SIV has finally been tracked down in wild west African chimps.credit: Ian C. Gilby

Scientists have spotted the signs of an HIV-like virus in chimpanzees in southern Cameroon, confirming the long-held suspicion that these animals are a natural reservoir for the virus in the wild.

The discovery bolsters the theory that the first people to contract HIV did so through contact with infected blood from wild chimps in the jungle, before eventually spreading the virus to nearby Kinshasa in the Democratic Republic of Congo and onwards from there.

Researchers led by Beatrice Hahn of the University of Alabama at Birmingham travelled to Cameroon to collect droppings from the chimpanzee subspecies Pan troglodytes troglodytes.

The team knew that a few captive chimps of this subspecies have been found carrying a strain of simian immunodeficiency virus (SIV) almost identical to the HIV-1 strain, but it was not clear how these animals came to have the virus. Better evidence that these creatures were responsible for the human AIDS pandemic would come from tracking down a reservoir of the human-like SIV in wild chimps in west Africa. The team's hopes of finding such a reservoir were high: a few years ago they found another version of SIV, one quite different from the human virus, in a different subspecies of wild chimp living in east Africa.

As hoped, analysis of the Cameroon samples revealed the presence of antibodies against human-like SIV and traces of the virus' genetic sequence. On the basis of their samples, the researchers calculate that some 30-35% of chimpanzees are carriers. The team reports the findings in Science¹.

The virus does not seem to cause any AIDS-like symptoms in the chimpanzees, says Hahn, as captive infected chimps do not seem to develop immune disease. "Lots of people are trying to find out why," says Paul Sharp, a viral geneticist at the University of Nottingham, UK, who also worked on the study.

Wild hunt

The virus probably got into humans as a result of bushmeat hunting, Hahn suggests. "The most likely route, based on the biology of these viruses, is human exposure to infectious chimp blood or body fluids during hunting and butchering," she says.

The genetics of the wild chimp SIV are very close to the human virus. This suggests that humans contracted the virus directly from chimps, rather than both humans and chimps contracting it from monkeys, as some experts had previously suggested.

It is unclear exactly how the virus arose in the chimps themselves, but it is probably derived from two viruses carried by monkey species on which the chimps prey, suggests Sharp.

River commute

Analysis of the path of the human pandemic has pinpointed Kinshasa as the epicentre of the outbreak. The first HIV-positive human blood was obtained here in 1959. The virus was almost certainly carried here by infected humans, says Sharp: "Chimps don't walk the streets of Kinshasa."

The theory is supported by the fact that rivers, the primary transport routes through the dense West African jungle, provide an easy means of travelling from southern Cameroon to Kinshasa. Once there, the urban environment would have been far more conducive to the transmission of the virus between people. But it will be a difficult story to verify with certainty. "We're talking about something thought to have happened more than 75 years ago," he says.

It seems to be largely a matter of chance that this strain became the one to terrorize the globe, says Sharp. HIV-2, thought to have come from sooty mangabey monkeys, also causes AIDS in humans, but this virus is confined almost entirely to West Africa.

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University of Alabama at Birmingham

• References

  1. Keele B. F., et al. Science, doi:10.1126/science.1126531 (2006).