Spillover: Animal Infections and the Next Human Pandemic (8 page)

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

H
iding 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?

By then they knew that the pathogen
was
a virus. That knowledge derived from isolations performed quickly on clinical samples shipped to overseas laboratories, including the CDC. (Johnson, before flying to Zaire, had led the CDC isolation effort himself.) They knew that this virus was similar to Marburg virus, another lethal agent, identified nine years before; the electron micrographs showed that it was equally filamentous and twisty, like an anguished tapeworm. But the lab tests also revealed Ebola virus as distinct enough from Marburg virus to constitute something new. Eventually these two wormy viruses, Ebola and Marburg, would be classified within a new family,
Filoviridae
: the filoviruses.

Johnson’s group knew also that the new agent, Ebola virus, must reside in some living animal—something other than humans—where it could exist less disruptively and maintain a continuous presence. But the question of its reservoir was less urgent than other concerns, such as how to break the chain of person-to-person transmission, how to keep patients alive, how to end the outbreak. “
Only limited ecological investigations
were made,” the team reported later, and the results of those investigations were all negative. No sign of Ebola virus appeared anywhere except in humans. But the negative data are interesting in retrospect, at least as a record of where these early researchers looked. They pureed 818 bedbugs collected from Ebola-affected villages, finding no evidence of the virus in any. They considered mosquitoes. Nothing. They drew blood from ten pigs and one cow—all of which proved Ebola-free. They caught 123 rodents, including 69 mice, 30 rats, and 8 squirrels, not one of which was a viral carrier. They read the entrails of six monkeys, two duikers, and seven bats. These animals also were clean.

The International Commission members were chastened by what they had seen. “
No more dramatic or potentially explosive epidemic
of a new acute viral disease has occurred in the world in the past 30 years,” their report warned. The case fatality rate of 88 percent, they noted, was higher than any on record, apart from the rate for rabies (almost 100 percent among patients not treated before they show symptoms). The Commission made six urgent recommendations to Zairian officialdom, among which were health measures at the local level and nationwide surveillance. But the identification of Ebola’s reservoir wasn’t mentioned. That was a scientific matter, slightly more abstract than the action items offered to President Mobutu’s government. It would have to wait.

The wait has continued.

Three years after Yambuku, Karl Johnson and several other members of the Commission were still wondering about the reservoir question. They decided to try again. Lacking funds to mount an expedition devoted solely to finding Ebola’s hideout, they hitched their effort to an ongoing research program on monkeypox in Zaire, coordinated by the World Health Organization. Monkeypox is a severe affliction, though not so dramatic as Ebola virus disease, and also caused by a virus that lurks in a reservoir host or hosts, at that time still unidentified. So it seemed natural and economical to do a combined search, using two sets of analytical tools to screen a single harvest of specimens. Again the field team collected animals from villages and surrounding forest in Bumba Zone, as well as in other areas of northern Zaire and southeastern Cameroon. This time their trapping and hunting efforts, plus the bounties they paid for creatures delivered alive by villagers, yielded more than fifteen hundred animals representing 117 species. There were monkeys, rats, mice, bats, mongooses, squirrels, pangolins, shrews, porcupines, duikers, birds, tortoises, and snakes. Blood was taken from each, and then snips of liver, kidney, and spleen. All these samples, deep-frozen in individual vials, were shipped back to the CDC for analysis. Could any live virus be grown from the sampled tissues? Could any Ebola antibodies be detected in blood serum? The bottom line, reported with candor by Johnson and coauthors in the pages of
The Journal of Infectious Diseases
, was negatory: “
No evidence of Ebola virus infection
was found.”

One factor making the hunt for Ebola’s reservoir especially difficult, especially hard to focus, is the transitory nature of the disease within human populations. It disappears entirely for years at a time. This is a mercy for public health but a constraint for science. Viral ecologists can look for Ebola anywhere, in any creature of any species, in any African forest, but those are big haystacks and the viral needle is small. The most promising search targets, in space and in time, are wherever and whenever people are dying of Ebola virus disease. And for a long interlude, no one
was
dying of that illness—no one whose death came to the attention of medical authorities, anyway.

