The Chimp and the River: How AIDS Emerged from an African Forest

by David Quammen

  One scientist who made the guess was Luc Montagnier, then a little-known molecular biologist at the Institut Pasteur in Paris. Montagnier’s research focused mainly on cancer-causing viruses, especially the group known as retroviruses, some of which cause tumors in birds and mammals. Retroviruses are fiendish things, even more devious and persistent than the average virus. They take their name from the capacity to move backward (retro) against the usual expectations of how a creature translates its genes into working proteins. Instead of using RNA as a template for translating DNA into proteins—the usual route by which genetic information becomes living reality—a retrovirus converts its RNA into DNA within a host cell; its viral DNA then penetrates the cell nucleus and gets itself integrated into the genome of the host cell, thereby guaranteeing replication of the virus whenever the host cell reproduces itself. Luc Montagnier had studied these things in animals—chickens, mice, primates—and wondered about the possibility of finding them in human tumors too. Another disquieting possibility about retroviruses was that the new disease showing up in America and Europe, AIDS, might be caused by one.

  There was still no solid proof that it was caused by a virus of any sort. But three kinds of evidence pointed that way, and Montagnier recalled them in his memoir, a book titled Virus. First, the incidence of AIDS among homosexuals linked by sexual interactions suggested that this was an infectious disease. Second, the incidence among intravenous drug users suggested a blood-borne infectious agent. Third, the cases among hemophiliacs implied a blood-borne agent that escaped detection in processed blood products such as clotting factor. So: It was contagious, blood-borne, infinitesimal. “AIDS could not be caused by a conventional bacterium, a fungus, or protozoan,” Montagnier wrote, “since these kinds of germs are blocked by the filters through which the blood products necessary to the survival of hemophiliacs are passed. That left only a smaller organism: the agent responsible for AIDS thus could only be a virus.”

  Other evidence hinted that, among all viral possibilities, it might be a retrovirus. This was new ground, but then so was AIDS. The only known human retrovirus as of early 1981 was something called human T-cell leukemia virus (HTLV), recently discovered under the leadership of a smart, outgoing, highly regarded, and highly ambitious researcher named Robert Gallo, whose Laboratory of Tumor Cell Biology was part of the National Cancer Institute in Bethesda, Maryland. HTLV, as its name implies, attacks T cells and can turn them cancerous. T cells are one of the three major types of lymphocyte of the immune system. (Later the acronym HTLV was recast to mean human T-lymphotropic virus, which is slightly more accurate.) A related retrovirus, feline leukemia virus, causes immunodeficiency in cats. So a suspicion arose among cancer-virus researchers that the AIDS agent, destroying human immune systems by attacking their lymphocytes (in particular, a subcategory of T cells known as T-helper cells), might likewise be a retrovirus. Montagnier’s group began looking for it.

  Gallo’s lab did too. And those two weren’t alone. Other scientists at other laboratories around the world recognized that finding the cause of AIDS was the hottest, the most urgent, and potentially the most rewarding quest in medical research. By late spring of 1983, three teams working independently had each isolated a candidate virus, and in the May 20 issue of Science, two of those teams published announcements. Montagnier’s group in Paris, screening cells from a thirty-three-year-old homosexual man suffering from lymphadenopathy (swollen lymph nodes), had found a new retrovirus, which they called LAV (for lymphadenopathy virus). Gallo’s group came up with a new virus also, one that Gallo took for a close relative of the human T-cell leukemia viruses (by now there was a second, called HTLV-II, and the first had become HTLV-I) that he and his people had discovered. He called this newest bug HTLV-III, nesting it proprietarily into his menagerie. The French LAV and the Gallo HTLVs had at least one thing in common: They were indeed retroviruses. But within that family exists some rich and important diversity. An editorial in the same issue of Science trumpeted the Gallo and Montagnier papers with a misleading headline: HUMAN T-CELL LEUKEMIA VIRUS LINKED TO AIDS, despite the fact that Montagnier’s LAV was not a human T-cell leukemia virus. Woops, mistaken identity. Montagnier knew better, but his Science paper seemed to blur the distinction, and the editorial occluded it entirely.

