AIDS Virus Jumped From Chimps.

Genetic studies confirm that the roots of the AIDS virus, which attacks people, can be traced to viruses infecting chimpanzees living in west-central Africa.

Researchers have long suspected that chimpanzees were the original reservoir for infection by HIV-1, the strain of HIV responsible for the global, human pandemic first recognized in the early 1980s. However, the three chimpanzee virus strains examined in past genetic studies varied so much that researchers could not rule out the case that chimpanzees and humans were separately infected through contact with a third, unidentified species.

Now, a newly discovered simian immunodeficiency virus (SIV) strain shows strong similarities to two of the previously analyzed simian viruses, which appear in the same chimpanzee subspecies, Pan troglodytes troglodytes. HIV-1 is also closely related to this cluster of simian viruses, they report.

The other, more divergent chimp virus came from another subspecies, Pan troglodytes schweinfurthii. Beatrice H. Hahn of the University of Alabama at Birmingham presented these findings this week in Chicago at the sixth annual Conference on Retroviruses and Opportunistic Infections and in the Feb. 4 NATURE.

"The puzzle of the origin of HIV-1 has been solved," says Hahn.

As more evidence for chimp-to-human viral transmission, she adds, the troglodytes subspecies lives in the area where researchers already have established that the HIV epidemic began and where the earliest known infection has been identified (SN: 2/7/98, p. 85).

"We believe that HIV was introduced into the human population through exposure to blood during hunting ... of these animals," Hahn says. When Hahn and her colleagues looked at one type of HIV-1 found so far only in Cameroon, they discovered that some of its genes are more closely related than others to simian viruses.

If the virus had jumped from a chimp to a human just once, all genes within that HIV strain should be similarly related to SIV genes. The researchers estimate that, instead, the virus has moved from chimps to humans at least three times, and they speculate that it may do so again.

"This [research] is probably the missing link demonstrating that this species of chimpanzee is the source of human infection," says Douglas D. Richman of the University of California, San Diego.

The virus has probably infected chimps for at least 100,000 years because different subspecies are infected with slightly different strains of SIV, Hahn says. Unlike humans with HIV-1, chimps infected with SIV show no symptoms of infection.

"This virus infects a primate species that is 98 percent related to humans," notes Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases in Bethesda, Md. "This may allow us--if done carefully and in collaboration with primatologists working to protect this endangered species--to study infected chimpanzees in the wild to find out why these animals don't get sick, information that may help us better protect humans from developing AIDS."

"We need to make a concerted effort to preserve this species and the viruses that the chimpanzees still harbor to get a better picture of what is going on," says Hahn.