- WASHINGTON (Reuters) - The
genetic union of pig and human influenza viruses triggered one of the most
deadly disease outbreaks in human history, the 1918 ``Spanish'' flu pandemic,
according to researchers who warned that another flu outbreak is inevitable.
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- In a study appearing in the journal Science on Thursday,
scientists at the Australian National University in Canberra said a key
gene in the virus responsible for the 1918 pandemic was a hybrid created
by the joining together of genetic sequences of pig and human influenza
viruses.
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- This ``recombination'' may account for the severity of
the outbreak, the researchers said.
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- The pandemic, whose outbreak came just as World War One
was drawing to a close, killed more than 20 million people as it spread
around the globe in 1918 and 1919.
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- Understanding the cause of past flu outbreaks is vital
in the quest to recognize threatening future outbreaks, said lead researcher
Mark Gibbs.
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- ``Another pandemic is inevitable,'' Gibbs told Reuters.
``They are triggered by changes in the virus, and the virus will change
again. ... There were major pandemics in the 1890s, 1918, 1957 and 1968,
and some scientists say that we are due for another one.''
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- The study was one of two concerning influenza appearing
in Science. In the second, researchers led by Yoshihiro Kawaoka of the
University of Wisconsin studied a more-recent deadly influenza outbreak,
the 1997 Hong Kong ``chicken'' flu. The outbreak killed six out of the
18 people it infected.
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- Influenza infects humans, pigs, some birds, horses and
seals. The Hong Kong outbreak was the first documented case of an influenza
virus jumping directly from chickens to people. Public health authorities
responded by ordering the slaughter of more than 1 million live poultry
to prevent further spread.
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- Using laboratory mice, Kawaoka's team showed that a tiny
change in one of the virus's 10 genes can make certain strains especially
virulent.
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- ``Because the influenza virus constantly mutates, and
because only a few changes can make a non-pathogenic virus highly pathogenic,
we should assume that an outbreak of any new strain or sub-type is potentially
dangerous to humans,'' Kawaoka said.
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- MEDICAL DETECTIVES
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- The virus from the 1918 ``Spanish'' flu was not preserved
at the time of the outbreak and long was believed to have been lost to
science. But American scientists in 1997 recovered some of its genetic
material from a female victim whose body was buried in permafrost in Alaska
and from samples taken in 1918 from two U.S. soldiers who died in the pandemic.
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- These scientists reconstructed part of the genetic data
of the virus and compared it to other strains of influenza virus. But this
analysis failed to reveal what triggered the pandemic and what made it
so bad.
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- Gibbs and colleagues John Armstrong and Adrian Gibbs
found that one of the genes of the virus actually was a hybrid that was
produced by recombination of parts of the genes of two strains of influenza
that were circulating just before the pandemic. Mark Gibbs said the gene
was constructed from these parts while the viruses were replicating.
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- The researchers said changes in this particular gene
can make the virus unrecognizable to the immune system -- which the body
uses to fight off disease -- and can ratchet up the virulence of the virus.
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- Gibbs said strains of influenza sometimes are transmitted
between pigs and people.
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- He said the recombination took place when a host -- one
of those pigs or people -- became infected with both strains at the same
time. The two viruses met up in some of the cells of the host and then
the mixture occurred as ``a kind of replication error,'' he added.
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- The 1918 flu killed about one in 40 of the people it
infected. Current influenza strains kill about one in 25,000 people infected.
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- ``We need to know why some influenzas are much more virulent
than others because this might help in medical treatment, in the control
of the virus through anti-viral drugs or help us to recognize threatening
new outbreaks,'' Gibbs said.
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