A virus called SIVcpz (Simian Immunodeficiency Virus from chimps) was believed to have been the precursor to HIV, but this virus had only been found in a few captive animals. The current study has been able to trace a natural reservoir of the virus.

    To solve the mystery of HIV's ancestry, scientists employed trackers to plunge through dense jungle and collect the fresh feces of wild apes - more than 1,300 samples in all.

    Scientists long have known that nonhuman primates carry their own version of the AIDS virus, called SIV or simian immunodeficiency virus. But, it was not known how prevalent the virus was in chimps in the wild, or how genetically or geographically diverse it was,

    The team tested chimp feces for SIV antibodies, finding them in a subspecies called Pan troglodytes troglodytes in southern Cameroon.

    By genetically analyzing the feces, researchers could trace individual infected chimps. The team found some chimp communities with infection rates as high as 35 percent, while others had no infection at all.

    There are three types of HIV-1, the strain of the human virus responsible for most of the worldwide epidemic. Genetic analysis let the team identify chimp communities near Cameroon's Sanaga River whose viral strains are most closely related to the most common of those HIV-1 subtypes.

    The Sanaga River is an important commercial gateway in this area and it is widely accepted that someone who was infected with HIV made his way to Kinshasa. The study suggests the virus passed from chimpanzees to people more than once.

    "We don't really know how these transmissions occurred," the researchers said.

    "We know that you don't get it from petting a chimp, or from a toilet seat, just like you can't get HIV from a toilet seat. It requires exposure to infected blood and infected body fluids. So if you get bitten by an angry chimp while you are hunting it, that could do it."

    The first human known to be infected with HIV was a man from Kinshasa in the nearby country of Congo who had his blood stored in 1959 as part of a medical study, decades before scientists knew the AIDS virus existed.

    In people, HIV leads to AIDS but chimps have a version called simian immune deficiency virus that causes them no harm. Humans are the only animals naturally susceptible to HIV.

    AIDS was only identified 25 years ago. The virus now infects 40 million people around the world and has killed 25 million.

    Spread via blood, sexual contact and from mother to child during birth or breast-feeding, HIV has no cure and there is no vaccine yet, although drug cocktails can help control it. Enditem

(Agencies)