The roster of simian immunodeficiency viruses was growing more crowded and complex. Now there were three known variants: one from African green monkeys, one from rhesus macaques (which they probably acquired in captivity), and one from sooty mangabeys. Needing a way to identify and distinguish them, someone hit upon the expedient of adding tiny subscripts to the acronym. Simian immunodeficiency virus as found in sooty mangabeys became SIVsm. The other two were labeled SIVagm (for African green monkeys) and SIVmac (for Asian macaques). This little convention may seem esoteric, not to mention hard on the eyes, but it will be essential and luminous when I discuss the fateful significance of a variant that came to be known as SIVcpz.
For now it’s enough to note the upshot of the leprosy experiment in Louisiana. One scientist from the Delta team, a woman named Michael Anne Murphey-Corb, collaborated with molecular biologists from other institutions to scrutinize the genomes of SIVs from sooty mangabeys and rhesus macaques, and to create a provisional family tree. Their work, published in 1989 with Vanessa M. Hirsch as first author, revealed that SIVsm is closely related to HIV-2. So is SIVmac. “These results suggest that SIVsm has infected macaques in captivity and humans in West Africa,” the group wrote, placing the onus of origination on sooty mangabeys, “and evolved as SIVmac and HIV-2, respectively.” In fact, those three strains were very similar, suggesting divergence fairly recently from a common ancestor.
“A plausible interpretation of these data,” Hirsch and her coauthors added, to make the point plainly, “is that in the past 30–40 years SIV from a West African sooty mangabey (or closely related species) successfully infected a human and evolved as HIV-2.” It was official: HIV-2 is a zoonosis.
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But what about HIV-1? Where did the great killer come from? That larger mystery took somewhat longer to solve. The logical inference was that HIV-1 must be zoonotic in origin also. But what animal was its reservoir? When, where, and how had spillover occurred? Why had the consequences been so much more dire?
HIV-2 is both less transmissible and less virulent than HIV-1. The molecular bases for those fateful differences are still secrets embedded in the genomes, but the ecological and medical ramifications are clear and stark. HIV-2 is confined mostly to West African countries such as Senegal and Guinea-Bissau (the latter of which, during colonial times, was Portuguese Guinea), and to other areas connected socially and economically within the former Portuguese empire, including Portugal itself and southwestern India. People infected with HIV-2 tend to carry lower levels of virus in their blood, to infect fewer of their sexual contacts, and to suffer less severe or longer-delayed forms of immune deficiency. Many of them don’t seem to progress to AIDS at all. And mothers who carry HIV-2 are less likely to pass it to their infants. The virus is bad, but not nearly so bad as it could be. HIV-1 provides the comparison. HIV-1 is the thing that afflicts tens of millions of people throughout the world. HIV-1 is the pandemic scourge. To understand how the AIDS catastrophe has happened to humanity, scientists had to trace HIV-1 to its source.
This takes us circling back to the city of Franceville, in southeastern Gabon, and its Centre International de Recherches Médicales (CIRMF), the same research institute at which Eric Leroy would later base his studies of Ebola. At the end of the 1980s, a young Belgian woman named Martine Peeters worked as a research assistant at CIRMF for a year or so, during the period between getting her diploma in tropical medicine and going on for a doctorate. The CIRMF facility maintained a compound of captive primates, including three dozen chimpanzees, and Peeters along with several associates was tasked with testing the captive animals for antibodies to HIV-1 and HIV-2. Almost all of the chimps tested negative—all except two. Both the exceptions were very young females, recently captured from the wild. Such baby chimps, like other orphan primates, are sometimes kept or sold off as pets after the killing and eating of their mothers. One of these animals, a two-year-old suffering from gunshot wounds, had been brought to CIRMF for medical treatment. She died of the wounds, but not before surrendering a blood sample. The other was an infant, maybe six months old, who survived. Blood serum from each of them reacted strongly when tested against HIV-1, less strongly when tested against HIV-2. That much was notable