Three veterinarians who work with cows have tested positive for prior infections of H5 bird flu, according to a study released today by the Centers for Disease Control and Prevention.
The finding may not seem surprising, given the sweeping and ongoing outbreak of H5N1 among dairy farms in the US, which has reached 968 herds in 16 states and led to infections in 41 dairy workers. However, it is notable that none of the three veterinarians were aware of being infected, and none of them worked with cows that were known or suspected to be infected with H5N1. In fact, one of them only worked in Georgia and South Carolina, two states where H5N1 infections in dairy cows and humans have never been reported.
The findings suggest that the virus may be moving in animals and people silently, and that our surveillance systems are missing infections—both long-held fears among health experts.
The authors of the study, led by researchers at the CDC, put the takeaway slightly differently, writing: “These findings suggest the possible benefit of systematic surveillance for rapid identification of HPAI A(H5) virus in dairy cattle, milk, and humans who are exposed to cattle to ensure appropriate hazard assessments.”
The study was carried out in September. Veterinarians were recruited at an in-person veterinary conference where they gave blood samples and reported cattle exposures in the previous three months. A total of 150 bovine veterinarians took part in the study, 143 from the US and seven from Canada.
Blood tests showed that three veterinarians (2 percent) had antibodies against H5-type flu viruses, indicating a recent infection. All three were from the US (Canada has not detected bird flu in any cows). None of the three reported having had a respiratory infection, flu-like symptoms, or conjunctivitis (pink eye), which has been a common symptom among documented cases of H5N1 in dairy workers. While none of the three reported working with cows with known or suspected bird flu infections, one had worked with H5-positive poultry.
Since September, the US Department of Agriculture has set up a national milk testing strategy to identify bird flu virus in bulk milk samples quickly. The surveillance strategy led to the recent identification of a second spillover event of H5N1 bird flu from wild birds to cows, which has spread to several herds in Nevada and at least one dairy worker.