Until recently, nobody knew what kind of sialic acid receptors cows had, because it was believed that they didn’t catch A-strain flu viruses like H5N1.
Larsen and his colleagues in the US and Denmark took tissue samples from the lungs, windpipes, brains and mammary glands of calves and cows and stained them with compounds that they knew would attach to different kinds of sialic acid receptors. They sliced the stained tissues very thinly and peered at them under a microscope.
What they saw was surprising: The tiny milk-producing sacs of the udder, called alveoli, were brimming with sialic acid receptors, and they had both the kind of receptors associated with birds and those that are more common in people. Almost every cell they looked at contained both types of receptors, said lead study author Dr. Charlotte Kristensen, a postdoctoral researcher in veterinary pathology at the University of Copenhagen.
That finding has raised concern because one way flu viruses change and evolve is by swapping pieces of their genetic material with other flu viruses. This process, called reassortment, requires that a cell be infected with two different flu viruses at the same time.
“If you get both viruses in the same cell at the same time, you can essentially get hybrid viruses coming out of it,” said study author Dr. Richard Webby, director of the World Health Organization Collaborating Centre for Studies on the Ecology of Influenza in Animals and Birds.
In order to be infected simultaneously with two flu viruses – a bird flu virus and a human flu virus – a cell would need to have both kinds of sialic acid receptors, which cows do, something that wasn’t known before this study.
“I think this is probably a pretty rare event,” said Webby, who has been studying the H5N1 virus for 25 years.
In order for something like that to happen, a cow infected with the bird flu virus would need to pick up a different flu strain from an infected human. Currently, human flu infections are low across the country and dropping as flu season winds down, making the possibility of something like this happening even more remote.