H5N1 Avian Influenza in Cattle: A Gradual Adaptation of the Virus to Its Host

A study shows that the H5N1 virus’s ability to replicate in bovine cells depends heavily on its “internal genomics”—the genes that drive replication and evasion of the innate immune system—far more than on the viral envelope, which typically attracts the most attention. This study shows that not all H5N1 viruses behave the same way when encountering bovine cells: some recent variants replicate significantly better than others, and this potential varies as these viruses evolve.  

The “entry point” isn’t the whole story

Since the beginning of the U.S. outbreak, the explanation put forward has been anatomical: the virus infected the udder, its epithelial cells, and its cellular receptors, and transmission was facilitated by milking. The study does not deny the importance of mammary tissue—on the contrary—but it shows that the core of the phenomenon lies elsewhere: in the virus’s internal “engine,” which governs its ability to replicate once it has entered cells. To isolate this effect, the researchers created “chimeric” viruses: the same viral envelope, but different internal genes depending on the H5N1 virus variants. The result? Very uneven performance was observed depending on the variant! The ability to replicate in bovine cells varies greatly among viruses, even within the currently dominant clade (2.3.4.4b). Certain genetic combinations, particularly those observed in the North American outbreak, exhibit more favorable replication kinetics in bovine cell models, and this effect is also observed in mammary tissues tested ex vivo. In other words, some viruses appear to be more capable than others of replicating in bovine cells. This is a difference in degree, not a clear barrier to adaptation to cattle. What the study reveals is a polygenic adaptation: several internal genomics segments that, together, make influenza viruses more infectious in bovine hosts. Therefore, searching for THE mutation that enabled H5N1 to infect cattle is a narrow perspective.