As H5N1 continues to be detected in U.S. dairy cattle, new research shows some modern bird flu viruses are genetically better equipped to infect bovine cells than earlier strains, helping explain why cattle are now part of the outbreak.
Highly pathogenic avian influenza (HPAI) is no longer just an avian problem. Recent detections of H5N1 in dairy cattle, including the latest confirmed case in a Wisconsin herd, have elevated concern among veterinarians, producers and animal health authorities. According to USDA, HPAI has been detected in dairy herds in at least 18 states since March 2024, with milk testing serving as a routine detection pathway.
A newly published Nature Communications study led by scientists at the MRC–University of Glasgow Center for Virus Research provides critical insight into why certain H5N1 viruses are now capable of infecting cattle and highlights that some recent H5N1 variants are better at infecting cow cells and mammary tissues than older viruses. This suggests recent spillover events are not random accidents but might reflect viral genetic traits that support infection in cattle.
“Our work shows that different bird flu viruses have very different abilities to infect cow cells and tissues,” says Professor Massimo Palmarini, from both the Erasmus Medical Center and the MRC–University of Glasgow Center for Virus Research. “While the strain currently spreading in U.S. cattle is clearly the best adapted so far, there are other bird viruses that could potentially infect cows if given the chance.”
Key Findings Veterinarians Should Know
- H5N1’s ability to infect cattle varies by viral lineage and the outbreak clade stands out
Researchers evaluated a wide range of historical and contemporary H5N1 viruses in bovine cell systems. The results were clear: Replication efficiency in bovine cells differed substantially between strains. Earlier H5N1 viruses often showed limited replication, while variants of the current outbreak from clade 2.3.4.4b, including genotypes B3.13 and D1.1, performed significantly better.
Why it matters: This variability helps explain why cattle infections are being detected now, after decades of HPAI circulation in birds. This also aligns closely with what field veterinarians are seeing: dairy cows developing clinical signs, such as reduced milk production and abnormal milk, often in the absence of severe respiratory disease.
- Internal genes, not just surface proteins, drive adaptation
The study highlights that synergistic interactions among internal gene segments, including viral polymerase complex and non-structural genes, play a critical role in determining how well H5N1 replicated in bovine cells. This shifts the focus away from viral surface protein hemagglutinin alone.
Why it matters: Viral evolution that improves replication in cattle could occur without obvious changes to classical avian influenza red flags, complicating surveillance and risk assessment.
- Adaptation does not mean inevitability, but it raises the stakes
The authors stop short of suggesting H5N1 is becoming a cattle-adapted virus. However, they do demonstrate a biological pathway for improved compatibility with bovine hosts.
Why it matters: If H5N1 continues circulating in cattle, even transiently, it carries opportunities for viral maintenance, farm-level spread, and additional spillover events.
Connecting Lab Findings to Field Observations
These results provide a critical biological framework for the unusual epidemiology seen in recent cattle detections. The finding that modern H5N1 variants replicate efficiently in bovine mammary cells explains why dairy herds, and not beef, have been the focus of this outbreak and why milk has emerged as a vital surveillance sample.
This shift in viral tropism is directly reflected in the field: Infections are typically identified not by respiratory distress but by sudden drops in milk yield and abnormal milk consistency. While commercial pasteurization ensures the general milk supply remains safe, the high viral loads in raw milk highlight a pressing need for enhanced biosecurity within the milking environment. Ultimately, when production anomalies coincide with local avian influenza activity, H5N1 testing should be considered an essential component of the diagnostic workup.
