MONTREAL, CANADA--Researchers have long assumed that SIVcpz, the chimpanzee virus that infected humans and triggered the AIDS epidemic, caused no harm to the apes. But new data presented here today at the 16th Conference on Retroviruses and Opportunistic Infections reveal that wild chimps infected with SIVcpz are more likely to die than uninfected chimps. The animals also show AIDS-like damage to their immune systems. The finding raises the possibility that chimps, too, are suffering from an AIDS epidemic.
More than 40 simian immunodeficiency viruses, or SIVs, infect African primates, yet they rarely cause disease. SIVcpz was discovered in 1989, and it soon became clear that it was closely related to HIV-1 and predated it. Several researchers soon proposed that chimps had long lived with the virus and that their immune systems had evolved to coexist with SIVcpz. But fewer than a dozen SIVcpz-infected chimps were identified until more than a decade later, when researchers led by Beatrice Hahn of the University of Alabama, Birmingham, developed a way to routinely test fecal samples from wild chimps for evidence of the virus. At the conference, Rebecca Rudicell, a graduate student in Hahn's lab, explained that the researchers have now amassed enough data to assess the impact of SIVcpz on wild chimps.
Rudicell, Hahn, and colleagues analyzed more than 1099 fecal samples collected between 2000 and 2008 from chimpanzees living in Gombe Stream National Park, the Tanzanian site that Jane Goodall made famous. From these samples, they found evidence of SIVcpz infection in 18 chimps. The prevalence of SIVcpz has fluctuated in the Gombe communities from a low of 9% to a high of 18%, which mirrors the devastating levels of infections seen in human populations in the hardest hit countries in sub-Saharan Africa.
Rudicell said that seven of the 18 infected chimps died during their study, compared with 10 of 76 uninfected animals. When they corrected for age and other variables, the scientists found that the SIVcpz-infected chimps had a 15-fold higher risk of death than did virus-free apes. Studies of lymph nodes from two of the infected chimps that died showed the type of immunologic destruction seen in HIV-infected humans. And these chimps also had low levels of CD4 cells, the lymphocytes that are the main targets of SIVcpz and HIV-1.
Although the researchers do not know that the chimpanzees died of AIDS, the data made a convincing case to primatologists at the meeting. "I think there's AIDS in that group," says virologist Preston Marx of the Tulane National Primate Research Center in Covington, Louisiana. The finding raises provocative questions about the relationship between HIV-1 and SIVcpz. For instance, why SIVcpz harms chimp immune systems but HIV-1 doesn't is a mystery, especially given the close similarities between humans and chimps. The work might offer clues to vaccine makers, too, about which immune responses to target. Also unknown is whether SIVcpz has contributed to the alarming chimp decline seen in Gombe and elsewhere. "It's interesting and new information," says Marx. "The rate of disease here is surprisingly high."