By studying chimpanzee droppings in remote African jungles, scientists reported yesterday, they have found direct evidence of a missing link between a chimpanzee virus and the one that causes human AIDS.
Chimpanzee Reservoirs of Pandemic and Nonpandemic HIV-1 (Sciencemag.org)
Scientists have long suspected that chimpanzees are the source of the human AIDS pandemic because at least one subspecies carries a simian immune deficiency virus closely related to H.I.V., the virus that causes AIDS.
But because the simian virus, known as S.I.V.cpz, was identified in chimpanzees in captivity, researchers could not be sure that the same simian virus existed among these apes in the wild.
It does, the team of American, European and Cameroonian scientists reported in the journal Science. They found it by testing hundreds of chimpanzee droppings collected in Cameroon.
The genetic and immunologic tests were developed in stages over the past seven years to help trace the evolution of H.I.V. and solve the mysterious origins of AIDS, said Dr. Beatrice H. Hahn, a virologist at the University of Alabama in Birmingham. Dr. Hahn led the international team that conducted the study, which combined genetics and epidemiology.
The new findings, she said in a telephone interview, do not explain the entire chain of events that led from the first human H.I.V. infection to the infection of 65 million people around the world.
But, Dr. Hahn reported, her team's findings show "for the first time a clear picture of the origin of H.I.V.-1 and the seeds of the AIDS pandemic." H.I.V.-1 is the virus that causes the vast majority of AIDS cases in the world. The first cases of AIDS were detected in the United States in 1981.
Studies estimate that the human AIDS virus jumped species 50 to 75 years ago. But no one knows who the first infected person was or how that person acquired H.I.V.
The earliest H.I.V. infection was documented in 1959 in an unidentified man in Kinshasa, in what was then the Belgian Congo and is now Congo. The man participated anonymously in a genetic study conducted by Dr. Arno Motulsky of the University of Washington in Seattle.
Dr. Hahn said her team theorized that H.I.V. was first transmitted locally somewhere in west-central Africa. Because the subspecies of chimpanzees, Pan troglodytes troglodytes, in which the simian virus had been found in captivity, lives in the wild in Cameroon, Gabon and the Congo Republic, the first infection could have been in any of those areas.
If the transmission occurred in southern Cameroon, where the new chimpanzee studies were conducted, an infected person or persons could have carried the virus traveling by river to Kinshasa. From there, it spread farther and eventually around the world, according to Dr. Hahn's hypothesis.
Wild chimps are reclusive, live in remote jungle areas and form geographically distinct communities. "You can hear them, but not see them," Dr. Hahn said.
So, with the permission of the government of Cameroon, Dr. Hahn's team asked hunters, members of expeditions and workers sent by local health officials to collect fecal samples from the forest floor, particularly near the base of fruit trees.
Dr. Hahn sent hundreds of test tubes containing a preservative. As the trekkers collected samples they put them immediately into the test tubes and then into their backpacks. They collected 599 fecal samples in 10 forest sites in the southern part of Cameroon.
The preservative allowed the specimens to be kept for weeks, until they ended up in Dr. Hahn's laboratory. There she tested for antibodies that detect S.I.V.cpz. She found evidence of infection with that virus in 5 of the 10 field sites.
Different DNA tests identified each individual chimp and its sex. Other tests found evidence of the simian virus.
The team found that there was widespread but uneven infection with that virus. The prevalence was up to 35 percent in three communities; 4 and 5 percent in two communities; and none in five communities.
The communities with a high prevalence of infected chimpanzees were located south of the Sangha River, which flows into the Congo River and on to Kinshasa. That led Dr. Hahn's team to the theory that some infected person carried H.I.V. from a remote area to Kinshasa, where it was then passed on.
It is not known whether chimpanzees infected with S.I.V.cpz become ill, Dr. Hahn said. She said more collections were needed in other vast areas of Africa to provide a clearer picture of the evolution of AIDS and to determine if there were other viruses that could cause epidemics like AIDS.