  Then again, neither was Gallo’s “HTLV-III” an HTLV, once it was clearly seen and correctly classified. It turned out to be something nearly identical to Montagnier’s LAV, of which Montagnier had given him a frozen sample. Montagnier had personally delivered that sample, carrying it on dry ice during a visit to Bethesda.

  Confusion was thus sown early—confusion about what exactly had been discovered, who had discovered it, and when. That confusion, irrigated with competitive zeal, fertilized with accusation and denial, would grow rife for decades. There would be lawsuits. There would be fights over royalties from the patent on an AIDS blood-screening test that derived from virus grown in Gallo’s lab but traceable to Montagnier’s original isolate. (Contamination from one experiment to another, or from one batch of samples to another, is a familiar problem in lab work with viruses.) It wasn’t a petty squabble. It was a big squabble, in which pettiness played no small part. What was ultimately at stake, besides money and ego and national pride, was not just advancing or retarding research toward an AIDS cure or vaccine but also the Nobel Prize in medicine, which eventually went to Luc Montagnier and his chief collaborator, Françoise Barré-Sinoussi.

  Meanwhile the third team of researchers, led quietly by a man named Jay A. Levy in his lab at the University of California School of Medicine, in San Francisco, also found a candidate virus in 1983 but didn’t publish until more than a year afterward. By summer of 1984, Levy noted, AIDS had affected “more than 4000 individuals in the world; in San Francisco, over 600 cases have been reported.” Those numbers sounded alarmingly high at the time, though in retrospect, compared with 36 million deaths, they seem poignantly low. Levy’s discovery was also a retrovirus. His group detected it in twenty-two AIDS patients and grew more than a half dozen isolates. Because the bug was an AIDS-associated retrovirus, Levy called it ARV. He suspected, correctly, that his ARV and Montagnier’s LAV were simply variant samples of the same evolving virus. They were very similar but not too similar. “Our data cannot reflect a contamination of our cultures with LAV,” he wrote, “since the original French isolate was never received in our laboratory.” Harmless as that may sound, it was an implicit jab at Robert Gallo.

  The details of this story, the near-simultaneous triple discovery and its aftermath, are intricate and contentious and seamy and technical, like a ratatouille of molecular biology and personal politics left out in the sun to ferment. They lead far afield from the subject of zoonotic disease. For our purposes here, the essential point is that a virus discovered in the early 1980s, in three different places under three different names, became persuasively implicated as the causal agent of AIDS. A distinguished committee of retrovirologists settled the naming issue in 1986. They decreed that the virus would be called HIV.

  3

  It’s quite appropriate and more than coincidental, given the zoonotic dimension of this story, that the next phase began with a veterinarian. Max Essex studied retroviruses in monkeys and cats.

  Dr. Myron (Max) Essex, DVM, PhD, was not your ordinary small-animal vet. He was a professor and a research scientist in the Department of Cancer Biology at the Harvard School of Public Health. He had worked on feline leukemia virus (FeLV), among other things, and cancer-causing viruses formed the broad frame of his interests. Having seen the effects of FeLV in wrecking the immune systems of cats, he suspected as early as 1982, along with Gallo and Montagnier, that the new human immune deficiency syndrome might be caused by a retrovirus.

  Then something strange came to his notice, by way of a grad student named Phyllis Kanki. She was a veterinarian like him, but now working on a doctorate there at the School of Public Health. Kanki grew up in Chicago, spent her adolescent sum
mers doing zoo work, then studied biology and chemistry on the way toward veterinary medicine and comparative pathology. During the summer of 1980, while still amid her DVM studies, she worked at the New England Regional Primate Research Center, which was part of Harvard but located out in Southborough, Massachusetts. There she saw a weird problem among the center’s captive Asian macaques—some of them were dying of a mysterious immune dysfunction. A macaque is a kind of monkey, and several of the Asian species, such as the rhesus macaque and the Formosan rock macaque, are highly valued as laboratory animals for medical research. At the primate center, which held almost eight hundred animals of the macaque genus, Formosan rock macaques in particular seemed to be suffering this immune-system failure. Their T-helper lymphocyte counts were way down. They wasted away from diarrhea or succumbed to opportunistic infections, including Pneumocystis carinii. It sounded too much like AIDS. Kanki later brought this to the attention of Essex, her thesis adviser, and together with colleagues from Southborough, they started to look for what was killing those monkeys. Based on their knowledge of FeLV and other factors, they wondered whether it might be a retrovirus infection.

