July 18, 2005

Ducks may silently pass along bird flu – study

By Maggie Fox, Health and Science Correspondent

WASHINGTON (Reuters) - The bird flu virus that experts fear
will jump from birds to humans seems to be mutating yet again,
and may be able to hide in healthy-looking ducks, thus putting
both other birds and people at risk, experts said on Monday.

They said the H5N1 virus could kill some ducks after
causing only mild symptoms -- which means it could lurk,
undetected, in flocks while spreading silently.

"There is a real possibility that if these H5N1 viruses
continue to circulate, further human infection will occur,
increasing the potential for human-to-human transmission," the
researchers write in this week's issue of the Proceedings of
the National Academy of Sciences.

The H5N1 strain has killed more than 50 people in Asia
since 2003. More than 140 million chickens have been
slaughtered in the region in a bid to halt the disease.

Public health experts say the avian flu virus is mutating
and fear it could develop the ability to spread easily from
person to person and kill millions in a flu pandemic.

Dr. Robert Webster of St. Jude Children's Research Hospital
in Memphis, Tennessee, and colleagues found more evidence of
the virus mutating.

"Wild waterfowl, including ducks, are natural hosts of
influenza A viruses," they wrote.

"These viruses rarely caused disease in ducks until 2002,
when some H5N1 strains became highly pathogenic," they said,
adding that their study showed the viruses were again becoming
harmless to ducks.

Webster's team tested the newer strains of the H5N1 virus,
including some taken from human patients in Vietnam.

They infected four-week-old mallard ducks, dripping various
strains of the virus into the throat, eyes and elsewhere. Then
they put uninfected ducks into the same cages.


All the various strains of H5N1 infected the new ducks,
including samples taken from human patients.

So then they tested each individual strain in more ducks.

"Viruses that caused the death of at least one duck could
cause very mild symptoms, such as cloudy eyes with no
neurological signs," the researchers wrote.

Health officials need to take note, the researchers said,
because since the virus began making ducks as well as chickens
sick in 2002, they had been looking for sick birds in checking
for outbreaks of avian influenza.

Not only that, but the ducks that survived infection also
spread the virus for weeks afterward, both in their droppings
and from the respiratory tract.

"Therefore, the duck may be resuming its role as a
reservoir of H5N1 viruses, transmitting them to other bird
species and potentially to mammals," the researchers concluded.

"There may be many more ducks infected with
low-pathogenicity viruses than are currently detected."

They recommended that health authorities start a survey to
see if the virus is infecting even healthy-looking birds across
the entire region.

Earlier this month researchers reported that H5N1 was
infecting and killing wild geese in a Chinese park -- wild
geese that migrate as far afield as Siberia and New Zealand.

The researchers said in a separate report on Monday that
Roche's influenza drug Tamiflu can help laboratory animals and,
it is hoped, people, survive the newest strain of H5N1 virus.

Government officials are buying up stocks of Tamiflu and
are also working to develop and stockpile a vaccine that works
against avian flu in case it does begin to infect people.