June 13, 2003
Scientists made the deduction by sequencing the genes of the simian immunodeficiency viruses in chimpanzees and 30 monkey species, and then compiling "family trees" to see which were most closely related. They believe two monkey viruses were involved because the virus from spot-nosed guenons was closest in the part of the genome that contains the code for the virus's protein envelope, while the virus from the mangabey was closest in a different segment.
There is no way to know when the two viruses merged inside one chimpanzee. Chimp virus has been found in two subspecies from central Africa, but not yet in the westernmost subspecies, nor in a related species found south of the Congo River. The virus's failure to spread to all chimpanzees before they diversified into subspecies suggests that it is relatively new, researchers said. Subspecies have been separated for eons by large rivers like the Congo and the Ubangi, since chimps cannot cross water.
The new conclusion is important, said Dr. Beatrice Hahn, a University of Alabama-Birmingham virologist and one of the authors, "because it shows that chimpanzees acquired their virus exactly the same way humans did, by hunting bushmeat." Neither chimps nor monkeys get sick from it.
It is still unclear exactly how chimpanzees infect each other and why the disease is not more rampant among them, since they have many sexual partners and fight frequently, often biting each other, which in rare human cases has passed the virus. Nursing is surely one route, Hahn said, because some chimps captured in infancy are infected. The full study, "Hybrid Origin of SIV in Chimpanzees," appears in the Friday issue of Science (2003;300(5626):1713).
New York Times
06.13.03; Donald G. McNeil Jr.