Two mutations now prevalent in H5N1 avian influenza viruses circulating in U.S. dairy cattle have enabled the virus to exploit a molecular receptor found abundantly in bovine tissue but absent in humans and birds — enhancing viral replication in cow mammary glands while appearing to pose limited additional zoonotic risk in current form. The findings, posted to bioRxiv on April 6, 2026, provide the first detailed glycomic characterization of cattle tissues in the context of H5N1 adaptation and carry important implications for pandemic surveillance, agricultural biosecurity, and public health preparedness planning.
Since H5N1 was first detected in U.S. dairy cattle in early 2024, the virus has continued evolving under host-specific selective pressure. Researchers from the Pirbright Institute, the Roslin Institute, and collaborating institutions across the United Kingdom analyzed approximately 4,000 H5N1 genomes sampled from cattle and humans, identifying two hemagglutinin mutations — D104G and V147M — that have undergone repeated independent emergence and are now reaching fixation in circulating bovine H5N1 lineages.
The Molecular Mechanism: A Species-Specific Sugar Lock
The mechanism underlying these mutations centers on a fundamental difference in cell surface chemistry between species. Influenza viruses attach to host cells by binding sialic acid-capped sugars on cell surfaces. Humans and birds produce only one form of sialic acid — N-acetylneuraminic acid (NeuAc) — while many mammalian species, including cattle, pigs, horses, and sheep, also produce a second form, N-glycolylneuraminic acid (NeuGc), through an enzyme called CMAH that humans lack.
The research team performed detailed glycomic profiling of four bovine tissue types — mammary gland, teat, trachea, and lung — and found that NeuGc-containing glycans are present in roughly equal abundance to NeuAc-containing glycans across all tissues. Early cattle H5N1 viruses bound poorly to NeuGc receptors. However, the D104G and V147M mutations together enabled efficient binding to both NeuAc- and NeuGc-containing receptors, confirmed through three independent methodologies: bio-layer interferometry, glycan microarray analysis, and pseudovirus cell entry assays.
Enhanced Cattle Replication, Attenuated Human Risk — For Now
The functional consequences of this receptor broadening are asymmetric across species in ways directly relevant to public health risk assessment. In bovine mammary explants, mutant viruses carrying NeuGc-adaptive mutations replicated to significantly higher titers than the early cattle strain. In human lung cells and primary human nasal epithelial cells, the same mutations showed similar or modestly attenuated replication compared to wild type — suggesting the NeuGc adaptation enhances bovine fitness without meaningfully increasing replicative capacity in human airways.
The authors frame this as a cattle-specific evolutionary filter rather than a stepping stone toward pandemic potential in its current form. Critically, the mutations broaden receptor usage rather than switch specificity — the virus retains NeuAc binding alongside its newly acquired NeuGc capability. As noted in accompanying commentary, this dual-sugar usage could still matter for human exposure risk: higher viral loads in cattle mammary tissue and potentially in respiratory secretions may mean that humans exposed to infected cattle encounter higher infectious doses, even if human-to-human transmission risk is not directly increased.
Why Other Livestock Species Matter
Because pigs, horses, and sheep also express NeuGc, the authors raise the concern that NeuGc-adapted H5N1 could more readily spill over into these species — which in the case of pigs represents a particularly significant pandemic risk given their established role as mixing vessels for influenza reassortment. The research team also notes that three independent 2025 spillovers of a different H5N1 genotype into cattle in Arizona, Nevada, and Wisconsin currently lack equivalent NeuGc adaptation.
The deep mutational scanning analysis identified multiple additional hemagglutinin positions that could independently confer enhanced NeuGc binding. The authors recommend close monitoring of these sites in cases of H5N1 spillover into any NeuGc-expressing host.
Operational Implications for Preparedness and Control
The research offers a nuanced but important signal. The current NeuGc-adaptive mutations do not appear to increase risk of efficient human infection or person-to-person transmission. However, the finding that prolonged circulation in cattle is generating host-specific molecular adaptations — and that those adaptations could increase viral loads in milk and respiratory secretions — reinforces the importance of rapid outbreak control in livestock, enhanced surveillance for novel hemagglutinin mutations at the human-animal interface, and continued monitoring of farmworkers and others with direct cattle exposure as a sentinel population.
The preprint has not yet completed peer review. The research was conducted under appropriate biosafety conditions, with high-pathogenicity H5N1 work performed at containment level 3/SAPO4 facilities.
Sources and Further Reading:
Hassard JA et al. Bovine H5N1 influenza viruses have adapted to more efficiently use receptors abundant in cattle. bioRxiv. April 6, 2026
Hesman Saey T. New mutations help the H5N1 bird flu virus infect cows but not people. Science News. April 14, 2026