Zoonotic Disease Discussion on National Geographic
By David Quammen 2007
When zoonotic diseases pass from animals to humans, pandemics can result. Scientists are tracking lethal new viruses.
"That’s it," Reid said. "That’s the bloody tree." That’s where the bats gathered, he meant.
In September 1994, a violent disease erupted among racehorses in a suburb of
The other horses suffered fever, respiratory distress, facial swelling, and clumsiness; in some, bloody froth came from the nostrils and mouth. Despite heroic efforts by the veterinarian, 12 more animals died within days. Meanwhile the trainer himself got sick; so did the stable foreman. The vet, who was following cautionary procedures but working amid the same mad circumstances, stayed healthy. After a few days in a hospital, the trainer died. His kidneys had failed and he couldn’t breathe. The stable foreman, a bighearted man named Ray Unwin, who had merely gone home to endure his fever in private, survived. He and the veterinarian told me their stories when I found them in Hendra last year. Ray Unwin is a middle-aged working bloke with a sandy red ponytail and a weary sadness in his eyes, who professed that he wasn’t a "whinger" (complainer) but said his health has been "crook" (not right) since it happened.
Laboratory analysis revealed that the horses and the men were infected by a previously unknown virus. At first the lab people called it equine morbillivirus, meaning a horse virus closely related to measles. Later, as its uniqueness became better appreciated, it was renamed after the place itself: Hendra. The veterinarian, a tall, gentle fellow named Peter Reid, told me that "the speed with which it went through those horses was unbelievable." At the height of the crisis, seven animals had succumbed to ugly deaths or required euthanasia within just 12 hours. One of them died thrashing and gasping so desperately that Reid couldn’t get close enough to give it the merciful needle. "I’d never seen a virus do anything like that before," he said. A man of understatement, he recalled it as "a pretty traumatic time."
Identifying the new virus was only step one in solving the immediate mystery of Hendra, let alone understanding the case in a wider context. Step two involved tracking that virus to its hiding place. Where did the thing exist when it wasn’t killing horses and people? Step three entailed asking a further cluster of questions: How did it emerge from its secret refuge, and why here, and why now?
After our first conversation, Peter Reid drove me out to the site where Drama Series took sick. Tract houses on prim lanes have been built over the original pasture. Not much of the old landscape remains. But toward the end of one street is a circle, called Calliope Circuit, in the middle of which stands a single mature tree, a native fig, beneath which the mare would have found shelter from eastern
Infectious disease is all around us. Infectious disease is a kind of natural mortar binding one creature to another, one species to another, within the elaborate edifices we call ecosystems. It’s one of the basic processes that ecologists study, including also predation, competition, and photosynthesis. Predators are relatively big beasts that eat their prey from outside. Pathogens (disease-causing agents, such as viruses) are relatively small beasts that eat their prey from within. Although infectious disease can seem grisly and dreadful, under ordinary conditions it’s every bit as natural as what lions do to wildebeests, zebras, and gazelles.
But conditions aren’t always ordinary.
Just as predators have their accustomed prey species, their favored targets, so do pathogens. And just as a lion might occasionally depart from its normal behavior—to kill a cow instead of a wildebeest, a human instead of a zebra—so can a pathogen shift to a new target. Accidents happen. Aberrations occur. Circumstances change and, with them, opportunities and exigencies also change. When a pathogen leaps from some nonhuman animal into a person, and succeeds there in making trouble, the result is what’s known as a zoonosis.
The word zoonosis is unfamiliar to most people. But it helps clarify the biological reality behind the scary headlines about bird flu, SARS, other forms of nasty new disease, and the threat of a coming pandemic. It says something essential about the origin of HIV. It’s a word of the future, destined for heavy use in the 21st century.
