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source (google.com.pk)The research puts to rest decades of speculation about the birthplace of most types of HIV and their animal "reservoir" in the wild. But there are still many questions that haven't yet been definitively settled -- questions such as:
When did HIV-1 first start spreading in humans?
HIV-1 is surprisingly old, and it probably "debuted" in humans at least three separate times -- one for each subtype, M, N, and O. Scientists' best guess is that the precursor of the most common "M" virus jumped from the Cameroon chimps to humans sometime before 1931. Using samples of HIV-infected tissue harvested over the last three decades, virologist Dr. Bette Korber of Los Alamos National Laboratory has calculated that an ancestral form of HIV started spreading, slowly at first, in humans about 75 years ago. The actual jump from chimps to humans probably occurred shortly before that, says Hahn: "There's no reason to believe this was just lingering around in people."
Korber's model estimates a virus' age based on how extensively different strains have mutated. HIV is an unusual virus; it changes its DNA by both mutation and, more often, recombination, when two strains merge within the body and exchange genetic material. Some scientists refer to this process as "viral sex," and it may partially explain why it is so hard for scientists to make a treatment or vaccine. Korber's model does not take recombination into account, but given a virus' DNA configuration, it can roughly predict the age of that strain. Korber has tested the oldest known HIV sample, taken in 1959, and derived the 1931 estimate.
Why do scientists look at recent samples of HIV to determine the virus' overall age? Wouldn't it be better to use older samples that haven't had as much time to mutate?
It would, but scientists don't have that luxury. Other than the 1959 sample, there are very few preserved specimens of HIV-infected tissue that predate the early '80s, when the virus was first recognized by health authorities. Researchers still hope there are forgotten samples in African freezers. "There has to be some serum or plasma somewhere, and given modern technology we could fish out the virus," says Dr. David Ho, director of the Aaron Diamond AIDS Research Center and one of the world's leading authorities on HIV.
But even if those samples are found someday, they won't necessarily yield definite answers about the virus' age, says Korber: "Often, you can't get anything out of samples like that." Most African samples are made of blood serum, and serum samples contain viral RNA, which degrades much faster than the DNA found in tissue samples. In fact, says Ho, the 1959 sample, which was sequenced by his laboratory, was kept in a freezer but still didn't survive the ravages of time. "It was completely dried up," he says. "We were only able to get small pieces [of genetic material], and we had to stitch them together."
So scientists have estimated when and where the most deadly type of HIV started infecting humans -- but how did it do that?
Most AIDS researchers believe that the "bushmeat trade" allowed the HIV-1 virus, and separately HIV-2, to enter the human bloodstream several times. Hunters who kill and butcher chimps and monkeys are regularly exposed to animal blood teeming with SIVs. If the hunters have cuts, bites, or scratches -- and given the nature of their work they almost always do -- they can catch the viruses from their prey. Hunters going after chimps in Cameroon could have caught the first strains of HIV-1. Sooty mangabeys, hunted and kept as pets in West Africa, could have transmitted HIV-2 to humans.
Africans have hunted chimps and monkeys and kept them as pets for centuries; they've presumably been exposed to SIVs during most of that time. But the conditions needed for HIV to spread widely weren't in place until after the continent was colonized and urbanized. The first victims would have found it easier to unwittingly spread the virus to sexual partners far and wide as roads and vehicles started connecting previously isolated villages and cities. Hospitals may have played a role, too. Strapped for cash, some of them probably re-used dirty needles, unknowingly infecting patients in the process.
Are there other theories about how the virus could have gotten into humans?
There are several competing theories, ranging from implausible conspiracies to arguments grounded in extensive research. The best-known of the latter, the "OPV/AIDS" theory, was exhaustively detailed in the 1999 book The River, by author Edward Hooper. As many as a million Africans were given oral polio vaccines (OPV) between 1957 and 1960. Hooper says witnesses have told him that a few batches of those vaccines were "grown" in chimp cells at a lab in Kisangani, a city in the Democratic Republic of the Congo -- and that the chimp cells, and thus the vaccines, could have contained SIVs that jumped into humans. "There are highly significant correlations between the places where this vaccine was administered and the places where … AIDS first appeared on the planet four to 20 years later," Hooper says.
Scientists' best guess is that the precursor of the most common 'M' virus jumped from the Cameroon chimps to humans sometime before 1931.
The majority of HIV researchers subscribe to the bushmeat theory and raise several arguments against the OPV theory. Hahn's recent research confirming that HIV-1 M and N arose from Pan troglodytes troglodytes chimps in Cameroon presents one problem: The Kisangani lab is in the Democratic Republic of the Congo, and it's home to a different subspecies of chimp than the one that was the source of HIV-1 M and N. However, it is possible that the chimps used in the Kisangani experiments were not from the area. In the spring of 2006, Hooper found a paper indicating that at least one of eight chimps at the Kisangani lab was a Pan troglodytes troglodytes.
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