A team of international researchers has found new evidence that an endangered subspecies of chimpanzee is the source of the virus that causes acquired immune deficiency syndrome (AIDS) in humans. Experts said the finding could lead to new treatments for AIDS and contribute to the development of a vaccine against the disease. The research team said the chimp -- a subspecies known as Pan troglodytes native to west central Africa -- carries a simian immunodeficiency virus (SIV) that is closely related to three strains of human immunodeficiency virus (HIV), the virus that causes AIDS. One of these strains, HIV-1, has caused the vast majority of the estimated 30 million HIV infections around the world. The researchers are uncertain when the chimp virus, called SIVcpz (for simian immunodeficiency virus chimpanzee), first infected humans, although the oldest documented case of HIV has been linked to a Bantu man who died in Central Africa in 1959. But they said the virus, which does not appear to harm the chimps, was most likely transmitted to humans when hunters were exposed to chimp blood while killing and butchering the animals for food. Once transmitted to humans, the researchers believe the virus mutated into HIV-1. Team leader Beatrice Hahn, an AIDS researcher at the University of Alabama in Birmingham, said the chimps have probably carried the virus for hundreds of thousands of years. Since humans have probably hunted the animals since prehistoric times, Hahn said the virus may have jumped to humans on many occasions, but was not transmitted widely among humans until the 20th century. Increased hunting of the chimpanzees, along with human migration to African cities and changing sexual mores, could help explain the recent epidemic, Hahn said. Scientists had long suspected that a nonhuman primate was the source of HIV-1. Earlier studies suggested that the sooty mangabey monkey, a native of West Africa, was the likely source of HIV- 2 -- a rarer form. of the AIDS virus that is transmitted less easily than HIV-1. However, only a few samples of SIV strains exist, making it difficult for researchers to confidently connect the strains to HIV-1. As part of their effort to discover the source of HIV-1, the research team studied the four known samples of SIVcpz. They learned that three of the four samples came from Chimps belonging to the subspecies P.t. troglodytes. The remaining sample came from another subspecies, Pan troglodytes schweinfurthii, which inhabits East Africa. The team then compared the SIVcpz strains to each other and found that all three of the viruses from P.t. troglodytes were closely related, while the virus from P.t. sehweinfurthii was genetically different. Next they compared the SIVcpz strains to the main subgroups of Hiv-1, known as M, N, and O. Their comparisons showed that the P.t. troglodytes viruses strongly resembled all three HIV-1 subgroups. Additional evidence that HIV-1 could be linked to P. t. troglodytes came when the researchers examined the chimps' natural habitat. The researchers quickly discovered that the chimps live primarily in the West African nations of Cameroon, the Central African Republic, Gabon, Equatorial Guinea, and the Republic of the Congo, the geographic region where HIV-1 was first identified. Upon closer study, the researchers learned that the chimps were being killed in growing numbers for the so-called bushmeat trade, a trend assisted by the construction of new logging roads in once remote forests. The researchers said that continued hunting of the animals meant that many people are still likely to be exposed to SIVcpz, increasing the risk of additional cross-species transmissions. Many AIDS researchers welcomed the team's finding, but said the new work had not proved the connection definitively. Most of the doubts centered on the difficulty