New study examines bird flu's genesis in China
- From: "1mitee" <haivtran@xxxxxxxxx>
- Date: 7 Feb 2006 10:18:45 -0800
New study examines bird flu's genesis in China
Updated Mon. Feb. 6 2006 6:58 PM ET
Canadian Press
TORONTO - Genetic analysis of H5N1 avian flu viruses collected from
wild and domestic birds in China suggests the virus has spread in
uncontrolled fashion in southern China for the past decade, igniting
outbreaks in Vietnam on at least three occasions and triggering spread
of the worrisome virus across Russia to eastern Europe, a study
released Monday suggests.
The study also provides what could be a smoking gun in the heated
dispute about whether migratory birds are actually playing a role in
moving the virus across the globe. A number of wildlife experts have
insisted migratory birds are not to blame, but this study contends
genetic analysis shows there can be no other explanation.
"I've spent most of my life working with wild birds and I'm
sympathetic. But you can't hide the facts that wild birds are most
probably involved in the spread of this thing now,'' said leading avian
influenza expert Dr. Robert Webster, one of the authors of the paper.
Webster and his co-authors -- researchers from institutions in China,
Hong Kong, Vietnam and Indonesia -- argue the only way to minimize the
damage done by the virus and reduce the risk it poses to human health
is by addressing the problem at the virus's birthplace, in the poultry
flocks of southern China.
"We have shown that H5N1 virus has persisted in its birthplace,
southern China, for almost 10 years and has been repeatedly introduced
into neighbouring (e.g. Vietnam) and distant (e.g. Indonesia) regions,
establishing 'colonies' of H5N1 viruses throughout Asia that directly
exacerbate the pandemic threat,'' they said in the article, published
in the Proceedings of the National Academy of Sciences.
"Therefore control of this regional epizootic and its attendant
pandemic threat requires that the source of virus in southern China be
contained.''
Doing that won't be easy, especially in light of another finding
Webster and his colleagues reported.
They discovered instances where seemingly healthy chickens in Chinese
poultry markets were shedding the virus. To date, chickens have been
like the proverbial canaries in the coal mine with H5N1, which has
killed millions of chickens across Asia.
It's not clear why the chickens didn't succumb to the virus. The
authors suggest they may have been vaccinated or protected by previous
exposure to a similar but not lethal virus. But one thing is clear: if
chickens are able to survive infection but continue to shed the virus,
tracing its spread will become much more difficult.
"It's not a case of `there are dead birds and they are hiding it'.
(That) there are no dead birds is the problem,'' Webster said in an
interview from Hong Kong.
"The pernicious thing is that there are these perfectly healthy birds
that you walk into the markets day after day after day and the virus is
always there.''
The researchers collected viruses over several years from wild birds in
the Mai Po marshes of Hong Kong and at Poyang Lake in eastern China,
and from swabs taken from apparently healthy poultry in markets across
southern China.
They generated genetic sequences of the H5N1 viruses, comparing them to
each other and to other previously sequenced viruses on what are called
phylogenetic trees. These charts, the equivalent of a family tree, can
identify where a virus came from by finding viruses to which it is most
closely genetically related.
The charts the group generated suggest viruses from domestic poultry in
southern China infected wild ducks in early 2005. Ducks took those
viruses to Poyang Lake in eastern China and from there to Qinghai Lake
in western China, more than 1,700 kilometres away. The viruses were
responsible for a large die-off of wild birds at Qinghai Lake -- a wild
bird reserve -- that occurred last May.
"That was the tipping point. The virus really got into the migratory
birds at that point,'' Webster, who is based at St. Jude Children's
Research Hospital in Memphis, Tenn., said of the Qinghai Lake incident.
Recent analysis of viruses from the first two human cases of H5N1
infection in Turkey showed they were closely related to viruses
isolated at Qinghai Lake.
"Genetic relatedness of gene segments from H5N1 viruses isolated at
Poyang and Qinghai Lakes strongly argues that migratory birds can
transfer the virus over long distances,'' the authors said.
"This possibility may provide insight into reported H5N1 outbreaks in
Mongolia, Siberian Russia and Europe that have been linked to migratory
birds.''
Earl Brown, a virologist who specializes in the evolution of influenza
virus virulence, said the work done by this group helps scientists
figure out what is going on with the H5N1 viruses.
"It puts details on the evolutionary story,'' said Brown, who is based
at the University of Ottawa.
"This links the dots from southern China to this Poyang Lake to
Qinghai. And then it sort of gets out from there even broader. But it's
very important to know where the virus is coming from, what it's doing
in the different places, like what birds (it infects) and what disease
(it causes) and that sort of thing.''
The study also noted there are a number of distinct clades or families
of viruses in birds. Previous genetic analysis of viruses isolated from
human cases showed two distinct clades.
That finding has implications for production of vaccines against H5N1,
as a vaccine produced using a virus from one clade might offer at best
partial protection against a virus from another.
The authors suggest multiple seed strains or vaccine starters should be
made and kept at the ready. The World Health Organization actually
commissioned that work last October.
A WHO avian flu expert suggested this work is a reminder of how
important it is to keep on top of how the virus is evolving.
"It's very important to do this kind surveillance so that we know
what's going on. And we're hopeful, since there are some clear
differences, that these viruses will be made available to other
investigators to do further analysis,'' said Michael Perdue, who is
with WHO's global influenza program.
.
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