| Vaccine-man 2005-11-11, 6:47 pm |
| Ra=E2=99=A5=C3=AF=E2=81=BFg L=E2=99=82=E2=99=80=E2=99=ABi=CE=B5 wrote:
> Avian flu has been around for a very long time. ... It has crossed over
> many times.
> Aside from us NOW being sensitized to 'bird flu', specifically ...
> ... What has changed?
Most human seasonal influenza A viruses arise by reassortment of bird
and mammalian (usually in pigs) influenza viruses. This almost always
happens in SE Asia because the culture is to have close proximity of
swine, chickens and humans on family farms. The influenza A viruses
have 8 pieces of RNA (termed "segments") that encode 10 distinct
polypeptides. The usual route to adaptation in humans is coinfection
with an avian influenza virus and a mammalian virus that already has
the ability to infect humans. Many mammalian influenza viruses are
already capable of infecting humans, often with very little pathology.
If a pig is coinfected with an avian virus (think "red" segments) and a
mammalian virus ("blue" segments) then you can get hybrid viruses that
possess avian (red) and mammalian (blue) segments. For example, if the
number represent the gene segment and the letter represents red or
blue, you might get a virus that is 1b2b3b4b5b6r7b8r. This virus would
be mammalian except for gene segments 6 and 8, which are avian. The
pathogenicity of these hybrid viruses can be rather serious. But, as a
rule, avian viruses (all red segments) rarely "jump" directly to
humans. When they do, it's usually a dead end for the virus. But in
this case the virus is capable of replicating in humans, and exhibits
human to human transmission (albeit, not very efficiently). This is
likely to become more efficient because it's just a numbers game.
Trillions of viruses, hundreds or thousands of humans - something is
going to give.
As was previously mentioned, this has occurred at least once before -
the 1918-1919 influenza pandemic was an avian influenza A virus. This
was not the case for the other two influenza pandemics of the 20th
century.
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