The Centers for Disease Control and Prevention (CDC) has uncovered evidence of undetected H5N1 avian influenza infections in humans, raising concerns about the adequacy of current surveillance systems. A recent CDC study revealed that 2% of surveyed bovine veterinarians showed antibodies to recent HPAI A(H5) infection despite reporting no symptoms, suggesting silent transmission of the virus.
"It means our surveillance is inadequate," warns Lauren Sauer, professor at the University of Nebraska Medical Center College of Public Health. "Any detection of asymptomatic or mild cases in this study just tells me we're missing cases."
Rising Human Cases and Emerging Strains
As of February 14, 2025, the CDC has documented 68 confirmed human cases of H5N1. The virus has caused several hospitalizations, including recent cases in Wyoming and Ohio, and one death in Louisiana. Of particular concern is the emergence of a new strain, D1.1, which has become dominant among bird and dairy cattle populations and was responsible for the Louisiana fatality.
The virus has shown concerning mutations in recent cases, with genetic changes that enhance its ability to attach to respiratory receptors in human lungs. While human-to-human transmission hasn't been documented, these adaptations have alarmed health professionals.
Expanding Host Range and Transmission Patterns
The study evaluated 150 veterinarians across 46 sites, finding that 55% practiced in states with virus-positive dairy herds. The three veterinarians who tested positive for antibodies had been exposed to multiple animals at sites with no known viral infection, indicating undetected spread in cattle populations.
The virus has demonstrated an expanding host range, affecting not only commercial and wild birds but also dairy cattle and domestic cats. A concerning development in Oregon saw two indoor cats contract H5N1 from raw pet food, leading to their euthanization and raising new concerns about transmission pathways through domesticated pets.
Government Response and Vaccine Development
In response to the growing threat, the U.S. government has initiated several countermeasures. The Department of Health and Human Services has granted Moderna $590 million to accelerate H5N1 mRNA vaccine development. Additionally, the US Department of Agriculture recently gave conditional approval for an updated flu vaccine to protect poultry, addressing the virus's impact on over 150 million commercial and backyard flocks.
"We should never underestimate flu," Sauer emphasizes. "If cases are occurring more frequently than detected in humans, we risk missing small changes that allow the virus to begin to spread much more easily in humans."
Surveillance and Prevention Measures
The discovery of asymptomatic cases among veterinarians has highlighted critical gaps in current monitoring systems. Health authorities are now calling for enhanced surveillance, particularly in high-risk populations and occupational settings where human-animal contact is frequent.
The situation continues to evolve, with health agencies working to strengthen detection methods and response protocols. The combination of asymptomatic spread, emerging strains, and expanding host range underscores the urgent need for improved monitoring and preventive measures to address this growing public health concern.
