Oct. 01, 2008 -- NEW YORK (AP) - The AIDS virus has been circulating among peoplefor about 100 years, decades longer than scientists had thought, anew study suggests.
Genetic analysis pushes the estimated origin of HIV back tobetween 1884 and 1924, with a more focused estimate at 1908.
Previously, scientists had estimated the origin at around 1930.AIDS wasn't recognized formally until 1981 when it got theattention of public health officials in the United States.
The new result is "not a monumental shift, but it means thevirus was circulating under our radar even longer than we knew,"says Michael Worobey of the University of Arizona, an author of thenew work.
The results appear in Thursday's issue of the journal Nature.Researchers note that the newly calculated dates fall during therise of cities in Africa, and they suggest urban development mayhave promoted HIV's initial establishment and early spread.
Scientists say HIV descended from a chimpanzee virus that jumpedto humans in Africa, probably when people butchered chimps. Manyindividuals were probably infected that way, but so few otherpeople caught the virus that it failed to get a lasting foothold,researchers say.
But the growth of African cities may have changed that byputting lots of people close together and promoting prostitution,Worobey suggested. "Cities are kind of ideal for a virus likeHIV," providing more chances for infected people to pass the virusto others, he said.
Perhaps a person infected with the AIDS virus in a rural areawent to what is now Kinshasa, Congo, "and now you've got the sparkarriving in the tinderbox," Worobey said.
Key to the new work was the discovery of an HIV sample that hadbeen taken from a woman in Kinshasa in 1960. It was only the secondsuch sample to be found from before 1976; the other was from 1959,also from Kinshasa.
Researchers took advantage of the fact that HIV mutates rapidly.So two strains from a common ancestor quickly become less and lessalike in their genetic material over time. That allows scientiststo "run the clock backward" by calculating how long it would takefor various strains to become as different as they are observed tobe. That would indicate when they both sprang from their mostrecent common ancestor.
The new work used genetic data from the two old HIV samples plusmore than 100 modern samples to create a family tree going back tothese samples' last common ancestor. Researchers got variousanswers under various approaches for when that ancestor virusappeared, but the 1884-to-1924 bracket is probably the mostreliable, Worobey said.
The new work is "clearly an improvement" over the previousestimate of around 1930, said Dr. Anthony Fauci, director of theNational Institute of Allergy and Infectious Diseases in Bethesda,Md. His institute helped pay for the work.
Fauci described the advance as "a fine-tuning."
Experts say it's no surprise that HIV circulated in humans forabout 70 years before being recognized. An infection usually takesyears to produce obvious symptoms, a lag that can mask the role ofthe virus, and it would have infected relatively few Africans earlyin its spread, they said.