Spillover

by David Quammen

  An earlier outbreak began during December 1994 in the gold-mining camps on the upper Ivindo, the same area from which Mike Fay later recruited his Gabonese crew. These camps lie about twenty-five miles upstream from Mayibout 2. At least thirty-two people got sick, showing the usual range of symptoms (fever, headache, vomiting, diarrhea, and some bleeding) that suggest Ebola virus disease. The source was hard to pinpoint, though one patient told of having killed a chimpanzee that had wandered into his camp and acted strangely. Maybe that animal was infected, inadvertently bringing the contagion to hungry humans. According to another account, the first case was a man who had come across a dead gorilla, took parts of it back to his camp, and shared. He died and so did others who touched the meat. Around the same time came some reports of chimps, as well as gorillas, seen dead in the forest. More generally, the miners (and their families—these camps were essentially villages) by their very presence, their needs for food, shelter, and fuel, had caused disturbance to the forest canopy and the creatures that lived in it.

  From the mining camps, those victims in 1994 were transferred downriver (as they would be again from Mayibout 2) to Makokou General Hospital. Then arose a wave of secondary cases, focused around the hospital or in villages nearby. In one of those villages was a nganga, a traditional healer, whose house may have been a point of transmission between a certain mining-camp victim of the outbreak, seeking folk medicine, and an unlucky local person visiting the healer about something less dire than Ebola. Possibly the virus was passed by the healer’s own hands. Anyway, by the time this sequence ended, forty-nine cases had been diagnosed, with twenty-nine deaths, for a case fatality rate of almost 60 percent.

  A year later came the outbreak at Mayibout 2, second in the series. Eight months after that, the CIRMF scientists and others responded to a third outbreak, this one near the town of Booué in central Gabon.

  The Booué situation had probably begun three months earlier, in July 1996, with the death of a hunter at a timber camp known as SHM, about forty miles north of Booué. In retrospect, this hunter’s fatal symptoms were recognized as matching Ebola virus disease, though his case hadn’t triggered alarm at the time. Another hunter died mysteriously in the same logging camp six weeks later. Then a third. What sort of meat were they supplying to the camp? Probably a wide range of wild species, including monkeys, duikers, bush pigs, porcupines, possibly even (despite legal restrictions) apes. And again there were reports of chimpanzees seen dead in the forest—fallen dead, that is, not shot dead. The three early human cases seem to have been independent of one another, as though each hunter contracted the virus from the wild. Then the third hunter broadened the problem, making himself a transmitter as well as a victim.

  He was hospitalized briefly at Booué but left that facility, eluded medical authorities, went to a nearby village, and sought help there from another nganga. Despite the healer’s ministrations the hunter died—and then so did the nganga and the nganga’s nephew. A cascade had begun. During October and into succeeding months there was a wider incidence of cases in and around Booué, suggesting more person-to-person transmission. Several patients were transferred to hospitals in Libreville, Gabon’s capital, and died there. A Gabonese doctor, having performed a procedure on one of those patients, fell sick himself and, showing little confidence in his own country’s health care, flew to Johannesburg for treatment. That doctor seems to have survived, but a South African nurse who looked after him sickened and died. Ebola virus had thereby emerged from Central Africa into the continent at large. The eventual tally from this third outbreak, encompassing Booué, Libreville, and Johannesburg, was sixty cases, of which forty-five were fatal. Rate of lethality? For that one, you can do the math in your head.

  Amid this welter of cases and details, a few common factors stand out: forest disruption at the site of the outbreak, dead apes as well as dead humans, secondary cases linked to hospital exposure or traditional healers, and a high case fatality rate, ranging from 60 to 75 percent. Sixty percent is extremely high for any infectious disease (except rabies); it’s probably higher, for instance, than mortalities from bubonic plague in medieval France at the worst moments of the Black Death.