  Taking blood samples from macaques, they did find a new retrovirus, and saw that it was closely related to the AIDS virus. Because this was 1985, they used Gallo’s slightly misleading label (HTLV-III) for what would soon be renamed HIV. Their monkey virus would be renamed too and become, by analogy, simian immunodeficiency virus: SIV. The group published a pair of papers in Science, which had grown hungry for AIDS breakthroughs. This discovery, they wrote, could help illuminate the pathology of the disease, maybe even advance efforts to develop a vaccine, by providing an animal model for research. Only a single sentence at the end of one of the papers, a modest but pertinent comment dropped in like an afterthought, noted that SIV might also be a clue toward the origin of HIV.

  It was. Phyllis Kanki performed the lab analysis of samples from the captive macaques and then made it her business to wonder whether the same virus might exist in the wild. Kanki and Essex looked at other Asian macaques, testing blood samples from wild-caught animals. They found no trace of SIV. They tested still other kinds of wild Asian monkey. Again, no SIV. This led them to surmise that the macaques at Southborough had picked up their SIV in captivity by exposure to animals of another species. It was a reasonable guess, given that the primate center at one point had a monkey playpen in its lobby, where Asian and African infant monkeys were sometimes allowed to mingle. But then which kind of African monkey was the reservoir? Where exactly had the virus come from? And how might it be related to the emergence of HIV?

  “In 1985, the highest rates of HIV were reported in the U.S. and Europe,” Essex and Kanki wrote later, “but disturbing reports from central Africa indicated that high rates of HIV infection and of AIDS prevailed there, at least in some urban centers.” The focus of suspicion was shifting: not Asia, not Europe, not the United States, but Africa might be the point of origin. Central Africa also harbored a rich fauna of nonhuman primates. So the Harvard group got hold of blood from some wild-caught African simians, including chimpanzees, baboons, and African green monkeys. None of the chimps or the baboons showed any sign of SIV infection. Some of the African green monkeys did. It was an epiphany. More than two dozen of the monkeys carried antibodies to SIV, and Kanki grew isolates of live virus from seven. That finding too went straight into Science, and the search continued. Kanki and Essex eventually screened thousands of African green monkeys, caught in various regions of sub-Saharan Africa or held captive in research centers around the world. Depending on the population, between 30 and 70 percent of those animals tested SIV-positive.

  But the monkeys weren’t sick. They didn’t seem to be suffering from immunodeficiency. Unlike the Asian macaques, the African green monkeys “must have evolved mechanisms that kept a potentially lethal pathogen from causing disease,” Essex and Kanki wrote. Maybe the virus had changed too. “Indeed, some SIV strains might also have evolved toward coexistence with their monkey hosts.” The monkeys evolving toward greater resistance, the virus evolving toward lesser virulence—this sort of mutual adaptation would suggest that SIV had been in them a long time.

  The new virus, SIV as found in African green monkeys, became the closest known relative of HIV. But it wasn’t that close; many differences distinguished the two at the level of genetic coding. The resemblance, according to Essex and Kanki, was “not close enough to make it likely that SIV was an immediate precursor of HIV in people.” More likely, those two viruses represented neighboring twigs on a single phylogenetic branch, separated by lots of evolutionary time and probably some extant intermediate forms. Where might the missing cousins be? “Perhaps, we thought, one could find such a virus—an intermediate between SIV and HIV—in human beings.” They decided to look in West Africa.

  With help from an international team of collaborators, Kanki and Essex gathered blood samples from Senegal and elsewhere. The samples arrived with coded labeling, for blind testing in the laboratory, so that Kanki herself didn’t know their country of origin, nor even whether they derived from humans or monkeys. She screened them using tests for both SIV and HIV. Despite one possible misstep involving a lab contamination, her team found what they had thought they might: a virus intermediate between HIV and SIV. With the code unblinded, Kanki learned that the positive results came from Senegalese prostitutes. In retrospect it made sense. Prostitutes are at high risk for any sexually transmitted virus, including a new one recently spilled into humans. And the density of the rural human population in Senegal, where African green monkeys are native, makes monkey-human interactions (crop-raiding by monkeys, hunting by humans) relatively frequent.

