Eight out of 115 dairy workers, or 7%, who worked with H5N1-infected cows in Michigan and Colorado have antibodies to bird flu, according to a new study from the US Centers for Disease Control and Prevention (CDC) – a rate significantly higher than known cases of the highly pathogenic virus, which means existing efforts are not protecting, diagnosing and treating people at risk, experts said.
It could become even harder to detect cases amid the fall migration of wild birds, the upcoming human flu season, and repercussions of the second Trump administration’s proposed policies to curtail public health and expand deportation of immigrants, who serve as the backbone of the agricultural workforce in the US.
The new survey from the CDC and state health departments looked at blood samples from people who worked with H5-infected cows in Michigan and Colorado between June to August 2024.
Out of the eight people who had previously undetected cases of the highly pathogenic bird flu, four remembered having symptoms, mostly conjunctivitis, and the other four did not recall having symptoms.
All eight workers were Spanish speakers who reported milking infected cows or cleaning milk parlors. None of them wore respirators, and less than half wore eye protection like goggles.
Notably, only one person said they had worked with infected cows, even though all of them were working with cows on farms with known infections – pointing to barriers in workers understanding the risks they face.
“It really speaks to the importance of more on-farm training around H5 as well as ways to protect from H5,” Demetre Daskalakis, director of the National Center for Immunization and Respiratory Diseases, told reporters on Thursday.
The news of cases that flew beneath the radar is “completely unsurprising”, said Jennifer Nuzzo, the director of the Pandemic Center and a professor of epidemiology at Brown University School of Public Health.
“When you test people at their place of work, and if the consequence of testing positive is that they have to stay home and possibly not earn an income, you should expect that people might not tell you if they’ve had symptoms. Also, everything we know about flu gives us the very strong suspicion that there would be asymptomatic infections,” Nuzzo said.
Until now, the CDC has recommended testing only people who report symptoms after having direct animal contact.
“We are not doing enough to make sure that we are protecting people from getting infected and certainly making sure that people who are infected get access to medicines that could potentially keep them from getting severely ill,” Nuzzo said.
The CDC is now bolstering measures to protect workers, including expanding recommendations to test farm workers who are exposed to the virus but don’t develop symptoms, and offering those workers access to flu antivirals.
“We in public health need to cast a wider net in terms of who is offered a test so that we can identify, treat and isolate those individuals,” Nirav Shah, the CDC’s principal deputy director, said on Thursday. Identifying cases and treating people helps to keep a mild infection from turning into a severe one – and it reduces the chances that the virus will spread onward among people.
“The less room we give this virus to run, the fewer chances it has to cause harm or to change,” Shah said. The agency is also improving guidance and education on the importance of personal protective equipment.
“Because we haven’t seen severe illness and deaths yet, I think there’s been some complacency around trying to control this virus, but I’ve always said we shouldn’t wait for farm workers to die before we take action to protect them,” Nuzzo said. “I just don’t think you should gamble with people’s lives like that.”
She believes existing stockpiles of H5N1 vaccines should be offered to farm workers, pending their authorization from regulatory agencies. Vaccines can help prevent severe illness, particularly among a population that may be hesitant to come forward with an illness that could jeopardize their job or even their ability to stay in the country.
“Just offer it for people who may want to protect themselves,” Nuzzo said. “This virus is not going away. This virus is going to represent an even greater threat to human health as it continues to find its way into more and more US farms.”
If the “moral imperative” to protect agricultural workers doesn’t move Americans, perhaps the economic effects of higher costs of milk, eggs and meat will, she said. “Nobody wants the cost of groceries to be any higher than they already are.”
So far, there have been 46 official cases of H5N1 diagnosed in people this year, more than half of which have been among dairy farmworkers. Another nine people have now been identified by blood testing, for a total of 55 people affected by bird flu in 2024.
Other influenza variants will soon begin circulation in people this fall, which raises the possibility of reassortment – a process where different flu variants combine and potentially gain worse attributes.
“By allowing this virus to circulate, we could give it a runway to develop the ability to more easily infect people, and crucially, to be able to spread easily between people,” Nuzzo said. “If the virus can do that, we will be in a new pandemic.”
