Spillover: Animal Infections and the Next Human Pandemic

by David Quammen

zoonosis is an animal infection transmissible to humans. There are more such diseases than you might expect. AIDS is one. Influenza is a whole category of others. 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.

The other horses suffered fever, respiratory distress, bloodshot eyes, spasms, and clumsiness; in some, bloody froth surged from the nostrils and mouth; a few had facial swelling.

African horse sickness (AHS), a disease carried by biting midges in sub-Saharan Africa.

“whinger”

hantavirus

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. This new virus differed from known paramyxoviruses in that each particle carried a double fringe of spikes.

“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 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.

Ebola is a zoonosis. So is bubonic plague. So was the so-called Spanish influenza of 1918–1919, which had its ultimate source in a wild aquatic bird and, after passing through some combination of domesticated animals (a duck in southern China, a sow in Iowa?)

All of the human influenzas are zoonoses. So are monkeypox, bovine tuberculosis, Lyme disease, West Nile fever, Marburg virus disease, rabies, hantavirus pulmonary syndrome, anthrax, Lassa fever, Rift Valley fever, ocular larva migrans, scrub typhus, Bolivian hemorrhagic fever, Kyasanur forest disease, and a strange new affliction called Nipah encephalitis, which has killed pigs and pig farmers in Malaysia. Each of them reflects the action of a pathogen that can cross into people from other animals. AIDS is a disease of zoonotic origin caused by a virus that, having reached humans through just a few accidental events in western and central Africa, now passes human-to-human by the millions. This form of interspecies leap is common, not rare; about 60 percent of all human infectious diseases currently known either cross routinely or have recently crossed between other animals and us. Some of those—notably rabies—are familiar, widespread, and still horrendously lethal, killing humans by the thousands despite centuries of efforts at coping with their effects, concerted international attempts

Smallpox, to take one counterexample, is not a zoonosis. It’s caused by variola virus,

In the United States, the polio problem peaked in 1952 with an outbreak that killed more than three thousand victims, many of them children, and left twenty-one thousand at least partially paralyzed. Soon afterward, vaccines developed by Jonas Salk, Albert Sabin, and a virologist named Hilary Koprowski (about whose controversial career, more later) came into wide use, eventually eliminating poliomyelitis throughout most of the world. In

Monkeypox is a disease similar to smallpox, caused by a virus closely related to variola.

Yellow fever, also infectious to both monkeys and humans, results from a virus that passes from victim to victim, and sometimes from monkey to human, in the bite of certain mosquitoes.

Lyme disease

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. Ebola, West Nile, Marburg, the SARS bug, monkeypox, rabies, Machupo, dengue, the yellow fever agent, Nipah, Hendra, Hantaan (the namesake of the hantaviruses, first identified in Korea), chikungunya, Junin, Borna, the influenzas, and the HIVs (HIV-1, which mainly accounts for the AIDS pandemic, and HIV-2, which is less widespread) are all viruses. The full list is much longer. There is a thing known by

catching bats on the wing, or even at their roosting sites, isn’t so simple as trapping rodents or possums in a meadow. The most conspicuous and far-ranging bats native to Queensland are the so-called flying foxes, which belong to four different species within the genus Pteropus, each one

infected with Hendra, yes, but not necessarily that they

15 percent of their flying foxes had tested positive for Hendra antibodies.

As the team continued testing, the seroprevalence rose.

flying foxes,

The identification of flying foxes as reservoir hosts, plus the high levels of seroprevalence within those bat populations, caused public-image trouble for a group of animals that had a legacy of such trouble already. Approval ratings for bats are never high. Now in Australia they went lower.

foot-and-mouth disease

picornaviruses, the same group that includes poliovirus and some viruses similar to those that cause the common cold. But infection with FMD virus is a rare misfortune in humans, seldom causing worse than a rash on the hands, the feet, or the mouth lining. More frequently and consequentially, it afflicts cloven-hoofed domestic animals such as cattle, sheep, goats, and pigs. (Cloven-hoofed wildlife such as deer, elk, and antelope are also susceptible.) The main clinical signs are fever, lameness, and vesicles (little blisters) in the mouth, on the snout, on the feet.

the fossil record in Queensland shows that small bats have been there for at least 55 million years, and flying foxes may have evolved in the region during the early Miocene, about 20 million years ago.

