HIV's Ancestry Traced to Wild Chimps

May 25, 2006 — Twenty-five years after the first AIDS cases emerged, scientists have confirmed that the HIV virus really did originate in wild chimpanzees, in a corner of Cameroon.

Solving the mystery of HIV's ancestry was dirty work. Scientists employed trackers to plunge through dense jungle and collect the fresh feces of wild apes - more than 1,300 samples in all.

Before that, it took seven years of research just to develop the testing methods to genetically trace the primate version of the virus in living wild chimps without hurting the endangered species.

Until now, "no one was able to look. No one had the tools," said Dr. Beatrice Hahn of the University of Alabama at Birmingham. She led the team of international researchers that reported the success in Friday's edition of the journal Science.

"We're 25 years into this pandemic," Hahn said. "We don't have a cure. We don't have a vaccine. But we know where it came from. At least we can make a check mark on one of those."

Scientists long have known that nonhuman primates carry a relative of the AIDS virus, called SIV or simian immunodeficiency virus. But with one exception, it had been found only in captive chimpanzees, particularly a subspecies originating from West Africa.

It was not known how prevalent the virus was in chimps in the wild, or how genetically or geographically diverse it was, complicating efforts to pin down how it jumped from chimp to man.

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

The team found some chimp communities with infection rates as high as 35 percent, while others had no infection at all. But every infected chimp had a genetic pattern that indicated a common ancestor, Hahn said.

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

"The genetic similarity was striking," Hahn said.

HIV's Beginnings

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

Presumably, someone in rural Cameroon was bitten by a chimp or cut while butchering one and became infected with the SIV virus. That person passed it to someone else.

The Sanaga River long has been a commercial waterway for transporting hardwood, ivory and other items to more urban areas.

"How many different transmission events occurred between that initial hunter and this virus making it to Kinshasa, I don't know. It could have been one, it could have been 10, it could have been 100," Hahn said. "Eventually, it ended up in an urban area, and that's where it really got going."

At some point, the virus became more deadly to people than it is to chimps, who seldom are bothered by SIV.

The research seems to settle any question of HIV's origin, said Dr. Anthony Fauci, the National Institutes of Health's AIDS chief.

When tracing a virus' evolution, "it's important to get as close to the source as you can," he said.