H5N1 bird flu may be infecting more people than we know. A new study from the Centers for Disease Control and Prevention shows that 7% of dairy farmers monitored in Michigan and Colorado tested positive for the virus.
As of 2022, highly virulent avian influenza A strains of H5N1 have begun to spread from birds to mammals. This year, H5N1 has spread widely among dairy cattle and other livestock in the United States. There have also been 46 confirmed human cases of H5N1 in the US this year, with most (except one) thought to be from cattle or chickens. The new results, published Thursday in the CDC’s Morbidity and Mortality Weekly Report, do not suggest that H5N1 is now spreading among humans, but they do indicate that more vigilance is needed to prevent such a potential disaster.
The CDC worked with health officials in Michigan and Colorado (both states with dairy cow disease) to interview and blood test local dairy farm workers. Of the 115 workers whose blood was tested, eight workers had antibodies to H5N1—a sign that they had been exposed to the virus in the past. But only four of these workers remember that they recently got sick.
On the bright side, these hidden forms do not mean that the virus has become familiar to humans, at least not yet. Infected workers reported milking cows or cleaning the milking parlor recently, and of the four sick, all worked on farms where H5N1 was found in nearby cows. This suggests that the virus may have recently spread from the cattle to these workers. Other data have not shown any signs of unusual colds in the US recently, further reducing the possibility of H5N1 spreading to the general population.
But outbreaks of H5N1 in dairy cattle have occurred in 48 states this year, and thousands of workers may have been exposed to the infected cattle. Therefore the confirmed human cases found so far are only a small approximation of the true number.
The more these viruses spread from birds to mammals, the more likely they are to evolve into strains that can easily spread among mammals in general. And the more chances that H5N1 is given to infect humans, the more likely it is that a strain will be able to pick up the right mix of mutations to become a fast-spreading, deadly pandemic in humans. So the worse we get at detecting these early cases, the harder it will be to prevent H5N1 from becoming a major problem.
CDC researchers say more is clearly needed to track and prevent the spread of H5N1 on dairy farms.
“The findings support the need for active surveillance of exposed workers and screening to detect and treat HPAI A(H5) infections, including those with mild symptoms,” they wrote. “These efforts must be accompanied by educating farm workers about the risks of infection and ways to protect themselves.”
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