  In the years since 1996, other outbreaks of Ebola virus disease have struck both people and gorillas within the region surrounding Mayibout 2. One area hit hard lies along the Mambili River, just over the Gabon border in northwestern Congo, another zone of dense forest encompassing several villages, a national park, and a recently created reserve known as the Lossi Gorilla Sanctuary. Mike Fay and I had walked through that area also, in March 2000, just four months before my rendezvous with him at the Minkébé inselbergs. In stark contrast to the emptiness of Minkébé, gorillas had been abundant within the Mambili drainage when we saw it. But two years later, in 2002, a team of researchers at Lossi began finding gorilla carcasses, some of which tested positive for antibodies to Ebola virus. (A positive test for antibodies is less compelling evidence than a find of live virus, but still suggestive.) Within a few months, 90 percent of the individual gorillas they had been studying (130 of 143 animals) had vanished. How many had simply run away? How many were dead? Extrapolating rather loosely from confirmed deaths and disappearances to overall toll throughout their study area, the researchers published a paper in Science under the forceful (but overconfident) headline: EBOLA OUTBREAK KILLED 5000 GORILLAS.

  10

  In 2006 I returned to the Mambili River, this time with a team led by William B. (Billy) Karesh, then director of the Field Veterinary Program for the Wildlife Conservation Society (WCS) of New York and now filling a similar role at the EcoHealth Alliance. Billy Karesh is a veterinarian and an authority on zoonoses. He’s a peripatetic field man, raised in Charleston, South Carolina, nourished on Marlin Perkins, whose usual working uniform is a blue scrub shirt, a gimme cap, and a beard. An empiricist by disposition, he speaks quietly, barely moving his mouth, and avoids categorical pronouncements as though they might hurt his teeth. Often he wears a sly smile, suggesting amusement at the wonders of the world and the varied spectacle of human folly. But there was nothing amusing about his mission to the Mambili. He had come to shoot gorillas—not with bullets but with tranquilizer darts. He meant to draw blood samples and test them for antibodies to Ebola virus.

  Our destination was a site known as the Moba Bai complex, a group of natural clearings near the east bank of the upper Mambili, not far from the Lossi sanctuary. A bai in Francophone Africa is a marshy meadow, often featuring a salt lick, and surrounded by forest like a secret garden. In addition to Moba Bai, the namesake of this complex, there were three or four others nearby. Gorillas (and other wildlife) frequent such bais, which are waterlogged and sunny, because of the sodium-rich sedges and asters that grow beneath the open sky. We arrived at Moba, coming upstream on the Mambili, in an overloaded dugout pushed by a 40-horse outboard.

  The boat carried eleven of us and a formidable pile of gear. We had a gas-powered refrigerator, two liquid-nitrogen freezer tanks (for preserving samples), carefully packaged syringes and needles and vials and instruments, medical gloves, hazmat suits, tents and tarps, rice, fufu, canned tuna, canned peas, several boxes of bad red wine, numerous bottles of water, a couple of folding tables, and seven stackable white plastic chairs. With these tools and luxurious provisions we established a field camp across the river from Moba. Our team included an expert tracker named Prosper Balo, plus other wildlife veterinarians, other forest guides, and a cook. Prosper had worked at Lossi before and during the outbreak. With his guidance, we would prowl the complex of bais, all full of succulent vegetation and previously famed for the dozens of gorillas that came there daily to eat and relax.

  Billy Karesh had visited the same area twice previously, before Ebola struck, seeking baseline data on gorilla health. During a 1999 trip, he had seen sixty-two gorillas here in one day. In 2000 he returned to try darting a few. “Every day,” he told me, “every bai had at least a family group.” Not wanting to
be too disruptive, he had tranquilized only four animals, weighed them and examined them for obvious diseases (such as yaws, a bacterial skin infection), and taken blood samples. All four apes had tested negative for Ebola antibodies. This time things were different. He wanted blood serum from survivors of the 2002 die-off. So we began, with high expectations. Days passed. As far as we could see, there were no survivors.

  Precious few, anyway—not enough to make gorilla-darting (which is always a parlous enterprise, with some risk for both the darter and the dartees) productive of data. Our stakeout at Moba lasted more than a week. Early each morning we crossed the river, walked quietly to one bai or another, concealed ourselves in thick vegetation along the edge, and waited patiently for gorillas to appear. None did. Often we hunkered in the rain. When it was sunny, I read a thick book or dozed on the ground. Karesh stood ready with his air rifle, the darts loaded full of tilletamine and zolazepam, drugs of choice for tranquilizing a gorilla. Or else we hiked through the forest, following closely behind Prosper Balo as he searched for gorilla sign and found none.