Ebola is a zoonosis. So is bubonic plague. So are yellow fever, monkeypox, bovine tuberculosis, Lyme disease, West Nile fever, Marburg, many strains of influenza, rabies, hantavirus pulmonary syndrome, and a strange new affliction called Nipah, which kills pigs and pig farmers in Malaysia. Each of them reflects the action of a pathogen that can cross to people from other species. This form of interspecies leap is common, not rare; about 60 percent of all human infectious diseases currently known are shared between animals and humans. Some of those—notably rabies—are widespread and famously lethal, still killing humans by the thousands despite centuries of effort at coping with their effects, concerted international attempts to eradicate or control them, and a clear scientific understanding of how they work. Others are new and inexplicably sporadic, claiming a few victims (as Hendra did) or a few hundred in this place or that, and then disappearing for years.
Smallpox, to take one counterexample, is not a zoonosis. It’s caused by a virus that infects Homo sapiens and, in very exceptional cases, certain nonhuman primates, but not horses or rats or other species. That helps explain why the World Health Organization’s global campaign to eradicate the disease was, as of 1979, successful. Smallpox could be eradicated because its virus, lacking ability to reside virtually anywhere other than in humans, couldn’t hide. Zoonotic pathogens can hide.
Monkeypox, though closely related to smallpox, differs in two crucial ways—its propensity to afflict monkeys as well as humans, and the ability of its virus to exist in still other species, some of which are so far unidentified. Yellow fever, also infectious to both monkeys and humans, and caused by a virus that hides in several species of mosquito, will probably never be eradicated. The Lyme disease perpetrator, a type of bacterium, hides effectively in white-footed mice and other small mammals. These pathogens aren’t consciously hiding, of course. For their purposes, such behavior merely constitutes a strategy of indirect transmission or inconspicuous survival.
The least conspicuous strategy of all is to lurk within what’s called a reservoir host, a species that carries the pathogen while suffering little or no symptomatic illness. When a disease seems to disappear between outbreaks (again, as Hendra did after the 1994 carnage), its causal pathogen may indeed have died out, at least from the region—but then again, maybe not. Maybe its still lingering nearby, all around, within some reservoir host. A rodent? A bird? A butterfly? Possibly a bat? To reside undetected within a reservoir host is probably easiest wherever biological diversity is high and the ecosystem is relatively undisturbed. The converse is also true: Ecological disturbance causes diseases to emerge. Shake a tree, and things fall out.
Some months after the deaths in
Then he took blood from Pteropus alecto, a species of fruit bat, big as a crow and commonly known as the black flying fox. Bingo: The lab team found molecular traces left by Hendra virus. Further sampling produced similar evidence from three other species of flying foxes, all native to the forests of
The lab work suggested that Hendra was an old virus, having probably existed within its reservoir host for thousands of years. Despite its age, it had never before—so far as historical records and human memory could say, anyway—caused disease in humans. What accounts for its emergence in 1994? Well, bad luck for Drama Series and those who knew her. Bats came to eat the figs in that solitary tree, and the poor mare, seeking shade, grazing too carelessly, evidently swallowed not just grass but also something of what they dropped, such as fruit pulp, feces, urine, afterbirth, and virus.
But there had to be a broader answer, too. Why did Hendra emerge in 1994, not decades or centuries earlier? Something was different. Some sort of change, or combination of changes, must have caused the virus to be transferred from its reservoir host into other species.
The fancy name for such transfer is spillover. Maybe the virus needed horses (which only reached
But proximity is one thing; spilling virus into horses is another. "How does transmission occur?" Field wondered aloud, at the end of our long conversation. "Well, we still don’t know."
