Scientists have detected a close relative of the Aids virus in a wild chimpanzee, raising the prospect that virologists may soon discover the source of HIV.
A few chimps kept in captivity have tested positive for the nearest simian relative of HIV, but finding the virus in wild animals supports the growing belief that Aids jumped from chimpanzee to man in the mid-20th century.
Aids researchers have been keen to discover the origin of HIV because that may lead to a group of wild chimpanzees with a natural immunity to the virus, which could help scientists to develop an effective vaccine for humans.
An international team of researchers found the Aids-like virus in a 23-year-old male chimpanzee living in the Gombe National Park on the eastern shore of Lake Tanganyika in Tanzania, an area studied by the primatologist Jane Goodall.
A genetic analysis of the simian immunodeficiency virus (SIV) isolated from the Gombe chimp revealed that it shared about 70 per cent of its identity with HIV, making it close enough to be related to the human virus but too distant to be its direct ancestor.
Although the researchers, led by Beatrice Hahn and George Shaw of the University of Alabama, have ruled out the virus as the ancestor of HIV, they believe its discovery increases the likelihood that other viruses which are direct progenitors to HIV exist in other parts of the central African jungle.
Dr Hahn said: "The finding of SIV in east African chimps indicates that chimps are naturally infected with SIV, that they are a natural host for the virus.
"We always assumed that the wild-caught captive chimps were infected in the wild but some people said that we couldn't prove it. This is now formal proof that wild chimps are infected."
If such viruses were found and proved to be genetically similar to HIV, infected chimps would be candidates for being the natural "reservoir" of the human virus, which has infected tens of millions of people over the past 20 years.
Dr Shaw said: "To find this virus for the first time in the wild opens a window of opportunity to begin to study the natural transmissibility of these types of viruses in their natural host.
"We also believe it may be important ultimately to understand the implications of the cross-species transmission that brought about the HIV epidemic."
The study, published in the journal Science, arose from a new test for SIV that could detect antibodies to the virus in the droppings and urine of chimps. It enabled the scientists to screen for SIV in 58 Gombe chimps without the need to take blood samples.
A further technical development enabled the scientists to extract the virus from the chimps' droppings, allowing them to sequence its entire genetic code.
Common chimpanzees, Pan troglodytes, are divided into four subspecies that range from the verus and vellerosus groups living in west Africa to the troglodytes and schweinfurthii subspecies living in central and eastern Africa.
The Gombe chimp belongs to the schweinfurthii subspecies but Dr Hahn believes the most likely host of the simian ancestor of HIV will be the troglodytes group living in forests to the west of the Congo river.
"We're already looking for SIV in the chimps of western central Africa and, now we have the new tests that do not require blood sampling, many primatologists are willing to help us," she said.
Dr Shaw said: "When we did our earlier work, we reasoned that since the virus from schweinfurthii was the most divergent from HIV, it must be that the human Aids virus came from the troglodytes subspecies. When we studied the virus from troglodytes animals it did resemble HIV-1."Reuse content