“The risk [for humans] is increasing in the sense that there is a lot of virus in poultry and wild birds,” Prof Munir Iqbal, a member of the UK government’s modelling group for bird flu, told the Telegraph recently.

“The virus can change at any time, and therefore the risk is higher when there’s more in the environment. That doesn’t mean [a human epidemic] is imminent… but no one has control of the virus,” he said.

'We have to stay on our toes'

So far, cases in people have been sporadic and health agencies have maintained the threat is low.

But the latest study points towards the “potential for this virus to infect and transmit between mammals including humans,” according to researchers at organisations including the Public Health Agency of Canada.

Prof Marion Koopmans, a Dutch virologist who was not involved in the research, said the preprint, which is yet to be peer reviewed, is a reminder of the threat posed when slightly different H5N1 strains combine.

“[The] most important message here is: let's not assume we know ‘the’ properties of viruses that are spreading so widely,” she told the Telegraph on Monday.

“With the widespread circulation among wild birds, we see mixtures of viruses arise, in which some of the internal genes are derived from other avian flu viruses. This can happen if you see co-infections of cells in an individual (bird) with two viruses.”

She added: “We have also seen this happening in Asia and in Europe. What the study adds is that this evolution can lead to strains with quite different traits. In this case, that is severity and ability to transmit in an animal model. There is no evidence for airborne transmission which would raise a big flag, but it is also not very comforting.

“To me it says: we really have to stay on our toes and enhance our surveillance, also in mammals like mink and pigs.”

Chilean case shows adaptation

The latest preprint comes after a sample taken from a Chilean man who caught H5N1 last month had two genetic mutations that point towards adaptation to mammals, according to the US Centers for Disease Control and Prevention.

The gene, called PB2, has previously been shown to help the virus replicate within – and therefore spread between – mammals.

Yet scientists stressed that previous studies, also in ferrets, have concluded that H5N1 needs to undergo more changes to efficiently spread in humans at a large scale.