AIDS denialists have claimed that the lack of disease progression in chimpanzees infected with HIV is evidence that the virus is at worst a passenger in human beings and cannot be the cause of AIDS. This was, for example, implied by the journalist Celia Farber in her AIDS denialist piece published in Harper's in 2006.
In the response to Farber [1], to which several of the AIDSTruth.org team contributed, we wrote:
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It is true that HIV replicates inefficiently in chimpanzees, to a much lower level than it does in humans so it usually does not cause disease. However, there are recorded examples of HIV causing immunodeficiency in these animals.
Many agents which cause disease in man are unable to cause disease in a host of other species because they fail to infect, or infect poorly, or produce a different response. HIV has probably been in the chimpanzee population for a very long time. Therefore it is plausible that natural selection has rendered it less harmful.
There is now new compelling evidence that we understated the extent of Farber's factual error. A team of researchers led by Brandon Keele and Beatrice Hahn have analysed mortality in wild chimpanzees infected with SIVcpz, the immediate precursor of HIV-1. They following 94 wild chimpanzees in Tanzania. They found a 10- to 16-fold higher age-corrected death hazard for SIVcpz-infected chimpanzees compared to uninfected chimpanzees. They also found that SIVcpz-infected females were less likely to give birth and had a higher infant mortality rate than uninfected ones. Furthermore, disease progression was associated with a declining CD4 T cell count and increased viral replication, which is characteristic of HIV infection in humans.
They therefore conclude:
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These findings challenge the prevailing view that all natural SIV infections are non-pathogenic and suggest that SIVcpz has a substantial negative impact on the health, reproduction and lifespan of chimpanzees in the wild.
Denialists have claimed that AIDS in humans is caused by poverty, antiretrovirals and recreational drugs such as poppers. These explanations are debunked on the AIDStruth myths [2] page and obviously cannot explain disease progression in SIVcpz-infected chimpanzees. This is further evidence that they are not the cause of AIDS in humans.
Here is the abstract of Keele et al.'s letter to Nature [3]:
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Keele, B.F. et al. Increased mortality and AIDS-like immunopathology in wild chimpanzees infected with SIVcpz. Nature 460, 515-519 (2009).
African primates are naturally infected with over 40 different simian immunodeficiency viruses (SIVs), two of which have crossed the species barrier and generated human immunodeficiency virus types 1 and 2 (HIV-1 and HIV-2). Unlike the human viruses, however, SIVs do not generally cause acquired immunodeficiency syndrome (AIDS) in their natural hosts. Here we show that SIVcpz, the immediate precursor of HIV-1, is pathogenic in free-ranging chimpanzees. By following 94 members of two habituated chimpanzee communities in Gombe National Park, Tanzania, for over 9 years, we found a 10- to 16-fold higher age-corrected death hazard for SIVcpz-infected (n = 17) compared to uninfected (n = 77) chimpanzees. We also found that SIVcpz-infected females were less likely to give birth and had a higher infant mortality rate than uninfected females. Immunohistochemistry and in situ hybridization of post-mortem spleen and lymph node samples from three infected and two uninfected chimpanzees revealed significant CD4+ T-cell depletion in all infected individuals, with evidence of high viral replication and extensive follicular dendritic cell virus trapping in one of them. One female, who died within 3 years of acquiring SIVcpz, had histopathological findings consistent with end-stage AIDS. These results indicate that SIVcpz, like HIV-1, is associated with progressive CD4+ T-cell loss, lymphatic tissue destruction and premature death. These findings challenge the prevailing view that all natural SIV infections are non-pathogenic and suggest that SIVcpz has a substantial negative impact on the health, reproduction and lifespan of chimpanzees in the wild.
