Bird flu viruses carry unique genes: study

By Maggie Fox, Health and Science Correspondent

WASHINGTON (Reuters) – Scientists may have found out what
makes the H5N1 influenza virus so deadly — bird flu viruses
have a gene that may make them especially destructive to cells,
U.S. researchers reported on Thursday.

All the bird flu viruses studied by the team at St. Jude
Children’s Research Hospital in Memphis had the gene and none
of the human influenza viruses did, they said.

People infected with the H5N1 bird flu virus in Vietnam and
Thailand had the “avian” version of the flu virus, as did the
victims of the 1918 influenza pandemic, which killed tens of
millions of people globally, the researchers said.

But the influenza viruses that cause the normal seasonal
human misery, and those that caused the less deadly 1957 and
1968 human flu pandemics, do not carry the avian genes.

The finding, published in the journal Science, may provide
a way to identify the more dangerous viruses and may also help
companies trying to make better flu drugs, said St. Jude’s
Clayton Naeve.

“We documented a clear difference between bird viruses and
human viruses. You need much more work to demonstrate this
actually contributes to virulence in nature,” Naeve said in an
interview.

Naeve and his colleagues have been working to sequence the
genomes of all known influenza viruses. No one has done this,
even though flu viruses have just eight genes and are
relatively simple organisms, the researchers said.

“This is information we expect will be very important in
understanding the attributes of this virus — how it will cross
from birds to humans. We are releasing this data so that other
investigators worldwide can mine it for information,” he said.

VIRUS COLLECTION

The researchers used a collection of samples of 11,000
influenza viruses, including 7,000 avian influenza viruses,
assembled by St. Jude’s Dr. Robert Webster over 30 years.

“We have sequenced a diverse sampling of 336 avian
influenza viruses from this collection including isolates from
ducks, gulls, shorebirds and poultry collected in North
American, Eurasian, and Australasian countries, primarily
during the years 1976 to 2004,” the researchers said.

The H5N1 virus has been found in birds for decades but it
first was seen to infect people in 1997, in Hong Kong. Since it
resurfaced in 2003, it has infected at least 152 people and
killed 83 of them, according to the World Health Organization.

It has killed or forced the culling of hundreds of millions
of birds.

H5N1 does not yet pass easily from person to person but
experts fear it will mutate into a form that does so, sparking
a pandemic that could kill millions or tens of millions.

One factor will be just what genetic changes the virus
makes, and no one can predict what they will be. But Naeve’s
team may have identified two proteins to watch.

They are called NS1 and NS2, for non-structural protein,
and they are only made once the virus has infected a cell.

The avian versions seem to allow the virus to do much more
damage to a cell than the human versions of NS, Naeve said.

“We were surprised to see a lot of variation in this NS
protein. That was the clue. We felt it must be playing an
important biological role,” he said.

It is possible that a mutation that would allow a flu virus
to more easily infect people will weaken the NS protein, Naeve
said. But no one knows.

“As time progresses one might expect that signature to
change into a less-virulent form,” he said, or it may not, as
was apparently the case in 1918.

“I don’t think anybody can predict at this time what the
future holds for the H5 outbreaks. It’s pretty scary,” Naeve
said.