering operations were taking place were probably even hotter,” said Nirav Shah, principal deputy director of the Centers for Disease Control and Prevention. Large industrial fans were used to cool the barns, but they also scattered feathers, which are known to carry viruses, he said. The slaughter method involved significant interaction with infected birds, requiring workers to place chickens in carts that kill them with carbon dioxide.
“Workers were having difficulty maintaining a good seal or fit, whether it was between the mask or with the eye protection,” Shah said. “That confluence of factors may play a role in explaining why this outbreak happened, where it happened, and when it happened.”
The cull at the Weld County poultry farm involves 160 workers and will continue for another 10 to 14 days to prevent further spread within the flock, said Eric Deeble, a senior U.S. Department of Agriculture official overseeing the bird flu response. About 55 poultry industry workers with symptoms have been tested, Shah said. All but five workers have tested negative for bird flu. Four of those have been confirmed by the CDC; one presumptive positive case is pending confirmation.
According to Colorado health officials, 16 more symptomatic poultry workers were tested Monday and are awaiting results.
Although more cases could be detected, the risk to the general public remains low, authorities said.
Human infection is rare. In the United States, all human cases have been linked to direct contact with infected cows or poultry, not human-to-human transmission, authorities said.
H5N1 avian flu is widespread in wild birds around the world and caused outbreaks in U.S. dairy cows for the first time this spring. Nearly 160 dairy herds have been infected in 13 states, according to the U.S. Department of Agriculture. This highly pathogenic strain of avian flu is deadly to domestic poultry and can wipe out entire herds in a matter of days. But it causes a less severe illness in cows.
Colorado is among the states hardest hit by the virus, with outbreaks in at least 37 dairy herds, including several in Weld County where poultry workers were Genetic sampling of the virus from chickens on the farm shows the same type of virus found in infected dairy herds nearby, the USDA’s Deeble said.
Earlier in July, a Colorado Dairy worker contracts bird flu after exposure cattle infected with the virus. Authorities are investigating links between the dairy industry worker and the five poultry industry workers.
In the United States, a total of nine people, including three other dairy workers – two in Michigan and one in Texas – have been infected with the H5N1 virus this year.
Direct exposure to infected birds increases the risk of contracting the disease because birds shed the influenza virus in their saliva, mucous membranes, and droppings. Dairy farm workers can contract the virus through contaminated milk or equipment.
The genetic sequence of the virus from one of the infected workers could provide more clues about how the virus spreads, authorities said. Some of the virus is the same as that found in the Texas worker and the first Michigan worker, Shah said. One hypothesis is that infected dairy cows from Texas were transported to Michigan and Colorado.
“What may be happening in some limited cases is spreading in very, very small regional or local areas,” Shah said. “And that would also explain why the virus that we’ve seen is largely the same, even though it’s emerged in disparate geographic areas, from Michigan to Colorado.”
The CDC does not recommend that livestock workers get vaccinated against avian flu because all workers who have contracted the disease have reported mild symptoms. Poultry workers have experienced eye inflammation and watering along with typical flu symptoms, including fever, chills, cough, sore throat and runny nose, the CDC said. None have been hospitalized.
The CDC has not identified any unusual influenza trends in laboratory data or emergency department visits at the national, state or local levels, Shah said.
Preliminary analysis of the genetic sequence of the virus from a Colorado poultry worker shows no changes in the virus which would increase the severity of the disease, facilitate human-to-human transmission or decrease the effectiveness of Tamiflu treatment, Shah said.
Federal health and agriculture officials have repeatedly stressed the importance of precautions — such as wearing personal protective equipment — when handling infected animals. Federal and state authorities have made supplies available to dairy farm owners but have not required their use.
Federal officials on Tuesday praised Colorado for its planning and response to the outbreak. In May, as the state’s dairy herds were infected, the state requested 5,000 goggles, 300,000 pairs of gloves and 150,000 N95 masks from the federal stockpile. Over the weekend, after tests indicated the virus had infected the five poultry workers, the state requested 500 doses of Tamiflu. More than 150 workers potentially exposed to the infected poultry were given antiviral drugs. State health officials also notified the CDC of worker infections in real time, allowing the agency to send a 10-person bilingual team to help with the investigation.
Nahid Bhadelia, director of Boston University’s Center for Emerging Infectious Diseases and a former senior adviser to the Biden administration’s White House coronavirus response team, said infectious disease experts are concerned about what could happen if the virus infects more people, increasing the chances of it mutating to become more transmissible from person to person and cause more severe illness.
“So far, yes, the cases are not that severe,” she said. “But it’s just a matter of time before it gets to someone who might have medical problems that could make it more difficult.”
The Colorado workers are the first cases of H5N1 infection among poultry workers since April 2022, when an inmate who slaughtered poultry as part of a pre-release employment program was infected with the same strain that caused the avian flu outbreak in dairy cows. The worker reported fatigue as his only symptom, was treated with Tamiflu and recovered.