The discovery by scientists of the apparent origin of the AIDS virus in a sub-species of chimpanzees in equatorial West Africa is a significant development in the fight against the disease, according to an Irish AIDS expert.
Full details of the 20-year study, which for the first time identifies a particular species as the likely source for an AIDS pandemic affecting 30 million people, will be published in the scientific journal, Nature, tomorrow.
Prof William Hall, head of the Virus Reference Laboratory in UCD, said yesterday that research could now focus on exactly why the virus did not cause disease in chimpanzees, which have been harbouring an ancestral version of the virus for several hundred thousand years, but did in humans.
For more than 15 years evidence has pointed to Africa as the birthplace of the virus. During that time virologists have believed human beings acquired it from primates. The new research narrows the virus's place of origin to the region near the countries of Gabon, Equatorial Guinea and Cameroon, on the Atlantic coast.
The research, which focused on a now-dead chimpanzee called Marilyn, sheds no light on the mystery of when or how the virus leapt the "species barrier", although genetic analysis suggests such an event occurred at least three times. It may, however, shed light on more practical and clinically relevant questions.
That is because of evidence suggesting the sub-species of chimpanzees, Pan troglodytes , does not become ill from the ancestral virus. If further study proves this, it might help explain why the virus is so deadly to its human cousins, who are 98 per cent identical to it.
"We want to focus on the naturally infected animals . . . and study them side by side with humans, looking at a number of immunological parameters," said Prof Beatrice H. Hahn of the University of Alabama, who led an international team of researchers. Increasing urbanisation, the breakdown of traditional lifestyles, population movements and sexual promiscuity are known to increase rates of sexually transmitted diseases, and thus may have triggered the AIDS pandemic, said Prof Hahn.
Prof Hall said it had been known for some time that the virus had originated in Africa from a number of monkey species. Chimpanzees, he said, carried viruses very similar to HIV, called SIV, simian immuno-deficiency virus. "If you analyse the genetic material of both, you can begin to tell where the virus may have come from."
In tracing the origin of the AIDS virus, Prof Hahn's team compared the genetic sequence of human immuno-deficiency virus 1 (HIV-1), which causes AIDS in humans, and simian immuno-deficiency virus (SIV), which often but not always causes a similar disease in primates. The SIV strain the researchers studied was SIVcpz, the last three letters denoting chimpanzee. Other strains infect other primates, such as African green monkeys.
HIV 1, explained Prof Hall, was the major AIDS virus in the US and Europe, and it has been known for a long time that, while not identical, it was closely related to SIV. Prof Hahn, he said, has spent many years researching this area. "She has found the SIV closely related or identical. It was a question of finding the right chimpanzee."
HIV 2, the type of virus restricted to western Africa, which is less aggressive, has already been traced to another breed of monkey, the Sooty Mangabay. "In the chimpanzees and Sooty Mangabays the virus does not cause AIDS. In fact there is no disease. It is carried from generation to generation, but once it gets outside the usual host it becomes pathogenic. She has spent years comparing genetic sequences of SIVs and HIVs."
The possibilities for transmission include a bite from a monkey, the bleeding when a monkey is hunted and killed, or eating the meat. The possible future benefits of the discovery, said Prof Hall, include the study of how the virus affected man and chimpanzees in a different way, including the immune response and the genetic background.
But the significance of the discovery is that it eliminates so many doubts about what caused the AIDS pandemic by providing convincing evidence of its source, according to Prof Paul Sharp of Nottingham University, who participated in the research.
The findings increase the likelihood that a vaccine will be developed, but this may be hampered by the chimpanzee becoming extinct before a cure can be found. Timber trade is destroying its jungle habitat, while commercial killing of chimps, monkeys and gorillas for food is accelerating their decline.