January 31, 1999
Beatrice H. Hahn, M.D., of the University of Alabama at Birmingham, a grantee of the National Institute of Allergy and Infectious Diseases (NIAID), led the international team of investigators. They describe their findings in the February 4 issue of Nature. The journal moved the normal press embargo ahead to coincide with Dr. Hahn's presentation of the study details on the opening night of the 6th Conference on Retroviruses and Opportunistic Infections in Chicago.
"This is an important finding with significant potential," notes Anthony S. Fauci, M.D., NIAID director. "We now have chimpanzee isolates of simian immunodeficiency virus [SIVcpz] that have been shown by careful molecular analysis to be closely related to HIV-1. Furthermore, this virus infects a primate species that is 98 percent related to humans. 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."
Until now, HIV-1's origin had been unclear. Although most scientists suspected that the virus descended from a primate species, only three chimpanzees infected with viruses related to HIV-1 had been documented, and one of these viruses correlated only weakly with HIV-1.
When Dr. Hahn and her collaborators recently identified a fourth chimpanzee infected with SIVcpz, they decided to use this opportunity to carefully examine all four viruses and the animals from which they were derived. With sophisticated genetic techniques, they analyzed the four SIVcpz isolates and compared them with various HIV-1 viruses taken from humans. They also determined the subspecies identity of the chimpanzees: three belonged to a subspecies native to west equatorial Africa, Pan troglodytes troglodytes. The fourth, the chimpanzee infected with a virus most unlike HIV-1, belonged to an east African subspecies known as Pan troglodytes schweinfurthii.
As it turns out, the three isolates from the Pan troglodytes troglodytes chimpanzees strongly resemble the different subgroups of HIV-1, namely groups M (responsible for the pandemic), N and O (both found only in west equatorial Africa). Their investigation also revealed that some of the viruses resulted from genetic recombination in the chimpanzees before they infected humans.
Their other significant find, Dr. Fauci notes, is that the natural habitat of these chimpanzees directly coincides with the pattern of the HIV-1 epidemic in this area of Africa. Putting all these pieces of the puzzle together, Dr. Hahn and her colleagues conclude that Pan troglodytes troglodytes is the natural reservoir of HIV-1 and has been the source of at least three independent occurrences of cross-species virus transmission events from chimpanzees to humans.
The authors believe that HIV-1 was introduced into the human population when hunters became exposed to infected blood. Furthermore, they speculate that humans might still be at risk for cross-species transmission because the bushmeat trade -- the hunting and killing of chimpanzees and other endangered animals for human consumption -- is still common practice in west equatorial Africa.
This new report suggests that preserving the wild chimpanzee populations will be crucial for further carefully designed studies to better understand how cross-species virus transmission occurs and how infected chimpanzees resist disease, studies that in turn may lead to new strategies for designing HIV drugs and vaccines.
RA Weis and RW Wrangham. From Pan to pandemic. Nature 397, 385-6 (1999).
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