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AIDS discovered in wild chimpanzees

July 22, 2009


Chimpanzee Echo with baby, Emela.

Echo with her infant Emela in March 2006. Echo was infected with an HIV-related virus for at least four years before dying.

Photo: Michael Wilson

Magnitude of threat to Gombe chimpanzees unknown

By Deane Morrison

Chimpanzees, like other wild African primates, naturally harbor HIV-related viruses, but the infection has been thought harmless. Now, a nine-year study of chimpanzees in Tanzania’s Gombe National Park challenges that view by showing that those with the virus can contract AIDS and die as a result.

The study was published July 23, 2009, in the journal Nature. Virologist Beatrice Hahn of the University of Alabama at Birmingham led the study, in collaboration with researchers from the University of Minnesota and other institutions.

Study co-author Anne Pusey, director of the Jane Goodall Institute’s Center for Primate Studies at the University of Minnesota, says the discovery gives researchers another avenue for studying, for example, how individuals either resist or succumb to AIDS.

Some humans and chimpanzees, she says, live a long time after infection, while others get sick and die quickly. Researchers are already examining the genomes of long- and short-term HIV survivors to identify “candidate genes” for resistance to the virus. If the same genetic pattern held in chimpanzees, that could confirm or refine ideas about the genes’ roles during infection in both species—key steps in developing therapies for the disease.

Complex infection pattern

More than 40 varieties of simian immunodeficiency virus, or SIV, are found in African primates. The chimpanzee-specific form, SIVcpz, is believed to have jumped to humans and become HIV-1, the AIDS virus.

In African monkeys, SIV appears to have co-evolved with host species for millions of years. Generally, long interaction with a host species weakens the harmful effects of a virus.

“Everyone is concerned about the decline in chimpanzee populations across Africa, which is due in large part to habitat destruction and the bush meat trade. We were not happy to find another thing causing mortality.”

Chimpanzees, however, have a virus that is a hybrid of viruses from two different monkey species, suggesting chimpanzees acquired it more recently from hunting monkeys—much as humans appears to have acquired the virus from hunting chimpanzees. Until now, researchers had assumed that chimpanzees and SIV had nonetheless been co-evolving long enough for the virus to be harmless in chimpanzees, but this new study changes that view.

“Everyone is concerned about the decline in chimpanzee populations across Africa, which is due in large part to habitat destruction and the bush meat trade,” says Pusey, who has studied the Gombe chimpanzees for decades in collaboration with Jane Goodall. “We were not happy to find another thing causing mortality.”

Even so, Pusey points out that SIVcpz appears to be less damaging than HIV-1 in humans, and the main study community in Gombe has so far maintained its size despite a certain level of disease-caused mortality. The “million dollar question” is to what extent SIVcpz may be contributing to population declines.

Elsewhere in Africa, variants of the SIV virus are patchily distributed.

For example, “the Gombe virus is very different from the form of SIV in Cameroon,” Pusey notes. “Cameroon is where, Beatrice [Hahn] figures, SIV jumped to humans and became HIV.”

Studying the virus everywhere it is found will help researchers construct its family tree and gain insights into the timeline of its spread. This could help answer the question of how much a threat the SIVcpz virus poses.

By the numbers

The researchers studied infection patterns in a total of 94 Gombe chimpanzees. During the study, seven of 17 infected chimpanzees and 11 of 77 uninfected ones died or disappeared. Taking into account uncertainties in the data, the researchers calculated that the death hazard for infected animals lay between 10 and 16 times the hazard for uninfected ones.

Infection with SIVcpz also was associated with reduced reproductive success. Of 30 uninfected females, 22 gave birth to a total of 30 infants during the study period. Of nine infected females, four produced a total of four infants. The odds of giving birth were three times lower for infected females.

Also, of the 30 infants born to uninfected chimpanzees, six, or 27 percent, died in their first year of life. Of those born to infected mothers, all died before their first birthday.

The researchers found that SIVcpz could be transmitted both sexually and through mother’s milk. They also identified the biological hallmark of AIDS—a dearth of immune cells known as CD4+ T cells—in tissues of several infected chimpanzees. In one of them, a female who died within three years of acquiring the virus, they observed weakness, lethargy, and muscle loss like that seen in acute cases of human AIDS.

Co-author Michael Wilson, an assistant professor of anthropology at the University of Minnesota, says that as a scientist he is fascinated that the virus affects humans and chimpanzees so similarly.

“But at the same time, having gotten to know these chimpanzees as individuals, it is hard watching them, knowing that some of their lives may be shortened by the virus, and there’s not really much we can do to help them.”

The researchers stress that they collected all biological samples from living chimpanzees by noninvasive means, such as by analyzing urine and fecal samples. They expect that continued study of the virus in a natural population will provide a better picture of the impact of SIVcpz infection on mortality and fertility and will inform HIV-1 research in humans.

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