Amidst the usual surge of winter respiratory illnesses, a recent report from the Centers for Disease Control and Prevention (CDC) sheds light on an emerging and largely undetected threat: the spread of highly pathogenic avian influenza (H5N1) to veterinarians.
During the past year, there have been at least 70 confirmed cases of symptomatic H5N1 bird flu in humans in 13 states, with one fatality. (That is in addition to the devastation in poultry and cows: "It has affected more than 165 million chickens, turkeys and other birds across almost all states since the outbreak began in poultry in 2022 and has been detected in more than 970 herds of cattle and dairy cows across 17 states.")
Most human cases have been mild, and there have been no known instances of person-to-person spread, but the CDC’s February 13 Morbidity and Mortality Weekly Report added a new wrinkle: It described three cases of H5N1 detected in U.S. veterinarians who work with cattle, with two cases lacking a clear source of exposure. These infections were identified through antibody testing rather than through symptomatic illness, reflecting silent spread of the virus. Although human-to-human transmission remains unconfirmed, a few genetic mutations could enable the virus to make this leap, which would increase significantly the public health risk.
More Spread Than Anticipated
The CDC's analysis of blood samples from 150 bovine veterinarians in 46 U.S. states and Canada during the current H5N1outbreak in dairy cows and poultry revealed that three asymptomatic U.S. practitioners (2%) had antibodies to H5N1 in September, suggesting recent infection.
Two of them reported no exposures to infected animals, and the third practiced in Georgia and South Carolina, which have had no known cases in cattle. "These findings suggest there could be U.S. states with A(H5)-positive people and animals that have not yet been identified," the study authors wrote.
Although the infected veterinarians reported wearing gloves or other protective gear when caring for cattle, none wore respiratory or eye protection, which are recommended when working with even uninfected animals in regions with confirmed cases. As the researchers wrote, "HPAI A(H5) virus is known to be present in high concentrations in milk produced by infected cattle, introducing infection risk through respiratory, ocular, and gastrointestinal exposure.”
Further compounding concerns, experimental studies have now confirmed that H5N1 can be transmitted through (unpasteurized) cow’s milk. In response, health officials are strongly advocating for nationwide milk testing initiatives to detect and limit potential sources of exposure. Without such testing, we risk missing small genetic changes in the virus that could allow it to spread much more easily in humans, which is already a concern.
An example is the emergence in Nevada of a new H5N1 variant, D1.1, which appears to be better adapted to mammalian cells, raising concerns about its potential to infect humans more efficiently. As the CDC works to track these developments, delays in federal health agency communications have hindered information sharing and response efforts. The World Health Organization (WHO) has expressed concern over the recent lack of reporting from the CDC to global flu monitoring platforms, emphasizing that global cooperation is vital to pandemic preparedness.
Misinformation and misinterpretation have already caused public confusion. Initial reports suggested cat-to-human transmission of H5N1, which was later debunked. Instead, the evidence indicates that dairy workers inadvertently infected cats via contaminated work clothing. Nevertheless, the risk of transmission of infection from animals to humans remains a pressing concern, particularly in settings where unpasteurized dairy products and live animal handling are prevalent.
Recommendations and Preventative Measures
To mitigate risks, the California Department of Public Health recommends these protective measures for “veterinary practitioners”:
• Avoid direct, unprotected contact with animals suspected or confirmed to have avian influenza, including carcasses, animal waste, and contaminated surfaces or water sources.
• Wear personal protective equipment (PPE), including NIOSH-approved respirators (N-95 masks), disposable gloves, safety goggles or face shields, and disposable or dedicated gowns when handling potentially infected animals.
• Refrain from consuming or feeding pets raw (unpasteurized) milk, dairy products, or raw pet food.
• Report signs of avian influenza in animals — such as fever, drowsiness, lack of coordination, circling movements, head tilting, and difficulty standing or flying — to local authorities.
The CDC’s survey for viral antibodies among veterinary bovine practitioners found that none of those with positive antibody tests had known exposure to infected cattle. This raises serious concerns about undetected transmission pathways and underscores the necessity for systemic surveillance of livestock and dairy products, in addition to the kinds of protective measures listed above.
A Call for Vigilance
The expansion of H5N1 into unexpected populations, the emergence of new variants such as D1.1, and its potential to evolve into a human-to-human transmissible virus necessitates immediate and comprehensive public health responses. Despite the current absence of symptomatic human cases in veterinarians, the silent spread to them calls for enhanced monitoring, robust communication among health agencies, and widespread preventative measures. We need to act before the virus acquires the mutations it needs to spread more efficiently among humans.
Henry I. Miller, MS, MD
Henry I. Miller, MS, MD, is the Glenn Swogger Distinguished Fellow at the American Council on Science and Health. His research focuses on public policy toward science, technology, and medicine, encompassing a number of areas, including pharmaceutical development, genetic engineering, models for regulatory reform, precision medicine, and the emergence of new viral diseases. Dr. Miller served for fifteen years at the US Food and Drug Administration (FDA) in a number of posts, including as the founding director of the Office of Biotechnology.