  On the morning of day 2, along a swampy trail to the bais, we saw leopard tracks, elephant tracks, buffalo tracks, and chimpanzee sign, but no evidence of gorillas. On day 3, with still no gorillas, Karesh said: “I think they’re dead. Ebola went through here.” He figured that only a lucky few, uninfected by the disease or else resistant enough to survive it, remained. Then again, he said, “those are the ones we’re interested in,” because they, if any, might carry antibodies. On day 4, separating from the rest of us, Karesh and Balo managed to locate a single, distraught male gorilla from the sound of his chest beats and screaming barks, and to crawl within ten yards of him in the thick underbrush. Suddenly the animal stood, only his head visible, in front of them. “I could have killed him,” Karesh said later. “Pitted him.” Drilled him between the eyes, that is, but not immobilized him with a safe shot to the flank. So Karesh held his fire. The gorilla let out another bark and ran off.

  My notes from day 6 include the entry: “Nada nada nary gorilla nada.” On our final chance, day 7, Balo and Karesh tracked another couple of animals for hours through the boggy forest without getting so much as a good glimpse. Gorillas had become desperately scarce, round about Moba Bai, and the stragglers were fearfully shy. Meanwhile the rain continued, the tents grew muddy, and the river rose.

  When we weren’t in the forest, I spent time in camp talking with Karesh and the three Africa-based WCS veterinarians on his team. One was Alain Ondzie, a lanky and bashful Congolese, trained in Cuba, fluent in Spanish as well as French and several Central African languages, with a likable tendency to dip his head and giggle joyously whenever he was teased or amused. Ondzie’s main job was to respond to reports of dead chimps or gorillas anywhere in the country, getting to the site as quickly as possible and taking tissue samples to be tested for Ebola virus. He described to me the tools and procedures for such a task, with the carcass invariably putrefied by the time he reached it and the presumption (until otherwise proven) that it might be seething with Ebola. His working costume was a hazmat suit with a vented hood, rubber boots, a splash apron, and three pairs of gloves, duct taped at the wrists. Making the first incision for sampling was dicey because the carcass might have become bloated with gas; it could explode. In any case the dead ape was usually covered with scavenging insects—ants, tiny flies, even bees. Ondzie told of one occasion when three bees from a carcass ran up his arms, under his hood flap, down across his bare body, and commenced to sting him as he worked on the samples. Can Ebola virus travel on the stinger of a bee? No one knows.

  Does this work frighten you? I asked Ondzie. Not anymore, he said. Why do you do it? I asked. Why do you love it? (as he clearly did). “Ca, c’est une bonne question,” he said, with the characteristic bob and giggle. Then he added, more soberly: Because it allows me to apply what I’ve learned, and to keep learning, and it might save some lives.

  Another member of the team was Patricia (Trish) Reed, who had come out to Africa as a biologist fifteen years earlier, studied Lassa fever and then AIDS, hired on with CIRMF in Franceville, gotten some field experience in Ethiopia, and then collected a DVM from the veterinary school at Tufts University in Boston. She was back at CIRMF, doing research on a monkey virus, when the WCS field vet working out here was killed in a plane crash coming into a backcountry Gabonese airstrip. Karesh hired Reed as the dead woman’s replacement.

  The scope of her work, Reed told me, encompassed a range of infectious diseases that threaten gorilla health, of which Ebola is only the most exotic. The others were largely human diseases of more conventional flavor, to which gorillas are susceptible because of their close genetic similarity to us: TB, poliomyelitis, measles, pneumonia, chickenpox, et cetera. Gorillas can be exposed to such infections wherever unhealthy people are walking, coughing, sneezing, and crapping in the forest. Any such spillover in the reverse direction—from humans to a nonhuman species—is known as an anthroponosis. The famous mountain gorillas, for instance, have been threatened by anthroponotic infections such as measles, carried by ecotourists who come to dote upon them. (Mountain gorillas constitute a severely endangered subspecies of the eastern gorilla, confined to the steep hillsides of the Virunga Volcanoes in Rwanda and neighboring lands. The western gorilla of Central African forests, a purely lowland species, is more numerous but far from secure.) Combined with destruction of their habitat by logging operations, and the hunting of them for bushmeat to be consumed locally or sold into markets, infectious diseases could push western gorillas from their current levels of relative abundance (maybe a hundred thousand in total) to a situation in which small, isolated populations survive tenuously, like the mountain gorillas, or go locally extinct.

