One of the many daunting questions facing researchers studying the AIDS virus is where, exactly, it came from. The prevailing theory suggests that it arose from a related diseasesimian immunodeficiency virus (SIV)that jumped from chimpanzees to humans. To date, however, only a few captive animals have tested positive for the virus, puzzling scientists looking for a natural reservoir of SIV in the wild. Now a new report published in Science describes the first case of SIV discovered in a wild chimpanzee, providing further clues as to where the origins of AIDS may lie.

Using a non-invasive test for SIV that detects the virus from antibodies present in urine and feces, Mario Santiago of the University of Alabama at Birmingham and colleagues tested 58 wild chimpanzees in Ivory Coast, Uganda and Tanzania. One animal, a healthy 23-year-old male in Tanzania, tested positive. "To find this virus for the first time in the wild opens a window of opportunity to begin to study the natural transmissibility of these types of viruses in their natural host," study co-author George Shaw explains. Moreover, the discovery of this animal, he says, is "one of the many pieces required to absolutely, definitely prove and ultimately understand how it is the chimp got the virus from something, carried it for a long period of time, and then transmitted it to humans."

The infected chimp resides in Gombe Stream National Park, a refuge made famous by primatologist Jane Goodall. Because Goodall and colleagues have studied him for nearly 20 years, the scientists can analyze all his previous mates and offspring to elucidate how the SIV virus propagates in the wild. "From such studies could come clues to why the disease is benign in chimpanzees but virulent in humans," Shaw says. According to the report, the prevalence of SIV infection among wild chimps is "surprisingly low" and additional field studies are required to better understand its epidemiology.

Analysis of the SIV strain the chimp carries found it different from all previously known strains of both SIV or HIV-1, the virus that causes AIDS. Because of this divergence, the scientists conclude that chimpanzees from the same subspecies (Pan troglodytes schweinfurthii), which reside in East Africa, did not pass the virus to humans. Instead, Shaw says, the finding provides support for the popular theory that chimpanzees from west central Africa, the subspecies Pan troglodytes troglodytes, transmitted it. The next step, he notes, will be to study wild chimps in west Africa for further clues to the circumstances and reasons behind the ascent of AIDS.