Mutations target a specific sugar on the surface of cow cells
H5N1 avian flu viruses have acquired a molecular trick that allows them to more easily infect the mammary glands of cattle, but this adaptation does not appear to affect humans.
To infect cells, flu viruses attach to certain sugars decorating the cell surface. Some H5N1 viruses have detected mutations that allow them to catch one of these viruses. sugar made by livestock but not by humans or birds, the researchers report April 6 on bioRxiv.org.
Specifically, two mutations frequently found in H5N1 virus infecting dairy cattle now let the avian flu viruses take over the sugar N-glycolylneuraminic acid, or NeuGc. Seizing this bovine sugar facilitated the infection and growth of the H5N1 avian influenza virus in the cows’ mammary tissues, the researchers found.
The sugar switch could also make it easier for the H5N1 virus to spread from cow to cow through the air and increase the risk of spread to other farm animals. like pigssheep and horses, which also make NeuGc, the researchers suggest.
Humans and birds lack an enzyme that produces the sugar NeuGc. Instead, they make acetylneuraminic acid, or NeuAc. The H5N1 viruses that latch onto cattle sugar can latch on to the version found in humans and birds. But in laboratory tests, the virus’s ability to take over livestock’s NeuGc had no effect or only slightly hindered viral growth in human nasal cells. This change therefore does not appear to have increased the risk of H5N1 avian flu. spreads easily between people.
Before that, scientists were only aware of one other example of a flu virus that had acquired the ability to take over NeuGc. In this case, a now-extinct equine flu virus shifted entirely from using NeuAc to using NeuGc, says Thomas Peacock, a virologist at the Pirbright Institute in England. Because he could no longer grasp NeuAc, “probably the [equine] the virus would have gotten worse [at] infecting birds or humans,” he says. But livestock-friendly H5N1 “has just learned to use the second type while using the first type just as well.”
This double consumption of sugar could be bad news for people. Although livestock-adapted viruses don’t have a growth advantage in human cells because they have both human and bovine sugars to latch on to, avian flu viruses can grow much faster and reach higher levels in cattle, increasing the number of viruses in milk and perhaps the air, Peacock says. So, “perhaps when humans are exposed to infected livestock, the doses they receive could be higher.”
