By Kate Kelland, Health and Science Correspondent
LONDON |
Mon Apr 15, 2013 3:11pm IST
(Reuters) - A new bird flu virus that has killed 13 people in China is
still evolving, making it hard for scientists to predict how dangerous
it might become.
Influenza experts say the H7N9
strain is probably still swapping genes with other strains, seeking to
select ones that might make it fitter.
If it succeeds, the world could be facing the threat of a deadly flu pandemic. But it may also fail and just fizzle out.
The virus' instability also raises questions about whether H7N9 might become resistant to antiviral drugs such as Roche's ( http://in.reuters.com/finance/stocks/overview?symbol=ROG.VX - ROG.VX ) antiviral drug Tamiflu, a possibility already suggested by analyses of genetic data available on the strain so far.
For a graphic http://link.reuters.com/wec47t - link.reuters.com/wec47t
"Even
with just the three (gene) sequences we have available, there's some
evidence that one doesn't quite fit with the other two. So we might
think this virus is still fishing around for a genetic constellation
that its happy with," said Wendy Barclay, a flu virologist at Imperial
College London.
"Maybe there are other viruses out there that it is still exchanging genes with until it gets to a stable constellation."
To
be able to say with any confidence whether this new strain, which
before March had never been seen in humans, could go on to cause a
pandemic, scientists need to know a lot more.
H7N9 A TRIPLE MIX BIRD FLU
So
far, genetic sequence data from samples from three H7N9 victims and
posted on the website of GISAID, the Global Initiative on Sharing All
Influenza Data, show the strain is a so-called "triple reassortant"
virus with a mixture of genes from three other flu strains found in
birds in Asia.
Writing in the New
England Journal of Medicine last week, researchers who conducted a
detailed analysis of the strain's origin said it seemed that so far the
reassortment of genes to make H7N9 had taken place in birds rather than
in humans or in any other mammal - a somewhat reassuring sign.
Barclay
said this may continue, and could mean it is some time before the
strain finds a form in which it can spread swiftly and efficiently in
bird populations.
Yet genetic
analyses also show the virus has already acquired some mutations that
make it more likely be able to spread between mammals, and more able to
spark a human pandemic.
A study in
the online journal Eurosurveillance by leading flu experts Yoshihiro
Kawaoka of the University of Wisconsin and Masato Tashiro at the
National Institute of Infectious Diseases in Tokyo, said the H7N9
sequences "possess several characteristic features of mammalian
influenza viruses, which are likely to contribute to their ability to
infect humans".
These features, the scientists wrote, "raise concerns regarding their pandemic potential".
That
sentiment was echoed on Saturday by the World Health Organization
(WHO), which said "genetic changes seen among these H7N9 viruses
suggesting adaptation to mammals are of concern" and warned: "Further
adaptation may occur".
PANDEMIC POTENTIAL
While
experts take some comfort in the lack of evidence so far that H7N9 is
passing from person to person - a factor that would dramatically
increase its pandemic potential - they are find little comfort in not
yet knowing how the 60 or so people confirmed as having this flu strain
became infected.
"We know H7
viruses can spill over into humans ... and for me the most important
thing to find out now is from which species do we think this H7N9 is
spilling over," said An Osterhaus, head of viroscience of the Erasmus
Medical Centre in the Netherlands.
"Is it one species? Are there different species? At this stage we are still lacking a lot of data."
He
said rigorous surveillance of wild birds, such as ducks and quail, and
poultry such as chickens, as well as well-known flu-carrying mammals
such as pigs, should yield answers.
Recent
pandemic viruses - including the H1N1 "swine flu" of 2009/2010 - have
been mammal and bird flu mixtures. Experts say these hybrids are more
likely to be milder, because mammalian flu tends to make humans less
severely ill than bird flu.
Pure
bird flu strains - like the new H7N9 strain and like the H5N1 strain
that has killed around 371 of 622 the people it has infected since 2003 -
are generally more deadly for people.
The
world's worst known pandemic, the "Spanish flu" of 1918 that killed
more than 50 million people, was a bird flu that had picked up gene
mutations that enabled it to spread efficiently in humans.
David
Heyman, a flu expert and head of Britain's Chatham House Centre on
Global Health Security, said it is important to put the discovery of
H7N9 in humans into the context of modern-day scientific capability.
He
said that in the years since the outbreak of Severe Acute Respiratory
Syndrome (SARS) in China in 2003, there has been a significantly
increased focus on detecting and reporting flu-like respiratory
infections in Asia and across the world.
The
harder scientists look, he said, the more likely they are to find
viruses that are potentially threatening but may equally be the sort of
events that in the past might have flared up and petered out again under
the flu surveillance radar.
That said, he stressed this is no time to relax.
"Influenza
viruses are very unstable. And (any) mutation is a random event - so
nobody can predict when it will happen," he said. "You can't take your
eye off anything. You have to keep you eye on everything." (Reporting by
Kate Kelland, additional reporting by Ben Hirschler in London and Giles
Elgood)