Nearly all zoonotic diseases result from infection by one of six kinds of pathogen: viruses, bacteria, protozoans, prions, fungi, and worms. Mad cow disease is caused by a prion, a weirdly folded protein molecule that triggers weird folding in other molecules, like Kurt Vonnegut’s infectious form of water, ice-nine, in his great early novel Cat’s Cradle. Sleeping sickness is a protozoan infection, carried by tsetse flies between wild and domestic mammals and people on the landscapes of sub-Saharan
Viruses are the most problematic. They evolve quickly, they are unaffected by antibiotics, they can be elusive, they can be versatile, they can inflict extremely high rates of mortality, and they are fiendishly simple, at least relative to other living or quasi-living creatures. Hanta, SARS, monkeypox, rabies, Ebola,
About the same time as the Hendra outbreak near
Those facts and numbers were collected by a team of medical researchers, some Gabonese, some French, who reached Mayibout II during the outbreak. Among them was a Frenchman named Eric M. Leroy, based at the Centre International de Recherches Médicales de Franceville (CIRMF), in
Four years later, I sat at a campfire near the upper
Amid the chaos and sorrow of the outbreak, M’Both told me, he and Etouck had seen something bizarre: 13 gorillas, all dead, lying in the forest. That image, of 13 gorilla carcasses strewn on the leaf litter, is lurid but plausible. Subsequent research has confirmed that gorillas are susceptible to Ebola. Being social creatures, they could easily pass the infection among group members by mutual grooming, infant care, or trying to rouse their sick or their dead.
In the years since 1996, other outbreaks of Ebola have struck both people and great apes (chimps as well as gorillas) within the region surrounding Mayibout II. One area hit hard lies along the
Last autumn I returned to the
The virus seemed to be gone, too. But we knew it was only hiding.
Hiding where? For a decade, the identity of Ebola’s reservoir host was one of the darkest mysteries in the world of disease science. Several sets of researchers were trying to solve it. Then, two years ago, Eric Leroy and some colleagues announced in the journal Nature: "We find evidence of asymptomatic infection by Ebola virus in three species of fruit bat, indicating that these animals may be acting as a reservoir for this deadly virus." Leroy’s group hadn’t captured any live virus, but they had established—with positive results from several kinds of molecular tests—that Ebola had passed through at least a few of the bats examined.
Leroy himself wants stronger evidence. "We continue to catch bats—to try to isolate virus from their organs," he said late last year, when I visited him in Franceville. Identifying the reservoir host with certitude, though, would still leave other questions unanswered.
For instance, how does Ebola emerge from that reservoir? "We don’t know if there’s direct transmission from bats to humans," Leroy said. "We only know there is direct transmission from dead great apes to humans." And how has the virus evolved, producing four distinct strains? Why is the Ebola-Zaire strain, the one found in
Hendra and Ebola are part of a much larger pattern: the recent emergence of new zoonotic diseases, variously lethal and horrific, more than a few of which seem to be associated with bats. Another part of the pattern is human-caused disruption of wild landscape. Nipah came next.
In September 1998, a pork seller in peninsular
The molecular profile of this new virus suggested a close kinship with Hendra. That provided a clue. Not long afterward, researchers found Nipah living sedately in a reservoir host: Pteropus hypomelanus, another species of fruit bat. They also noted that fruit bats, deprived of habitat elsewhere, had been congregating in orchards near the pig farms.
And then there was SARS. It came out of southeastern
There’s more. Australian bat lyssavirus, a newly identified virus closely related to rabies, has killed at least two people with rabies-like symptoms after the victims were bitten by bats. Menangle and Tioman are also bat-carried viruses, of the same family as Hendra, that scientists are watching carefully. Rabies itself and rabies-like viruses, found in bat reservoirs around the world, are still probably the most lethal of all viral pathogens if untreated—with nearly 100 percent mortality among humans. In northern
At this point, you’re entitled to ask: Damn, what is it about bats?
I asked that myself, in conversation with Charles Rupprecht, a virologist and veterinarian who leads the rabies section at the Centers for Disease Control and Prevention, in
Another informed view came from Xavier Pourrut, a research veterinarian based at CIRMF in
Contact is crucial. Close contact between two species represents an opportunity for a pathogen to expand its horizons and possibilities. The pathogen may be well adapted to its quiet, secure life within a reservoir host; spilling over into a new species presents a chance, at some risk, of vastly increasing its abundance and its geographic reach. The risk is that, by killing the new host too quickly, before getting itself transmitted onward, the pathogen will come to a dead end. But evolutionary theory suggests that some pathogens, on some occasions, will accept that risk in exchange for the chance of a big payoff. Long-term survival is only one form of evolutionary success. Gross abundance and broad distribution is another.
