Bird Flu Snapshot: This is the latest installment in a series of regular updates on the H5N1 bird flu that STAT publishes on Monday morning. To read future updates, you can also subscribe to STATs Morning newsletter.
Public health experts who have been tracking the surprising spread of H5N1 bird flu into U.S. dairy herds now have all eyes on Colorado, waiting to see if a cluster of human cases there could snowball into something bigger.
On July 14, Colorado officials made the announcement that five workers involved in culling 1.8 million chickens at a large H5N1-infected egg farm in Weld County had tested positive for the virus. And the strain infecting the workers appears to be closely related to the virus that infects cows in Colorado and at least a dozen other states.
On Friday, the Centers for Disease Control and Prevention confirmed a sixth case among Colorado poultry workers. Nearly 70 people involved in the “depopulation operation” were tested for H5N1 after showing symptoms of the disease, a spokesperson for the Colorado Department of Public Health and the Environment said. On Saturday, the state announced that it was a possible seventh human case; The CDC, which must conduct confirmatory testing, is expected to receive the sample on Tuesday.
The six cases in Colorado were all mild, with some showing more traditional flu symptoms such as fever and coughing, and others having conjunctivitis, a symptom that has been seen in some dairy workers who became infected during the outbreak. But it’s the first time multiple human cases have been reported on a single U.S. farm, raising questions about whether the virus has changed or whether environmental factors provide unique opportunities to spread.
A recent study led by noted flu virologist Yoshihiro Kawaoka of the University of Wisconsin-Madison found evidence that the virus now circulating in cows has acquired some ability to bind to receptors found in the upper respiratory tract of humans, although other laboratories have produced conflicting data. . The concern with a large number of human cases is the increased potential for these people to transmit the virus to others, especially to immunocompromised individuals.
But Michael Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy, said without more data it’s too early to say what the risk is from the situation in Colorado. “If we get another seven or even seventy cases of conjunctivitis, what does that mean? Could this be a harbinger of a respiratory infection, of influenza that is transmitted from person to person? Nobody knows.”
He pointed to the situation in Michigan earlier this summer, where 54 farm workers who had been exposed to infected cows and showed flu-like symptoms were tested by state public health officials. Only two of these individuals tested positive for H5N1.
Results of a serological study of agricultural workers in Michigan, released Friday by the CDC, provide additional assurance that asymptomatic human infections do not go undetected. None of the blood samples taken from people who had been exposed to infected dairy cows at two farms but showed no symptoms were found to contain H5N1 antibodies, meaning they were not infected.
Even further back in time, in February 2003 there was an outbreak of another variant of bird flu on commercial poultry farms in the Netherlands. Nearly 500 agricultural workers registered health complaintsand although some complained of flu-like symptoms and one vet died, the majority suffered only conjunctivitis. Eighty-nine of those people tested positive for the bird flu virus and all cases were related to direct contact with poultry. But the outbreak never spread more widely; the following year it had largely burned out.
“It’s clear that we are vulnerable to H5N1 if it is airborne,” Osterholm said. “Our naked eyeballs are a perfect landing spot for this. But there is a big difference between that and the virus establishing itself in the human respiratory tract.”
At a news briefing Tuesday, federal officials said Weld County workers faced challenging conditions at the poultry facilities. Temperatures above 104 degrees and high-powered fans made it difficult to wear the protective gear, including full-body suits and N95 masks, meant to protect them from the virus, especially if it becomes aerosolized.
“Workers found it difficult to maintain a good seal or fit, either between the mask or with eye protection,” said Nirav Shah, deputy director of the CDC. “This confluence of factors may play a role in explaining why this outbreak occurred, where it occurred and when it occurred.”
Initial genetic analyzes have shown that the virus that is making poultry workers sick is linked to the version spreading among cows, but it is still unclear which dairy farm it came from. The state is in the early stages of an investigation to understand these links and has requested additional epidemiological support from the U.S. Department of Agriculture. The CDC said Friday that genetic sequencing of the virus that infected one of the poultry workers showed it was closely related to the first case in Michigan and shows no changes associated with antiviral resistance.
Weld County, just northeast of Denver, has a population of 350,000 and the largest concentration of dairies in the state. So it’s no surprise that it has been particularly hard hit by H5N1, Colorado State veterinarian Maggie Baldwin told STAT in an interview last week. “Geography is a very big factor,” she said. “The fact that most of our dairies in Colorado are in the same region will lead to increased transmission of this virus.”
Colorado has been dealing with H5N1 on its poultry farms since early 2022, but until this summer these outbreaks were sporadic and linked to wild birds. “What we have now is ongoing mammal-to-mammal transmission of H5N1 in dairy cows, leading to a potential source of ongoing spillover effects to our poultry farms,” Baldwin said. “So this is even riskier than what we’ve seen in the last two and a half years.”
Since the first reported case of bird flu in dairy cattle in late April, Colorado has recorded 41 additional H5N1-positive herds, including six in the past week alone. That means infections have been reported in 40% of Colorado herds. It now leads the nation, accounting for nearly a quarter of the 163 livestock outbreaks in the USDA’s official count.
Correction: An earlier version of this story stated that an incorrect number of people tested positive for bird flu during the 2003 outbreak in the Netherlands.