  But the forests of Central Africa are still relatively vast, compared to the small Virunga hillsides that harbor mountain gorillas; and the western gorilla doesn’t face many ecotourists in its uncomfortable, nearly impenetrable home terrain. So measles and TB aren’t the worst of its problems. “I would say that, without a doubt, Ebola is the biggest threat” to the western species, Reed said.

  What makes Ebola virus among gorillas so difficult, she explained, is not just its ferocity but also the lack of data. “We don’t know if it was here before. We don’t know if they survive it. But we need to know how it passes through groups. We need to know where it is.” And the question of where has two dimensions. How broadly is Ebola virus distributed across Central Africa? Within what reservoir species does it lurk?

  On the eighth day, we packed up, reloaded the boats, and departed downstream on the Mambili, taking away no blood samples to add to the body of data. Our mission had been thwarted by the very factor that made it relevant: a notable absence of gorillas. Here was the curious incident of the dog in the nighttime again. Billy Karesh had seen one gorilla at close range but been unable to dart it, and had tracked two others with the help of Prosper Balo’s keen eye for spoor. The rest, the many dozens that formerly frequented these bais, had either dispersed to parts unknown or they were . . . dead? Anyway, once gorillas had been abundant hereabouts, and now they were gone.

  The virus seemed to be gone too. But we knew it was only hiding.

  11

  Hiding where? For almost four decades, the identity of Ebola’s reservoir host has been one of the darkest little mysteries in the world of infectious disease. That mystery, along with efforts to solve it, dates back to the first recognized emergence of Ebola virus disease, in 1976.

  Two outbreaks occurred in Africa that year, independently but almost simultaneously: one in the north of Zaire (now the Democratic Republic of the Congo), one in southwestern Sudan (in an area that today lies within the Republic of South Sudan), the two separated by three hundred miles. Although the Sudan situation began slightly earlier, the Zaire event is the more famous, partly because a small waterway there, the Ebola River, eventually gave its name to the virus.

  The focal point of the Zaire outbreak was
a small Catholic mission hospital in a village called Yambuku, within the district known as Bumba Zone. In mid-September, a Zairian doctor there reported two dozen cases of a dramatic new illness—not the usual malarial fevers but something more grisly, more red, characterized by bloody vomiting, nosebleeds, and bloody diarrhea. Fourteen of the patients had died, as of the doctor’s cabled alert to authorities in Kinshasa, Zaire’s capital, and others were in danger. By the start of October, Yambuku Mission Hospital had closed, for the grim reason that most of its staff members were dead. An international response team of scientists and physicians converged on the area several weeks later, under the direction of the Zairian Minister of Health, to do a crash study of the unknown disease and give advice toward controlling it. This group, consisting of members from France, Belgium, Canada, Zaire, South Africa, and the United States, including nine from the CDC in Atlanta, became known as the International Commission. Their leader was Karl Johnson, the same American physician and virologist who had worked on Machupo virus in Bolivia back in 1963, barely surviving his own infection with that disease. Thirteen years later, still intense, still dedicated, and not noticeably mellowed by near-death experience or professional ascent, he was head of the Special Pathogens Branch at the CDC.

  Johnson had helped solve the Machupo crisis by his attention to the ecological dimension—that is, where did the virus live when it wasn’t killing Bolivian villagers? The reservoir question had been tractable, in that case, and the answer had quickly been found: A native mouse was carrying Machupo into human households and granaries. Trapping out the mouse effectively ended the outbreak. Now, amid the desperate and befuddling days of October and November 1976, in northern Zaire, confronting a different invisible and unidentified killer, as the death toll rose into the hundreds, Johnson and his fellow researchers found time to wonder about Ebola virus as he had wondered about Machupo virus: Where did this thing come from?