http://www.nytimes.com/2003/06/12/science/12CND-CHIM.html?ei=5070&en=1b465702891afcc5&ex=1056427200&pagewanted=print&position=

Origin of AIDS Traced Back Another Step, Researchers Say

By DONALD G. McNEIL Jr.
 

our years after arguing that humans probably got the AIDS virus from butchering chimpanzees for food, the same researchers say they have traced the origin back one step further: to the monkeys that the chimpanzees ate.

They believe the simian precursor to the AIDS virus was created in chimps that ate two kinds of monkeys with different but related viruses: red-capped mangabeys and spot-nosed guenons.

They made the deduction by sequencing the genes of the simian immunodeficiency viruses in chimpanzees and 30 different monkey species and then compiling "family trees" to see which were most closely related.

The study, done jointly by researchers at the University of Nottingham, the University of Alabama at Birmingham, Duke University, Tulane University and the University of Montpelier in France, appears in the Friday issue of Science magazine.

The conclusion is important, said Dr. Beatrice Hahn, a virologist at the University of Alabama at Birmingham and one of the authors, "because it shows that chimpanzees acquired their virus exactly the same way humans did — by hunting bushmeat."

Unlike the other great apes, chimpanzees are formidable hunters. Troops of males often work together; some chase monkeys through forest canopies while others wait in nearby trees to swat their prey off branches, and yet others follow on the ground, leaping on fallen monkeys and battering them to death.

Hunting males tear their catches limb from limb and eat them on the spot, share the carcasses or trade them to females for sex, so blood-to-blood contact from "open cuts or chomping on bones" is easy to imagine, one researcher said.

The prevailing theory about the origin of H.I.V. is that somewhere in central Africa, probably between 1910 and 1950, a chimpanzee hunter picked up its virus by cutting himself while butchering a carcass. The simian virus then mutated into H.I.V. and spread among humans, mostly through sex.

However, "a lot of people just don't buy this and say it was polio vaccine or dirty needles or tattooing or crazy tribal practices," Dr. Hahn said. "This shows the flaw in their argument."

Experts not connected with the study called its conclusions plausible. Dr. Ronald Desrosiers, a professor of genetics at Harvard Medical School, said it "looks like it makes sense" and demonstrates how easily diseases transfer between species.

Another expert, Edward Hooper, argued in a 1999 book, "The River" (Little, Brown), that a chimpanzee virus passed into humans when an experimental oral polio vaccine was grown in a medium containing chimpanzee cells and used in parts of the former Belgian Congo in 1957 to 1960. He called the study "reasonably plausible, though based on limited data."

"I have no problem with the idea that chimps got it by eating other monkeys," he said.

There is no way to know when the two viruses merged inside one chimpanzee. "It could have been hundreds of years ago or tens of thousands of years ago," Dr. Hahn said.

A related study of the virus in wild chimpanzees by many of the same authors, due to appear in The Journal of Virology next month, shows that it is much less common than in monkeys. None of the chimps studied in Uganda's Kibale National Park were infected. An estimated 13 percent of those in Tanzania's Gombe National Park were.

By contrast, 50 to 90 percent of most adult monkeys are infected with their versions, said Dr. Paul M. Sharp, a professor of genetics at the University of Nottingham.

Neither in chimps nor in monkeys "was there any evidence that the virus causes disease," he said.

Because wild chimpanzees, which grow nearly six feet tall, can easily kill humans, getting blood samples is dangerous, so researchers watch chimps closely enough to be able to test their feces and urine.

It is still unclear exactly how chimpanzees infect one another and why the disease isn't more rampant among them, since they have many sexual partners and fight frequently, often biting, which in rare human cases has passed the virus.

Nursing is surely one route, Dr. Hahn said, because some chimps captured in infancy are infected.

By observing the rate at which the human virus mutates, researchers have estimated that it jumped from chimpanzees sometime between 1910 and 1950. Mr. Hooper disputes that, arguing that they underestimated the effects of DNA recombination, which would have sped the process.

The earliest known H.I.V.-positive person was a man who died in what is now Kinshasa, in the Democratic Republic of Congo, in 1959 and whose blood was frozen.