After the Yambuku outbreak of 1976, and then two small episodes in Zaire and Sudan between 1977 and 1979, ebolaviruses barely showed themselves anywhere in Africa for fifteen years. There may have been some scattered cases during the early 1980s, retrospectively suspected, but there was no confirmed outbreak that evoked emergency response; and in each of those minor instances the chain of infection seemed to have burned itself out. Burning out is a concept with special relevance to such highly lethal and moderately contagious pathogens
.
It means that a few people died, a few more got infected, a fraction of those also died but others recovered, and the pathogen didn’t continue to propagate. The incident expired on its own before shock troops from WHO, the CDC, and other centers of expertise had to be mustered. Then, after an interval, it returned—with the outbreaks at Mayibout 2 and elsewhere in Gabon, and even more alarmingly at a place called Kikwit.

Kikwit, in Zaire, lay about three hundred miles east of Kinshasa. It differed from Yambuku, and Mayibout 2, and the timber camp outside Booué in one crucial way: It was a city of two hundred thousand people. It contained several hospitals. It was connected to the wider world in a way that those other outbreak sites weren’t. But like them it was surrounded by forest.

The first identified case in the Kikwit outbreak was a forty-two-year-old man who worked in or near that forest and probably, to some small extent, disturbed it. He farmed several patches of cleared land, planting corn and cassava, and made charcoal from timber, all at a spot five miles southeast of the city. How did he get his wood supply, how did he clear daylight for his gardens? Presumably by cutting trees. This man fell sick on January 6, 1995, and died of a hemorrhagic fever a week later.

By that time he had directly infected at least three members of his family, all fatally, and launched the infection into his wider circle of social contacts, ten more of whom died within coming weeks. Some of those contacts evidently carried the virus into the city’s maternity hospital, where it infected a laboratory technician, and from there into Kikwit General Hospital. The technician, while being treated at Kikwit General, infected several doctors and nurses who did surgery on him (suspecting a gut perforation related to typhoid, they cut open his abdomen), as well as two Italian nuns who helped with his care. The technician died, the nuns died, and local officials hypothesized that this was epidemic dysentery, a misdiagnosis that allowed the virus to spread further among patients and staff at other hospitals in the Kikwit region.

Not everyone accepted the dysentery hypothesis. One doctor at the Ministry of Health thought it looked instead like a viral hemorrhagic fever, which suggested Ebola. That good guess was confirmed quickly from blood specimens received by the CDC, in Atlanta, on May 9. Yes indeed: It was Ebola virus. By the end of the outbreak, in August, 245 people had died, including 60 hospital staff members. Performing abdominal surgery on Ebola patients, when you thought they were suffering from something else (such as gastrointestinal bleeding from ulcers), was risky work.

Meanwhile, another international team came out to search for the reservoir, converging on Kikwit in early June. This group consisted of people from the CDC, from a Zairian university, from the US Army Medical Research Institute of Infectious Diseases (USAMRIID, formerly a bioweapons lab but now committed to disease research and biodefense) in Maryland, and one fellow from the Danish Pest Infestation Laboratory, who presumably knew a lot about rodents. They began work at the site to which the spillover seemed traceable—that is, at the charcoal pit and crop fields of the unlucky forty-two-year-old man, the first victim, southeast of the city. From that site and others, over the following three months, they trapped and netted thousands of animals. Mostly those were small mammals and birds, plus a few reptiles and amphibians. All the traps were set within forest or savanna areas outside the city limits. Within Kikwit itself, the team netted bats at a Sacred Heart mission. They killed each captured animal, drew blood, and dissected out the spleen (in some cases other organs too, such as a liver or a kidney), which went into frozen storage. They also took blood from some dogs, cows, and pet monkeys. The total yield included 3,066 blood samples and 2,730 spleens, all shipped back to the CDC for analysis. The blood samples, after having been irradiated to kill any virus, were tested for Ebola virus antibodies, using the best available molecular method of the time. The spleens were transferred to a biosafety level 4 (BSL-4) laboratory, a new sort of facility since Karl Johnson’s early work (and of which he was one of the pioneering designers), with multiple seals, negative air pressure, elaborate filters, and lab personnel working in spacesuits—a containment zone in which Ebola virus could be handled without risk (theoretically) of accidental release. No one knew whether any of these Zairian spleens contained the virus but each had to be treated as though it did. From the spleen material, minced finely and added to cell cultures, the lab people tried to grow the virus.