Colorado workers exposed to bird flu while killing infected chickens ...

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Health officials detail bird flu transmission to humans. What we know.

Health officials are urging caution with where consumers get their milk as they investigate bird flu deaths in humans. Here's what we know.

Triple-digit temperatures, industrial fans and carts used to kill millions of sick chickens with gas may have caused the largest bird flu outbreak among U.S. workers to date, federal health officials said Tuesday.

A commercial egg-laying facility in northeast Colorado now has four workers confirmed infected with bird flu, and another presumed to have contracted the virus, pending test results. The sickened Colorado workers, announced over the weekend, doubled the number of U.S. human cases since agricultural workers began catching bird flu this year, first from dairy cows.

Workers at the poultry farm in Weld County had been in the process of killing 1.8 million chickens, some of which were infected with the dangerous H5N1 influenza, to prevent it from spreading, federal officials said in a briefing with reporters Tuesday.

Their goggles or N95 face masks apparently slipped as industrial fans blew feathers and other infected items through the sweltering barn, exposing them to the virus, said Dr. Nirav Shah, principal deputy director of the Centers for Disease Control and Prevention.

The workers all suffered some combination of fever, chills, coughing and/or eye infections, Shah said. They were given the anti-viral Tamiflu and none required hospitalization.

The virus' risk to the general public remains low, Shah said. The workers' illnesses were relatively mild, he said, and Tamiflu remains effective. Also, the genetic sequence of the virus has not changed substantially since December, as the virus moved from Texas to a dozen other states, including Colorado ‒ and from birds to cows and then, apparently back to chickens, as well as people.

"We've not seen severe illness, which is reassuring," Shah said. "The other thing we look at, in addition to severity is whether the virus is able to transmit from animals to humans and/or from humans to humans with greater ease or with greater efficiency. We haven't seen changes, particularly on the human-to-human side."

The unfolding route of transmission

Officials said they don't know for sure how the H5N1 virus arrived at the Weld County poultry farm. In early July, Colorado Gov. Jared Polis issued a disaster emergency for the outbreak at the facility, the largest flock of chickens affected in the state and among the largest across the U.S.

Days earlier, a dairy worker, also in northeastern Colorado, contracted bird flu, the state’s first case, while working with infected cows.

Cattle in the same region of Colorado had been known to be infected with the specific variant of the virus, so it's likely, though not yet confirmed, that the virus was passed from cattle to chickens, said Dr. Eric Deeble, acting senior adviser for the U.S. Department of Agriculture’s H5N1 Response.

"Poultry are very susceptible and easily infected," Deeble said on the call with reporters. "It does not take much in order to introduce this into a flock."

Once any poultry on a farm are infected with H5N1, the only way to contain the virus is to kill the entire flock, USDA has said. Killing all 1.8 million chickens could take up to two weeks, Deeble said. 

Sweltering barns, feathers flying

Around 160 workers inside barns at the facility were tasked with pulling out chickens, placing several at a time in a cart that would then be filled with carbon dioxide, killing them in under 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 on the call.

Temperatures in northeast Colorado have hit 104 degrees, though Shah said temperatures inside the barns were even hotter. Workers wore light paper protective suits over their clothes, N95 respirators, goggles, boots and gloves. Industrial fans pushed air through the barns. 

The fans made it uncomfortable to wear PPE, all while feathers ‒ a means in which bird flu virus can spread ‒ blew around. This likely caused the infections in five workers, officials said.

This suggests that risks can be better controlled in the future with more systematic use of PPE, in addition to measures to improve ventilation, Shah said. A 10-person CDC team, which includes bilingual speakers and an industrial hygienist, arrived in Colorado on Saturday to monitor and test workers and suggest safety improvements.

In total, 60 workers were tested for bird flu, with 55 testing negative, though many had symptoms consistent with other respiratory illnesses like rhinovirus, Shah said.

Sixteen workers who are symptomatic had been tested Monday, said AnneMarie Harper, a spokesperson for the Colorado Department of Public Health and Environment, in an email. Testing remains ongoing and Shah said more infections might come to light.

What preparedness looks like

Part of the virus found on its surface remained unchanged from the first person infected in Texas to the first infected in Michigan and the workers sickened recently in Colorado, Shah said, suggesting the virus has not mutated to become more worrisome.

The Department of Health and Human Services’ Administration for Strategic Preparedness and Response (ASPR) is pushing ahead with production of millions of bird flu vaccine doses through July as part of a pre-pandemic stockpile, said David Boucher, director of ASPR’s infectious disease preparedness and response.

There is no recommendation yet for people to receive the vaccine, which has not been fully tested or approved for use.

The agency also distributed 5,000 goggles, 300,000 gloves, 150,000 N95 masks and 528 courses of Tamiflu to Colorado, Boucher said.

"This is what preparedness looks like," Shah said. "It's making sure they have adequate stocks and supplies in the event that they may need them rather than trying to scurry and scramble at the last minute to acquire them."

Karen Weintraub of USA TODAY contributed to this report.