The risk of the bird flu outbreak among dairy cows in the US triggering a human flu pandemic may be lower than feared.
Cow udders have lots of bird-like flu virus binding sites but no human-like ones, a study has found, meaning there is no reason for the virus to evolve inside cows to become better at infecting people.
However, if farm workers continue to be infected, the virus will still have a chance of evolving to become better at spreading among humans, says study leader at Utrecht University in the Netherlands.
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“For me, it is quite a scary idea to have a virus with these [characteristics] being maintained in a very significant livestock,” says de Vries.
For flu viruses to infect cells, they have to bind to a sugar molecule called sialic acid that protrudes from the outside of many cells. However, there are many different variants of sialic acid, and flu viruses can bind only to specific ones.
For instance, the sialic acid variant found in the upper respiratory tract of birds is different from the one found in the upper respiratory tract of humans. Some other tissues in the human body, such as the lungs, do have the so-called bird-like variant, which is why bird flu viruses can sometimes infect people.
Crucially, though, as long as bird flu viruses cannot infect the throats and noses of people, they are very unlikely to spread from person to person, because people don’t spray these viruses around when they cough or sneeze.
In the past few years, a particularly nasty strain of H5N1 bird flu has spread all around the world. Around 60 per cent of people known to be infected with this virus . So far, people have only caught the virus from animals, but the big fear is that the virus will evolve to bind to the human-like sialic acid, allowing it to spread among people.
This evolution is most likely to happen in a mammal, hence the concern about the H5N1 outbreak in cows.
On 3 May, at St. Jude Children’s Research Hospital in Tennessee and his colleagues published preliminary results suggesting that in the udders of cows, explaining why H5N1 is able to infect udders.
Alarmingly, these findings suggested that the human-like variant is also common in the udder. This would mean that any H5N1 viruses with mutations that allow them to bind the human-like variant could survive and thrive in udders, and potentially spread to people.
However, de Vries’s team has now done a more detailed study that suggests the human-like receptor is not present in udders after all.
“With the lack of human-type receptors in the mammary gland, where these viruses replicate, the chances of adapting to them are quite slim,” says de Vries.
The issue is that finding out which sialic acid variant is present in different parts of the body is tricky. Webby’s team used only proteins called lectins that bind to specific sialic acids. De Vries’s team used the flu proteins that bind to sialic acids – called haemagglutinins – as well as lectins.
“The Dutch team did use a more sophisticated methodology,” says Webby. “I much prefer their conclusion.”
De Vries and his colleagues also found that neither the bird-like nor human-like sialic acid was present in the upper respiratory tract of cows. This adds to the growing evidence that H5N1 is being spread by milking machinery, rather than via respiratory infections, says at the Pirbright Institute in the UK.
It also means that cows are unlikely to be infected by human flu viruses, says Peacock, which is more good news.
“One of the worries is that cows could be full of human influenza viruses that we’ve never really looked for,” he says. This would mean a cow might be infected by H5N1 and a human flu virus simultaneously, allowing the viruses to swap genes and potentially generate a new lethal strain capable of causing a human pandemic.
So de Vries’s results suggest that cows are unlikely to be a breeding ground for dangerous new viruses, but he, Webby and Peacock all say further studies using a variety of methods will be needed to confirm this.
“Virus evolution is unpredictable, and it’s hard to say we shouldn’t worry about this because these viruses keep on doing weird things and surprising us in ways that we’d rather they didn’t,” says Peacock.
And eliminating the virus from cows is still important to avoid human infections, which do provide an opportunity for H5N1 to gain the ability to bind to the human-like sialic acid, or to swap genes with human flu viruses.
bioRxiv
