Gorillas have emerged as the source of a strain of HIV that infects humans.

Credit: Cecile Neel/IRD

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PARIS: Gorillas are the source of a strain of HIV that infects humans, the origins of which had puzzled scientists, according to a new French study.

Of the three strains of HIV known to infect humans, scientists had established that two - the one causing the global AIDS epidemic and another that has infected a small number of people in Cameroon - came from a chimpanzee virus called SIV.

The source of the third strain, which infects people in western central Africa, has remained a mystery until now.

French scientists made the startling discovery - which has wide implications for the illegal market in bushmeat - as they were looking for traces of the simian immunodeficiency virus (SIV) among chimpanzees.

Chimpanzees are believed to have initially spread SIV to humans, where the agent mutated into a form that adapted to a human host - the human immunodeficiency virus (HIV).

Martin Peeters and Eric Delaporte of the Institute for Development Research and the University of Montpellier, southern France, analysed more than 500 faecal samples deposited by chimps and gorillas in remote forests in Cameroon.

Forty of the 323 chimp faeces had antibodies that reacted against the HIV-1 strain, which was consistent with previous findings. But, quite unexpectedly, so did six of the 213 gorilla samples.

Genetic analysis of the infected gorilla faeces showed that the virus, called SIVgor, was very close to the subtype O of HIV-1 lineage, which is found among humans in central west Africa.

As the faecal samples came from gorillas that live nearly 400 kilometres apart, it is likely that the apes' infections were not coincidental but a sign that SIV is endemic among their species, the authors said.

Gorillas are classified into two species - Gorilla gorilla, the western African species, and Gorilla berengei, the eastern species, which in turn is sub-divided into four sub-species. Whether the eastern species is also infected by the virus remains unclear.

The researchers, who published their findings in the British journal Nature, believe that chimpanzees caused the gorilla infections. Pan troglodytes probably infected gorillas as well as humans with SIV, said the study.

"Gorillas are hunted for food and medicinal use, and it is possible that these practices may have been responsible for the HIV-1 group O [strain]," the authors wrote, adding that SIV-infected gorillas "could pose an ongoing risk to humans."

At the end of last year, 38.6 million people were living with the human immunodeficiency virus, the agency UNAIDS reported in May. The disease was first detected among a small number of homosexuals in California in 1981. It has since killed around 25 million people.

One theory, based on a 'molecular clock' which is derived from the rate at which the virus mutates, suggests the virus leapt from ape to man in the 1920s or 30s. This is thought to have occurred through a bite, or by a hunter who handled infected meat and picked up the virus through a cut.