Spillover: Animal Infections and the Next Human Pandemic

by David Quammen

A zoonosis is an animal infection transmissible to humans.

Pondering them as a group tends to reaffirm the old Darwinian truth (the darkest of his truths, well known and persistently forgotten) that humanity is a kind of animal, inextricably connected with other animals: in origin and in descent, in sickness and in health.

Her cause of death remained uncertain. Had she been bitten by a snake? Had she eaten some poisonous weeds out in that scrubby, derelict meadow? Those hypotheses crumbled abruptly, thirteen days later, when her stable mates began falling ill. They went down like dominoes. This wasn’t snakebite or toxic fodder. It was something contagious.

They found a virus. It wasn’t a hantavirus. It wasn’t AHS virus. It was something new, something the AAHL microscopist hadn’t seen before but which, from its size and its shape, resembled members of a particular virus group, the paramyxoviruses.

The matching subgroup was the morbilliviruses, which include rinderpest virus and canine distemper virus (infecting nonhuman animals) and measles (in humans). So the creature from Hendra was classified and given a name, based on those provisional identifications: equine morbillivirus (EMV). Roughly, horse measles.

“That’s it,” Reid said. “That’s the bloody tree.” That’s where the bats gathered, he meant.

When a pathogen leaps from some nonhuman animal into a person, and succeeds there in establishing itself as an infectious presence, sometimes causing illness or death, the result is a zoonosis.

It’s a mildly technical term, zoonosis, unfamiliar to most people, but it helps clarify the biological complexities behind the ominous headlines about swine flu, bird flu, SARS, emerging diseases in general, and the threat of a global pandemic.

Smallpox, to take one counterexample, is not a zoonosis. It’s caused by variola virus, which under natural conditions infects only humans. (Laboratory conditions are another matter; the virus has sometimes been inflicted experimentally on nonhuman primates or other animals, usually for vaccine research.) That helps explain why a global campaign mounted by the World Health Organization (WHO) to eradicate smallpox was, as of 1980, successful. Smallpox could be eradicated because that virus, lacking ability to reside and reproduce anywhere but in a human body (or a carefully watched lab animal), couldn’t hide.

The eradication campaign for poliomyelitis, unlike other well-meant and expensive global health initiatives, may succeed. Why? Because vaccinating humans by the millions is inexpensive, easy, and permanently effective, and because apart from infecting humans, the poliovirus has nowhere to hide. It’s not zoonotic.

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.

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 fatality, and they are fiendishly simple, at least relative to other living or quasi-living creatures.

Laboratory virologists are not generally knockabout people. You don’t meet them in bars, waving their arms and bragging lustily about the perils of their métier. They tend to be focused, neat, and still, like nuclear engineers.

The virus hunter is a field biologist, possibly with advanced training in human medicine, veterinary medicine, ecology, or some combination of those three—a person who finds fascination in questions that must be answered by catching and handling animals.

He began his search by going back to the index case—the first equine victim, its history and locale. That was Drama Series, the pregnant mare, fallen ill in the paddock at Cannon Hill.

The reality was this: that very close contact, plus bad luck, plus maybe one or two other factors were necessary for a person to become infected, and nobody knew what the other factors were.

After the early bat surveys, about 15 percent of their flying foxes had tested positive for Hendra antibodies. This parameter—the percentage of sampled individuals showing some history of infection, either present or past—is called seroprevalence. It constitutes an estimate, based on finite sampling, of what the percentage throughout an entire population might be.

Not a single antibody-positive person out of 128 carers. What did that tell you, I asked her, about the nature of this virus? “That it needed some sort of amplifier,” she said. She was alluding to the horse.

One experimental study found that pig breath carried thirty times as much FMD virus as the breath of an infected cow or sheep, and that once airborne it could spread for miles. That’s why pigs are considered an amplifier host of this virus.

An amplifier host is a creature in which a virus or other pathogen replicates—and from which it spews—with extraordinary abundance.

The amplifier host becomes an intermediate link between a reservoir host and some other unfortunate animal, some other sort of victim—a victim requiring higher doses or closer contact before the infection can take hold.

Until that day there was no record, either fossil or historic, of members of the genus Equus in Australia. Nor were there any oral traditions (none shared with the world so far, anyway) of Hendra virus outbreaks among aboriginal Australians.

Make no mistake, they are connected, these disease outbreaks coming one after another. And they are not simply happening to us; they represent the unintended results of things we are doing. They reflect the convergence of two forms of crisis on our planet. The first crisis is ecological, the second is medical. As the two intersect, their joint consequences appear as a pattern of weird and terrible new diseases, emerging from unexpected sources and raising deep concern, deep foreboding, among the scientists who study them.

One: Mankind’s activities are causing the disintegration (a word chosen carefully) of natural ecosystems at a cataclysmic rate.

Two: Those millions of unknown creatures include viruses, bacteria, fungi, protists, and other organisms, many of which are parasitic.

Three: But now the disruption of natural ecosystems seems more and more to be unloosing such microbes into a wider world.

A parasitic microbe, thus jostled, evicted, deprived of its habitual host, has two options—to find a new host, a new kind of host . . . or to go extinct. It’s not that they target us especially. It’s that we are so obtrusively, abundantly available.

Most people don’t know that the real, full story of AIDS doesn’t begin among American homosexuals in 1981, or in a few big African cities during the early 1960s, but at the headwaters of a jungle river called the Sangha, in southeastern Cameroon, half a century earlier.

Diseases of the future, needless to say, are a matter of high concern to public health officials and scientists.

Some knowledgeable and gloomy prognosticators even speak of the Next Big One as an inevitability. (If you’re a seismologist in California, the Next Big One is an earthquake that drops San Francisco into the sea, but in this realm of discourse it’s a vastly lethal pandemic.)

Will the Next Big One be caused by a virus? Will the Next Big One come out of a rainforest or a market in southern China? Will the Next Big One kill 30 or 40 million people?

The chief difference between HIV-1 and the NBO may turn out to be that HIV-1 does its killing so slowly. Mo...

The one I prefer simply says that an emerging disease is “an infectious disease whose incidence is increasing following its first introduction into a new host population.”

A re-emerging disease is one “whose incidence is increasing in an existing host population as a result of long-term changes in its underlying epidemiology.”

“Spillover” is the term used by disease ecologists (it has a different use for economists) to denote the moment when a pathogen passes from members of one species, as host, into members of another. It’s a focused event.

Emergence is a process, a trend.

Why do strange new diseases emerge when they do, where they do, as they do, and not elsewhere, other ways, at other times?

Leroy and his colleagues found evidence of Ebola virus in samples from some patients, and they deduced that the butchered chimpanzee had been infected with Ebola.

The chimpanzee seems to have been the index case for infecting 18 primary human cases,” they wrote. Their investigation also turned up the fact that the chimp hadn’t been killed by village hunters; it had been found dead in the forest and scavenged.

Minkébé’s gorillas, once abundant, had disappeared. The inescapable inference was that something had killed them off.

The four major lineages of virus that showed themselves during those emergence events are collectively known as ebolaviruses.

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.

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.)

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.

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.

Because it allows me to apply what I’ve learned, and to keep learning, and it might save some lives.

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.

Any such spillover in the reverse direction—from humans to a nonhuman species—is known as an anthroponosis.

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?

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.