CDC: Asymptomatic Cases of Bird Flu in Humans Are Flying Under the Radar

Suspicions that there may be more bird flu going around undetected were backed by a voluntary serosurvey among veterinary practitioners who work with cattle, CDC investigators said.

Out of 150 participating veterinary practitioners across the country, three had antibodies indicating recent infection with Type A H5N1 bird flu, based on blood samples collected in September 2024, noted Samantha Olson, MPH, of the CDC’s National Center for Immunization and Respiratory Diseases, and colleagues.

Of those three people, none reported exposure to dairy cattle with known or suspected bird flu infection, though one practitioner did work with bird flu-positive poultry. Notably, one had been practicing in Georgia and South Carolina — two states with no known H5N1-infected cattle, the researchers reported in Morbidity and Mortality Weekly Report.

“These findings suggest that there might be HPAI [highly pathogenic avian influenza] A(H5)-infected dairy cattle in states where infection in dairy cattle has not yet been identified, highlighting the importance of rapid identification of infected dairy cattle through herd and bulk milk testing as recently announced by the U.S. Department of Agriculture,” the authors wrote.

Olson and colleagues made the case that surveillance of symptomatic exposed workers would likely underestimate human infection, since none of the three people flagged with prior bird flu infection reported any accompanying respiratory or influenza-like symptoms, including conjunctivitis.

The three exposed veterinary practitioners said they wore gloves or a clothing cover when providing veterinary care to cattle; none reported wearing respiratory or eye protection.

“Neither respiratory nor eye protection is recommended when working with uninfected animals in regions without confirmed cases; however, safety goggles and a respirator are recommended when working with uninfected animals in regions where there are confirmed or potentially infected animals,” Olson and team noted.

For their serosurvey, the investigators went to an annual American Association of Bovine Practitioners conference and sought volunteers among bovine veterinary practitioners with cattle exposure in the previous 3 months. Of the sample of 150 individuals, 55% practiced in states with bird flu-infected dairy herds and 17% worked with dairy cattle with known or suspected infections.

“These data highlight the possible benefit of national seroprevalence assessments of recent HPAI A(H5) infection among practitioners at increased risk for exposure, which might help assess occupational risk in states without confirmed HPAI A(H5) virus detections in dairy cattle,” the authors wrote.

The CDC maintains that the current risk to the general public remains low.

Since the first cow-to-human transmission documented in April 2024, there have been 68 confirmed total reported human cases in the U.S. These cases have typically been mild, though one person in Louisiana died with severe respiratory symptoms in January. The bulk of known human cases have been reported out of California and in people with exposure to commercial agriculture.

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