ohh geez... lol people...
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What type of virus is HIV?
HIV is a lentivirus, and like all viruses of this type, it attacks the
immune system. Lentiviruses are in turn part of a larger group of
viruses known as retroviruses. The name 'lentivirus' literally means
'slow virus' because they take such a long time to produce any adverse
effects in the body. They have been found in a number of different
animals, including cats, sheep, horses and cattle. However, the most
interesting lentivirus in terms of the investigation into the origins
of HIV is the Simian Immunodeficiency Virus (SIV) that affects
monkeys.
So did HIV come from an SIV?
It is now thought that HIV came from a similar virus found in chimpanzees.
It is now generally accepted that HIV is a descendant of a Simian
Immunodeficiency Virus because certain strains of SIVs bear a very
close resemblance to HIV-1 and HIV-2, the two types of HIV.
HIV-2 for example corresponds to SIVsm, a strain of the Simian
Immunodeficiency Virus found in the sooty mangabey (also known as the
green monkey), which is indigenous to western Africa.
The more virulent strain of HIV, namely HIV-1, was until recently more
difficult to place. Until 1999, the closest counterpart that had been
identified was SIVcpz, the SIV found in chimpanzees. However, this
virus still had certain significant differences from HIV.
What happened in 1999?
In February 1999 a group of researchers from the University of
Alabama1 announced that they had found a type of SIVcpz that was
almost identical to HIV-1. This particular strain was identified in a
frozen sample taken from a sub-group of chimpanzees known as Pan
troglodytes troglodytes, which were once common in west-central
Africa.
The researchers (lead by Paul Sharp of Nottingham University and
Beatrice Hahn of the University of Alabama) made the discovery during
the course of a 10-year long study into the origins of the virus. They
claimed that this sample proved that chimpanzees were the source of
HIV-1, and that the virus had at some point crossed species from
chimps to humans.
Their final findings were published two years later in Nature
magazine2. In this article, they concluded that wild chimps had been
infected simultaneously with two different simian immunodeficiency
viruses which had "viral sex" to form a third virus that could be
passed on to other chimps and, more significantly, was capable of
infecting humans and causing AIDS.
These two different viruses were traced back to a SIV that infected
red-capped mangabeys and one found in greater spot-nosed monkeys. They
believe that the hybridisation took place inside chimps that had
become infected with both strains of SIV after they hunted and killed
the two smaller species of monkey.
How could HIV have crossed species?
It has been known for a long time that certain viruses can pass
between species. Indeed, the very fact that chimpanzees obtained SIV
from two other species of ape shows just how easily this crossover can
occur. As animals ourselves, we are just as susceptible. When a viral
transfer between animals and humans takes place, it is known as
zoonosis.
Below are some of the most common theories about how this 'zoonosis'
took place, and how SIV became HIV in humans:
The 'Hunter' Theory
The most commonly accepted theory is that of the 'hunter'. In this
scenario, SIVcpz was transferred to humans as a result of chimps being
killed and eaten or their blood getting into cuts or wounds on the
hunter. Normally the hunter's body would have fought off SIV, but on a
few occasions it adapted itself within its new human host and become
HIV-1. The fact that there were several different early strains of
HIV, each with a slightly different genetic make-up (the most common
of which was HIV-1 group M), would support this theory: every time it
passed from a chimpanzee to a man, it would have developed in a
slightly different way within his body, and thus produced a slightly
different strain
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go to http://www.avert.org/origins.htm for the rest
Elí |