Think of tortoises and rats. Tortoises tend to live by a conservative strategy, remaining within their preferred habitat and reproducing slowly. Rats tend to be opportunists, fanning out, traveling across land and sea as stowaways, arriving in new places and reproducing fast. Similarly, pathogens may differ in their degree of adventurousness. Spillover from a reservoir host isn’t necessarily an accident, always leading to the dead end. It may be a strategy, leading to evolutionary success. Simian immunodeficiency virus (SIV) achieved that sort of success when it spilled over from one subspecies of chimpanzee into humans, probably in west-central
Close contact between humans and other species can occur in various ways: through killing and eating of wild animals (as in Mayibout II), through caregiving to domestic animals (as in Hendra), through fondling of pets (as with monkeypox, brought into the American pet trade by way of imported African rodents), through taming enticements (feeding bananas to the monkeys at a Balinese temple), through intensive animal husbandry combined with habitat destruction (as on Malaysian pig farms), and through any other sort of disruptive penetration of humans into wild landscape—of which, needless to say, there’s plenty happening around the world. Once the contact has occurred and the pathogen has crossed over, two other factors contribute to the possibility of cataclysmic consequences: the sheer abundance of humans on Earth, all available for infection, and the speed of our travel from one place to another. When a bad new disease catches hold, one that manages to be transmissible from person to person by a handshake, a kiss, or a sneeze, it might easily circle the world and kill millions of people before medical science can find a way to control it.
But our safety, our health, isn’t the only issue. Another thing worth remembering is that disease can go both ways: from humans to other species as well as from them to us. Measles, polio, scabies, influenza, tuberculosis, and other human diseases are considered threats to non-human primates. The label for those infections is anthropozoonotic. Any of them might be carried by a tourist, a researcher, or a local person, with potentially devastating impacts on a tiny, isolated population of great apes with a relatively small gene pool, such as the mountain gorillas of
That’s why Billy Karesh and his colleagues at the Wildlife Conservation Society label their program with the slogan "One World, One Health." The guiding principles come from ecology, of which human and veterinarian medicine are merely subdisciplines. "It’s not about wildlife health or about human health or about livestock health," he told me. "There’s really just one health"—the health and balance of ecosystems throughout the planet.
After our fruitless stakeout along the
LES ANIMAUX TROUVES
MORTS EN FORET
(Don’t ever touch dead animals found in the forest.)
Mbomo was Balo’s hometown. Visiting his house, we met his wife, Estelle, and some of his many children. We learned that Estelle’s sister, two brothers, and another close relative all died of Ebola in 2003, and that Estelle herself was shunned by townspeople because of her association with the disease. No one would sell food to her. No one would touch her money. She had to hide in the forest. She would have died herself, Balo said, if he hadn’t taught her the precautions he’d learned from Eric Leroy and the other scientists for whom he’d worked during the outbreak—sterilize everything with bleach, wash your hands, and don’t touch corpses. But now the bad time was past and, with Balo’s arm around her, Estelle was a smiling, healthy young woman.
Balo remembered the outbreak in his own way, mourning Estelle’s losses and some of a different sort. He showed us a book, a botanical field guide, on the endpapers of which he had written a list of names: Apollo, Cassandra, Afrodita, and almost 20 others. They were gorillas, an entire group that he had known well, that he had tracked daily and observed lovingly at Lossi. Cassandra was his favorite, Balo said. Apollo was the silverback. "Sont tous disparus en deux-mille trois," he said. All of them, gone in the 2003 outbreak. He’d lost his gorilla family, and also members of his own family. It was very hard, Balo said.
For a long time he stood holding the book, opened for us to see those names. He comprehended emotionally what the scientists know from their data: That we—people and gorillas, horses and pigs and bats, monkeys and rats and mosquitoes and viruses—are all in this together.
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