A virus that is killing chimpanzees in the wild may be an intermediate stage in the evolution of the deadly human strain

Scientists believe they have found a “missing link” in the evolution of the virus that causes Aids. It bridges the gap between an infection that does no harm to most non-human primates and one that kills millions of people.

The suspected link is a virus that is killing chimpanzees in the wild at a disturbingly high rate, according to a study in tomorrow’s issue of the journal Nature. Chimpanzees are the first primate shown to get sick in the wild in significant numbers from a virus related to HIV. They are also humans’ closest relative among primates.

The discovery of the disease killing chimps may help doctors to come up with better treatments or a workable vaccine for humans, experts said.

The primate version of the virus that causes AIDS is called simian immunodeficiency virus (SIV), but most apes and monkeys that are infected with it show no symptoms of illness. “If we could figure out why the monkeys don’t get sick, perhaps we could apply that to people,” said study lead author Beatrice Hahn, a professor of medicine at the University of Alabama in Birmingham.

The nine-year study of chimps in their natural habitat at Gombe National Park in Tanzania found chimps infected with SIV had a death rate 10 to 16 times as high as uninfected chimps. And postmortems of infected chimps showed unusually low T cell counts that are just like the levels found in humans with AIDS, said Hahn.

And when scientists looked at the strain infecting the chimps, they found that it was a close relative of the virus that first infected humans.

“From an evolutionary and epidemiological point of view, these data can be regarded as a ‘missing link’ in the history of the HIV pandemic,” said Aids researcher Dr Daniel Douek of the National Institute of Allergy and Infectious Diseases, who was not involved in the Nature study.

Monkeys and apes other than chimps seem to have an evolutionary adaptation, probably at the level of their cell receptors, that allows them to survive the virus, Douek said. The infection in chimps is more recent so they haven’t adapted.

Hahn said chimps and people probably caught the virus the same way, by eating infected monkeys. And they both spread it the same way, through sexual activity.

Chimps are already endangered in the wild. Many factors are causing Africa’s chimp population to dwindle, said study co-author Michael Wilson, a professor of anthropology at the University of Minnesota and former director of field research at the Jane Goodall Institute in Tanzania.

Hunting, loss of habitat and disease are decreasing chimp numbers and it’s hard to figure out how much of a factor SIV is, he said.

“It is a concern,” Wilson said. “The last thing these chimps need is another source of mortality.”

Wilson, who spent years observing chimps in Tanzania as part of the study, said that when researchers realised the virus was fatal and they knew which chimps were infected, it became hard to watch some of their activities in the wild.

He recalled wanting to warn one female chimp: “Don’t mate with those guys … But of course I can’t do that.”