A second farmworker in the U.S. has tested positive for bird flu, public health officials from the patient’s home state of Michigan announced May 22.
During a press conference, health officials said the case was “not unexpected,” given the ongoing spread of avian influenza among birds and mammals in the U.S., and that risk to the general public remains low. But the case raises the question: is human-to-human transmission of bird flu a serious concern?
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Not at this point, according to Dr. Nirav Shah, principal deputy director of the U.S. Centers for Disease Control and Prevention (CDC). “We have not seen evidence of other cases in this area or elsewhere in any of our monitoring systems, let alone any evidence of human-to-human transmission,” Shah said during the May 22 press briefing. “This is reassuring.”
“Risk is incredibly low right now,” agrees Andrew Pekosz, a professor of molecular microbiology and immunology at the Johns Hopkins Bloomberg School of Public Health. “There is no real evidence of spread from human to human.”
Avian influenza has been spreading widely among birds around the world in recent years, affecting nearly 91 million in the U.S. since January 2022. The virus has also affected mammals, infecting cows, goats, cats, and more in the U.S. alone. So far, two human infections in the country—one in Texas in April, and the new one in Michigan—have been detected in people working on dairy farms where there are infected cows. Both individuals reportedly experienced only mild conjunctivitis-like eye symptoms, then recovered.
The person in Michigan tested negative on a nasal swab but positive on an eye swab, according to the CDC. That’s a promising sign, Shah said during the press briefing, because it suggests the virus is not spreading like a traditional respiratory virus. The seasonal influenza virus, for example, takes root in the upper airways, which is why a sick person can spew potentially infectious respiratory particles when they cough or sneeze.
The avian influenza virus, by contrast, attaches to a type of receptor that is common in animals and birds and is also found in the human eye, but that is not prevalent in the human respiratory tract, explains Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. “Where I’d get concerned is if we see genetic changes that allow this virus to [easily] attach to the respiratory tract cells of a human,” Osterholm says.
It’s possible, he says, that the virus could eventually change in a way that allows it to do that, but “we’re not seeing that at all yet.”
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To answer questions about potential risks to humans, it’s important to better understand how the virus is spreading among cows, Pekosz says. If it’s spreading mostly through exposure to contaminated milking equipment, for example, “that’s probably not relevant to how the virus would spread in humans,” he says. That research remains ongoing, as does genetic analysis of the viral sample taken from the patient in Michigan.
Public-health officials are still taking precautions to reduce the chances of continued spread. Health care workers in Michigan are carrying out contract tracing and distributing influenza antivirals to people close to the infected person, Shah said during the press briefing. A spokesperson from the Michigan Department of Health and Human Services declined to provide additional details.
In April, the Texas patient was told to isolate from others and was treated with antivirals. They were also given antivirals to distribute to people in their household, according to a spokesperson from the Texas Department of State Health Services. “There has been no evidence of onward spread among people so the risk to the general public is low,” the spokesperson added.
The CDC also recently urged states and cities to keep their influenza surveillance efforts at peak-season levels to catch any signs of increased transmission. And the federal government is prepping nearly 5 million vaccine doses in case they need to be distributed quickly, officials said on the press call.
Measures like extra surveillance are precautionary, not reason for alarm, Pekosz says. “It’s preventive,” he says. “It’s trying to understand what’s happening so far—it’s not because we’re seeing evidence” of human transmission.
Public-health officials also said during the press briefing that while the risk to the general public remains low, farmworkers have been told to take additional safety precautions, such as wearing personal protective equipment around sick or potentially infected animals. But the average person doesn’t need to do much at this point, Pekosz says. The most important preventive measure most people can take is avoiding raw milk, which is not treated to remove viruses.
