An H5N1 outbreak that recently infected five poultry workers and 1.8 million chickens in northeast Colorado may have been fueled in part by heat wave conditions and slaughtering methods, according to federal health authorities.

At a press conference Tuesday, Nirav Shah, principal deputy director of the U.S. Centers for Disease Control and Prevention, said the human infections occurred as poultry workers culled infected birds in 104-degree heat — a condition that may have made wearing protective clothing and equipment nearly intolerable, and necessitated the use of large fans, which may have promoted the virus’s spread via feathers, dust and other poultry detritus.

In addition, the method used to kill the infected chickens — carbon dioxide gassing — required that workers move “from chicken to chicken” increasing their “degree of interaction with each potentially infected bird.”

“This confluence of factors may play a role in explaining why this outbreak occurred where it did and when it did,” said Shah, noting that a state and federal investigation is still underway.

He said these observations potentially “highlight a pathway for prevention,” which would include more systematic use of protective equipment as well as engineering adaptations that could help reduce exposure risk.

This weekend, Colorado and federal health officials reported five cases of bird flu in poultry workers at a single farm in northeast Colorado. Four of the cases have been confirmed by the CDC, and a fifth is considered presumptive as officials wait for the final results.

The poultry farm was infected by bird flu earlier this month. The virus is particularly deadly to poultry, and highly transmissible. Standard practice in the industry is to cull all potentially infected birds and clean the premises.

Federal officials said the chickens were slaughtered with carbon dioxide, which a 2016 Meat and Poultry magazine article described as the “gas of choice” in North America due to its availability, low cost, and track record for “attaining consistency in terms of good animal welfare and meat quality.”

Birds infected with H5N1 are discarded and do not enter the food supply.

The technique requires that workers place chickens in a sealed, portable unit in which anywhere from 20 to “several dozen” are exposed to the gas. At first the CO2 is emitted at a concentration that will render the birds unconscious — a phase of slaughter known as “the induction of insensibility.” Once the birds are knocked out, the concentration is increased, and the animals suffocate and die.

The whole thing takes “less than a minute and a half,” said Julie Gauthier, executive director for field operations at the USDA’s Animal and Plant Health Inspection Service.

Maurice Pitesky, an expert in poultry health and food safety epidemiology at UC Davis, said for “houses” as big as the one in Colorado, culling can take weeks.

The process requires that workers handle both live and dead birds. And officials on Tuesday’s call hypothesized that if their PPE was not on properly due to the excessive heat, or had been made less effective by large cooling fans (which were also kicking up dust), they may have been exposed and vulnerable to the virus.

“The heat is an issue,” Pitesky said. “The expectation that dairy workers, poultry workers, under those current heat conditions — or California’s Central Valley, for example, when it was over 110 degrees — that they would wear PPE like Tyvek suits that don’t breathe at all, and the N95 masks that USDA is offering for free, is unrealistic.”

He said there was “no way” anyone was going to wear PPE in those conditions. Instead, he said, the USDA should provide things like visors or surgical masks — protective items that might actually be worn.

“Then there’s the culture, which is probably the bigger issue,” he said, noting in his experience, most workers won’t wear masks — even for particulate matter. So, “while the USDA intentions were good, I think the practicality of what they were trying to bring about wasn’t very sensitive to that reality.”

Federal officials also noted that DNA sequencing of virus obtained from one of the patients is closely related both to infected chickens from that farm, as well as to the first dairy worker infected in Texas in April and to infected dairy herds located near the Colorado poultry farm.

The finding raises “the possibility that this virus was transmitted from a dairy herd in Colorado to the poultry farm,” said Shah, from the CDC. “That is a hypothesis … that needs and requires a full investigation.”

Pitesky said the finding implies the virus may be moving between workers employed at multiple farms, or equipment that’s being shared, “or there’s potentially some environmental connection through groundwater or some kind of habitat-type transmission.”

He said birds and rodents can be mechanical transmitters, and wild birds are common visitors in both dairy and poultry farms. He said he works with poultry farmers to keep birds from nesting inside — “that’s a no-no” — but birds, such as swallows, can and do fly through.

He also suggested that while poultry farmers have really upped their biosecurity in the past several years, the dairy industry is “light-years” behind when it comes to creating physical barriers.

He said with every update he hears, it’s becoming increasingly clear “there’s no way to model or predict how this virus is going to move when it’s in this many different species and in this many different environments.”

And it’s anyone’s guess, he said, what’s going to happen this fall when fall migration begins and things potentially get even more complex.