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Officials found no evidence of silent infections in 35 Michigan dairy workers, but experts noted that much more data was necessary.
Since an unusual bird flu outbreak was first detected in dairy cows in March, experts have warned that the virus could be infecting more farm workers than have been officially detected. Testing has been severely limited, constraining what health officials know about the ways that the virus is spreading from sick cows and contaminated equipment and how often it is spilling over into humans.
In recent weeks, state and federal officials have been working to unravel part of that mystery: whether silent, undetected infections are occurring in farm workers. Officials in Michigan recently began testing the blood of 35 dairy workers for antibodies to the virus, which would provide evidence of past infection.
On Friday, the Centers for Disease Control and Prevention shared preliminary results, which showed that none of the workers were carrying antibodies to the virus, known as H5N1. All worked on dairy farms in Michigan that had suffered outbreaks, and many worked directly with sick cows, the agency said.
Dr. Natasha Bagdasarian, Michigan’s chief medical executive, noted that the study was small and that the results were not definitive. The farm workers tested were also volunteers, which means that they may not be representative of dairy workers more broadly.
Still, she said, the results suggested that asymptomatic transmission “is likely not widespread, and that this disease is not something that is spreading with minimal contact.”
In its online update, the C.D.C. noted that the results supported its approach to testing, which has focused on symptomatic people who had been in contact with sick animals.