During a news briefing Tuesday, federal officials said temperatures soaring above 104 degrees made it difficult for workers to wear the required full-body suits, goggles and N95 masks to protect them from the virus.
“The barns in which the culling operations occur were no doubt even hotter,” said Nirav Shah, principal deputy director of the Centers for Disease Control and Prevention. Large-scale industrial fans were used to help cool the barns, but they also spread feathers around, which are known to carry virus, he said. The culling method involved extensive interaction with infected birds, requiring workers to put chickens in carts that kill them with carbon dioxide gas.
“The workers were finding it hard to maintain a good seal or a good fit, either between the mask or with eye protection,” Shah said. “This confluence of factors may play a role in explaining why this outbreak occurred, where it did, and when it did.”
The culling on the poultry farm in Weld County involves 160 workers and will continue for another 10 to 14 days to prevent further spread among the flock, said Eric Deeble, a senior official at the U.S. agriculture department overseeing the bird flu response. About 55 poultry workers with symptoms have been tested, Shah said. All were negative for bird flu except for the five workers. Four have been confirmed by the CDC; one presumed positive case is pending confirmation.
An additional 16 symptomatic poultry workers were tested Monday and are awaiting results, according to Colorado health officials.
While more cases may be detected, the risk to the general public remains low, officials said.
Human infection is rare. All U.S. human cases have been linked to direct contact with infected cows or poultry — not spread person to person, officials have said.
H5N1 bird flu is widespread in wild birds worldwide 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. agriculture department. This strain of highly pathogenic avian influenza is deadly to domestic poultry and can wipe out entire flocks in days. But it causes less severe illness in cows.
Colorado is among the states hit hardest by the virus, with outbreaks in at least 37 dairy herds, including several in Weld County where the poultry workers were infected. Genetic sampling of the virus from chickens at the farm shows the same type of virus found in nearby infected dairy herds, said USDA’s Deeble.
Earlier in July, a Colorado dairy worker was sickened with bird flu after being exposed to cattle infected with the virus. Officials are investigating links between that dairy worker and the five poultry workers.
Across the United States, a total of nine people, including three other dairy workers — two in Michigan and one in Texas — have been infected with H5N1 this year.
Direct exposure to infected birds increases the risk of contracting the disease because birds shed flu viruses in their saliva, mucous and feces. Dairy workers can contract the virus through contaminated milk or equipment.
The genetic sequence of the virus from one of the infected poultry workers may offer more clues about how the virus is spreading, officials said. One part 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 instances is spread within those very, very tight regional or local areas,” Shah said. “And that would also explain why the virus that we’ve seen is largely the same one, even though it’s popped up in disparate geographies from Michigan to Colorado.”
The CDC is not recommending livestock workers be vaccinated against bird flu because all workers who have contracted the disease reported mild symptoms. The poultry workers experienced eye inflammation and watery eyes along with typical flu symptoms including fever, chills, coughing, sore throat and runny nose, the CDC said. None were hospitalized.
The CDC has not identified any unusual flu trends in laboratory data or emergency department visits at the national, state or local levels, Shah said.
Preliminary analysis of the virus’ genetic sequence from the poultry worker in Colorado does not show any changes in the virus that would increase the severity of illness, ease person-to-person transmission or lessen the effectiveness of Tamiflu treatment, Shah said.
Federal health and agriculture officials have repeatedly emphasized the importance of precautions — such as wearing personal protective equipment — when working with infected animals. Federal and state officials 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 dairy herds in the state became infected, the state had requested 5,000 goggles, 300,000 pairs of gloves and 150,000 N95 masks from the federal stockpile. Over the weekend, after testing indicated that the virus had infected the five poultry workers, the state requested 500 courses of Tamiflu. More than 150 workers who had potential exposure to the infected poultry received antiviral medication. State health officials also notified the CDC of worker infections in real time, allowing the agency to send a 10-person bilingual team to assist in the investigation.
Nahid Bedalia, director of Boston University’s Center on Emerging Infectious Diseases and a former senior adviser on the Biden administration’s White House coronavirus response team, said infectious-disease experts are concerned about what could happen as the virus infects more people, increasing the chances for it to mutate to become more transmissible person-to-person and cause more serious illness.
“So far, yes, the illnesses have not been that severe,” she said. “But it’s only a matter of time before the disease may find somebody who may have medical conditions that could make this a tougher course.”
The Colorado workers are the first cases of H5N1 infection in poultry workers since April 2022, when a prison inmate culling poultry as part of a prerelease employment program became infected with the same strain causing the bird flu outbreak among dairy cows. That worker reported fatigue as the only symptom, was treated with Tamiflu, and recovered.
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