Research has found that bird flu viruses circulating in nature could be just a small step away from unleashing a pandemic as potentially ferocious as that which struck in 1918.
The scientists found that the viral proteins making up the 1918 flu strain differed by only a few molecular building blocks from those now found in bird populations around the world.
They said that a handful of key mutations could be all it took to allow a 1918-like virus to spread freely between humans.
Experts in the UK said the findings highlighted the need to closely monitor flu strains in animals, but said there was no immediate cause for concern.
The last flu pandemic - swine flu in 2009 - was linked to 200,000 death around the world. The first UK cases of swine flu and the first UK death were reported in Scotland.
The researchers, writing in the journal Cell Host and Microbe, came to their latest conclusions after testing the ability of a reverse-engineered 1918-like virus made from components circulating in birds to transmit to ferrets, whose susceptibility to flu mimics that of humans.
Lead scientist Professor Yoshihiro Kawaoka, from the University of Wisconsin-Madison in the US, said: “The worst-case scenario is the emergence of a novel avian influenza virus that exhibits high pathogenicity in humans, like H5N1 (avian flu) viruses, and efficient transmissibility in humans, like seasonal influenza viruses.
“Our findings demonstrate the value of continued surveillance of avian influenza viruses and reinforce the need for improved influenza vaccines and antivirals to prepare for such a scenario.”
Prof Kawaoka’s team generated a flu virus composed of eight gene segments creating proteins resembling those of the 1918 virus. All eight segments were in current circulation among bird populations.
The resulting 1918-like virus caused illness in ferrets, and the addition of seven mutations in a few key proteins allowed it to spread efficiently from animal to animal.
A similar natural virus acquiring the same mutations could cause a human pandemic in the near future, said the scientists.
The research showed that the genetic ingredients for such a pandemic already exist in nature and could combine to present a potentially deadly threat.
Another key finding was that the current seasonal flu vaccine may offer protection against a new 1918-like virus.
In addition, the 1918 virus was expected to be sensitive to the antiviral drug oseltamivir (Tamiflu).
Dr Kawaoka added: “The point of the study was to assess the risk of avian viruses currently circulating in nature.
“We found genes in avian influenza viruses quite closely related to the 1918 virus and, to evaluate the pandemic potential should such a 1918-like avian virus emerge, identified changes that enabled it to transmit in ferrets.
“With each study, we learn more about the key features that enable an avian influenza virus to adapt to mammals and become transmissible.
“Eventually, we hope to be able to reliably identify viruses with significant pandemic potential so we can focus preparedness efforts appropriately.”
Dr Ben Neuman, a virologist at the University of Reading, said the researchers had identified some “potentially very dangerous flu viruses” that are circulating in birds, and had shown that they could potentially spread between people with a few small changes. But he said it was worth remembering that past pandemics, including the swine flu pandemic, came from unexpected sources.
“The viruses described in this paper resemble one that killed millions at the end of World War I, which sounds quite worrying at first,” Dr Neuman said.
“However, it is important to remember that these viruses are not new – just newly discovered. Much of what makes a virus go pandemic is not related to disease, but to mundane changes that make the virus a little bit more efficient at growing in people.
“These are probably direct descendants of the virus that led to the 1918 Spanish flu, but if so, that means that we have been sharing the planet with them for nearly a century without major incident.”
Prof Peter Openshaw, director of the Centre for Respiratory Infection at Imperial College London, added: “These very well-performed studies underscore the need for close monitoring of the viruses that are currently circulating in birds.
“There is certainly no reason to be complacent – the swine flu outbreak may have been mild by comparison to the sorts of outbreaks that these viruses could cause, but it still stretched our resources to the limit.”