The highly pathogenic H5N1 avian influenza virus that spread from wild birds to U.S. dairy cows late last year may have recently jumped from a Colorado dairy farm to a nearby poultry farm, where it subsequently infected five workers charged with culling the infected chickens.
At a press conference Tuesday, federal officials said four of the bird flu cases have been confirmed by the Centers for Disease Control and Prevention, while the fifth is presumptive positive and has yet to be confirmed by the CDC.
All five individuals had mild illnesses, although they had varying symptoms. Some cases involved conjunctivitis, as seen in other human cases linked to the H5N1 outbreak in dairy cows. Others in the cluster of five had respiratory and typical flu symptoms, including fever, chills, sore throat, runny nose and cough. None of the five cases required hospitalization.
The virus infecting the five people is closely related to the virus infecting chickens at the poultry farm, which in turn is closely related to the virus seen in infected dairy herds and in other human cases linked to the dairy outbreak. The affected farm is in northern Colorado’s Weld County, where about two dozen outbreaks of avian influenza in dairy herds have also been reported.
Dairy-to-poultry hypothesis
In one fell swoop, the outbreak on Colorado poultry farms has more than doubled the number of human avian flu cases linked to spillovers from dairy cows, bringing the previous tally of four cases to nine. While officials have previously noted cases in which H5N1 appeared to have moved from dairy farms to nearby poultry farms, this appears to be the first time such a spread has led to documented human infections.
The link between the cases on the poultry farm and nearby dairy farms is still only a hypothesis, however, Nirav Shah, the CDC’s deputy director, told reporters Tuesday. “It’s a hypothesis that needs and requires a full investigation. But that’s a hypothesis at this point,” he said of the link between the dairy farms and the poultry farm. So far, there’s no direct evidence of a specific source of the infection on the poultry farm, and the route of infection is also unclear.
During the H5N1 outbreak on dairy farms, officials noted that the primary way the virus appears to spread to new farms is through the movement of cows, people and machinery between those facilities. There is still no evidence of human-to-human transmission. But milk from infected cows has been found to be loaded with high levels of infectious virus, and milk-contaminated equipment is a prime suspect in the spread.
During Tuesday’s press conference, Eric Deeble, acting senior advisor for H5N1 response at the U.S. Department of Agriculture, noted that poultry is highly susceptible to avian influenza and can easily become infected. “It doesn’t take much to introduce this into a flock,” Deeble said. The USDA is now working on a “trace-back” investigation into how the Colorado poultry farm was infected.
Scorching spread
As for how the farm workers specifically became infected with the virus, health officials pointed to the high temperatures that prevented workers from wearing protective gear. The poultry farm is a commercial laying hen farm with approximately 1.8 million birds. Given the presence of avian influenza on the premises, all 1.8 million birds must be killed, or “depopulated.” This is done using mobile carts with carbon dioxide gas chambers, a common killing method. Workers are tasked with placing the birds into the chambers, which can only hold a few dozen birds at a time. Overall, the method requires workers to have a lot of contact with the infected birds, moving from bird to bird and from batch to batch with the carts.
During this grim task, temperatures in the area reached over 100 degrees Fahrenheit and massive industrial fans were turned on inside the facility to try to cool things down. Between the heat and the fans, the approximately 160 people involved in the depopulation struggled to use personal protective equipment (PPE). The required PPE for the depopulation included a full Tyvek suit, boots, gloves, safety glasses and an N95 respirator.
“You can imagine how difficult it is to wear all that gear in that heat,” said Julie Gauthier, executive director of field operations for the USDA’s Animal and Plant Health Inspection Service (APHIS). Industrial fans blowing large volumes of air made it even harder for workers to keep their goggles and respirators on their faces, she said.
The CDC and USDA are both involved in further investigating the outbreak at the poultry farm. The CDC’s Shah noted that the team the agency sent to Colorado included an industrial hygienist, who can work on strategies to prevent further transmission.
So far, at least 161 flocks in 13 states have tested positive for bird flu since the dairy outbreak was confirmed in March. Since January 2022, when U.S. birds first tested positive for the H5N1 virus, 99 million birds have been affected in the U.S. across 48 states, representing 1,165 individual outbreaks.