Chimpanzees do not show symptoms of the virus

CHICAGO (CNN) -- A chimpanzee named Marilyn that died years ago has helped scientists solve the lingering mystery of the origin of AIDS.

Researchers at the University of Alabama at Birmingham said Sunday they have conclusive evidence that the HIV virus has spread on at least three separate occasions from chimpanzees to humans in Africa. One of the interspecies transmissions launched the epidemic that now infects about 35 million people worldwide, they said.

Chimps, which probably have carried the virus for hundreds of thousands of years, apparently do not get sick from it. Understanding why could help in the search for a cure, AIDS experts say.

Although scientists have long suspected chimps as the source of AIDS, the absence of clear evidence prevented them from making the conclusion, said Dr. Beatrice Hahn of the University of Alabama. Her report is published in this week's issue of the journal Nature.

Whatever its origins, HIV, the human immunodeficiency virus that causes AIDS, is a recent affliction for humans. At last year's Conference on Retroviruses and Opportunistic Infections, Dr. David Ho and others from the Aaron Diamond AIDS Research Center at Rockefeller University presented evidence that the virus probably first infected people around 50 years ago.

People in some African nations contract the virus by eating 'bushmeat'

At the opening of this year's meeting, Hahn made the case that this event almost certainly occurred in west equatorial Africa when someone caught the virus from a chimp, perhaps after killing the ape for food.

Hahn said her team confirmed the connection by analyzing frozen tissue saved from Marilyn, who died in 1984 at age 26 from childbirth complications at a U.S. Air Force primate center.

The chimp version of the AIDS virus, thought to be the grandfather of HIV, is called SIVcpz. It is extremely rare among chimps in U.S. lab colonies, apparently because they are removed from the wild at a young age and never exposed to the virus sexually.

"This is an important finding with significant potential," said Dr. Anthony Fauci of the U.S. National Institute of Allergies and Infectious Diseases (NIAID), which helped fund the study.

"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 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," he said.

Researchers at the University of Alabama at Birmingham presented their findings Sunday

Chance discovery helped scientists

Until recently, SIVcpz had been isolated only three times. The fourth turned up when a colleague cleaning out a lab freezer ran across Marilyn's specimens and sent them to Hahn. Her team was able to perform various kinds of genetic analysis that were unavailable when the chimp died.

Then the Alabama team used molecular analysis techniques to study all four examples of the virus. They found that three were nearly identical genetically to the human AIDS virus.

All three samples came from Pan troglodytes troglodytes. The chimp subspecies lives in Cameroon, Equatorial Guinea, Congo and Central African Republic, the region where AIDS is thought to have started.

The fourth sample, much less like HIV, came from a separate chimp subspecies native to East Africa.

Among humans, there are three major groups of HIV, one of which has spread around the world. The others exist in west- Central Africa. The researchers believe that each group arose from a separate chimp-to-human transmission of SIVcpz.

"We conclude that this subspecies is the natural host and reservoir for HIV-1," the AIDS virus, said Hahn.

She said a French team, headed by Dr. Phillippe Mauclere of the Pasteur Institute, recently found three more chimps infected with SIVcpz at a sanctuary in Cameroon. One sample has been genetically analyzed and it, too, closely resembles HIV.

Many viruses come from animals. Flu, for example, comes from ducks and pigs. Often the virus does not make its natural host sick. It must undergo genetic changes to infect another species such as humans, changes that can cause illness.

There is a second strain of HIV that infects people, known as HIV-2. It is believed to have started out in a monkey known as a sooty mangabey.

The Associated Press and Reuters contributed to this report.