Scientists searching for the origin of HIV, the global pandemic infecting more than 40 million people, believe they have finally tracked its original source to two colonies of chimpanzees in a corner of Cameroon.

The finding represents the culmination of a 10-year hunt for the source of the pandemic and provides a crucial link between HIV, which causes AIDS in humans, and the simian immunodeficiency virus (SIV), a strikingly similar virus that infects monkeys and chimpanzees.

Researchers believe that southeast Cameroon is where the virus first jumped from chimpanzees to humans before HIV infection began spreading among people as far back as the 1930s.

At the time, HIV, which destroys the immune system leaving those infected vulnerable to myriad diseases, was difficult to diagnose. Carried by people travelling in the areas, it spread unnoticed to Kinshasa, where the first human epidemic began to grow.

Researchers at Nottingham University in the UK joined scientists from Montpellier, France and Alabama, US to search for signs of the virus in 10 chimpanzee populations throughout Cameroon, where SIV was known to be circulating.

Cameroon is home to two different subspecies of chimpanzee, separated by the Sanaga river which slices the country into north and south. North of the river, tests on chimpanzee faeces found no traces of SIV, but it was present in colonies immediately on the southern side.

As the search for the source of HIV moved further south, researchers located two colonies, living deep in the southeast corner of the country near the Ngoko River bordering the Democratic Republic of Congo.

Tests on chimpanzee faeces here revealed an SIV strikingly similar to the HIV that causes AIDS in humans. In some communities they detected SIV specific antibodies and viral genetic information in as many as 35 per cent of chimps.

"For us, this is really the last piece of the puzzle," said Paul Sharp, a professor of genetics at Nottingham University. "This is where it probably all started. We've got these viruses in southeast Cameroon, which are so close to HIV, and it's difficult to envisage there could be any which could be closer." The research appears in the journal Science over the weekend.

"There was a nagging doubt that chimps had picked it up from monkeys, because monkeys carry the virus and chimps sometimes eat them," he added. "We didn't know for sure if humans picked up the virus in parallel from the same type of monkey."

Researchers have various theories on how the virus jumped to humans; the most widely held view is that hunters became infected when they caught and butchered infected chimps. SIV appears to cause no outward signs of illness, so hunters would not have known if they were catching infected animals.

"When people hunt chimpanzees they tend to butcher them on the spot and then there's a lot of blood flying around," said Sharp. "If the hunter has any open wounds then, that's an opportunity to get infected.

"Chimps and humans are extremely similar genetically, but here we have a virus that is seemingly harmless in chimps, jumps into humans and suddenly causes AIDS."

(China Daily May 30, 2006)