  Furthermore, the new bug from Senegalese prostitutes wasn’t just halfway between HIV and SIV. It more closely resembled SIV strains from African green monkeys than it did the Montagnier-Gallo version of HIV. That was important but puzzling. Were there two distinct kinds of HIV?

  Luc Montagnier now reenters the story. Having tussled with Gallo over the first HIV discovery, he converged more amicably with Essex and Kanki on this one. Using assay tools provided by the Harvard group, Montagnier and his colleagues screened the blood of a twenty-nine-year-old man from Guinea-Bissau, a tiny country, formerly a Portuguese colony, along the south border of Senegal. This man showed symptoms of AIDS (diarrhea, weight loss, swollen lymph nodes) but tested negative for HIV. He was hospitalized in Portugal, and his blood sample was hand-delivered to Montagnier by a visiting Portuguese biologist. In Montagnier’s lab, the man’s serum again tested negative for antibodies to HIV. But from a culture of his white blood cells Montagnier’s group isolated a new human retrovirus, which looked very similar to what Essex and Kanki had found. In another patient, hospitalized in Paris but originally from Cape Verde, an island nation off the west coast of Senegal, the French team found more virus of the same type. Montagnier called the new thing LAV-2. Eventually, when all parties embraced the label HIV instead, it would be HIV-2. The original became HIV-1.

  The paths of discovery may be sinuous, the labels may seem many, and maybe you can’t tell the players without a scorecard; but these details aren’t trivial. The difference between HIV-2 and HIV-1 is the difference between a nasty little West African disease and a global pandemic.

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  During the late 1980s, as Phyllis Kanki and Max Essex and other scientists studied HIV-2, a flurry of uncertainty arose about its provenance. Some challenged the idea that it was closely related to (and recently derived from) a retrovirus that infects African monkeys. An alternative view was that such a retrovirus had been present in the human lineage for as long as—or longer than—we have existed as a species. Possibly it was already with us, a passenger riding the slow channels of evolution, when we diverged from our primate cousins. But that view left an unresolved conundrum: If the virus was an ancient parasite upon humans, unnoticed for millennia, how had it suddenly become so pathogenic?

  Recent spillover see
med more likely. Still, the case against that idea got a boost in 1988, when a group of Japanese researchers sequenced the complete genome of SIV from an African green monkey. That is, they detected and assembled the linear message of nucleotide bases—represented by the letters A, T, C, and G—comprising that SIV’s genetic inscription. The host animal came from Kenya. The nucleotide sequence of its retrovirus proved to be substantially different from the sequence for HIV-1, and different in roughly the same degree from HIV-2. So the monkey virus seemed no more closely related to the one human virus than to the other. That contradicted the notion that HIV-2 had lately emerged from an African green monkey. A commentary in the journal Nature, published to accompany the Japanese paper, celebrated this finding beneath a dogmatic headline: HUMAN AIDS VIRUS NOT FROM MONKEYS. But the headline was misleading to the point of falsity. Not from monkeys? Well, don’t be so sure. It turned out that researchers were just looking at the wrong kind of monkey.

  Confusion came from two sources. For starters, the label “African green monkey” is a little vague. It encompasses a diversity of forms, sometimes also known as savannah monkeys, that occupy adjacent geographical ranges spread across sub-Saharan Africa, from Senegal in the west to Ethiopia in the east and down into South Africa. At one time those forms were considered a “superspecies” under the name Cercopithecus aethiops. Nowadays, their differences having been more acutely gauged, they are classified as six distinct species within the genus Chlorocebus. The “African green monkey” sampled by the Japanese team, because it was “of Kenyan origin,” probably belonged to the species Chlorocebus pygerythrus. The species native to Senegal, on the other hand, is Chlorocebus sabaeus. Now that you’ve seen those two names you can forget them. The difference between one African green monkey and another is not what accounts for the genetic disjunction between SIV and HIV-2.