Hendra virus in 1994 was just one thump in a drumbeat of bad news. The drumbeat has been sounding ever more loudly, more insistently, more rapidly over the past fifty years. When and where did it start, this modern era of emerging zoonotic diseases?

Machupo virus among Bolivian villagers between 1959 and 1963.

The first recorded case of the disease came and went, almost unnoticed, as a bad but nonfatal fever afflicting a local farmer. This was during the wet season of 1959. More such illnesses, and worse, occurred in the same region over the following three years. Symptoms included fever and chills, nausea and vomiting, body aches, nosebleeds, and bleeding gums. It became known as El Tifu Negro (the Black Typhus, for the color of vomit and stool),

Karl Johnson, pungently candid with his opinions, deeply enthralled by the dangerous beauty of viruses, who caught the disease himself and nearly died of it.

If you assembled a short list of the highlights and high anxieties of that saga within recent decades, it could include not just Machupo but also Marburg (1967), Lassa (1969), Ebola (1976, with Karl Johnson

again prominently involved), HIV-1 (inferred in 1981, first isolated in 1983), HIV-2 (1986), Sin Nombre (1993), Hendra (1994), avian flu (1997), Nipah (1998), West Nile (1999), SARS (2003), and the much feared but anticlimactic swine flu of 2009.

A person might construe this list as a sequence of dire but unrelated events—independent misfortunes that have happened to us, to humans, for one unfathomable reason and another. Seen that way, Machupo and the HIVs and SARS and the others are “acts of God” in the figurative (or literal) sense, grievous mishaps of a kind with earthquakes and volcanic eruptions and meteor impacts, which can be lamented and ameliorated but not avoided. That’s a passive, almost stoical way of viewing them. It’s also the wrong way.

Make no mistake, they are connected,

Human-caused ecological pressures and disruptions are bringing animal pathogens ever more into contact with human populations, while human technology and behavior are spreading those pathogens ever more widely and quickly. There are three elements to the situation.

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

logging, road building, slash-and-burn agriculture, hunting and eating of wild animals (when Africans do that we call it “bushmeat” and impute a negative onus, though in America it’s merely “game”), clearing f...

settlement, suburban sprawl, chemical pollution, nutrient runoff to the oceans, mining the oceans unsustainably for seafood, climate change, international marketing of the exported goods whose production requires any of the above, and other “civilizing” incursions upo...

Two: Those millions of unknown creatures include viruses, bacteria, fungi, protists, and other organisms, many of which are parasitic. Students of virology now speak of the “virosphere,” a vast realm of organisms that probably dwarfs every other group.

Three: But now the disruption of natural ecosystems seems more and more to be unloosing such microbes into a wider world. When the trees fall and the native animals are slaughtered, the native germs fly like dust

from a demolished warehouse. 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.

All these factors have yielded not just novel infections and dramatic little outbreaks but also new epidemics and pandemics, of which the most gruesome, catastrophic, and infamous is the one caused by a lineage of virus known to scientists as HIV-1 group M.

Diseases of the future, needless to say, are a matter of high concern to public health officials and scientists. There’s no reason to assume that AIDS will stand unique, in our time, as the only such global disaster caused by a strange microbe emerging from some other animal. Some knowledgeable and gloomy prognosticators even speak of the Next Big One as an inevitability.

Among the experts, they’re certainly common parlance. There’s even a journal dedicated to the subject, Emerging Infectious Diseases, published monthly by the CDC.

Tuberculosis is re-emerging as a severe problem, especially in Africa, as the TB bacterium exploits a new opportunity: infecting AIDS patients whose immune systems are disabled. Yellow fever re-emerges among humans wherever Aedes aegypti mosquitoes are allowed to resume carrying the virus between infected monkeys and uninfected people. Dengue, also dependent on mosquito bites for transmission and native monkeys as reservoirs,

Emergence and spillover are distinct concepts but interconnected. “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.

Spillover leads to emergence when an alien bug, having infected some members of a new host species, thrives in that species and spreads among it.

Not all emerging diseases are zoonotic, but most are.

Let’s keep an eye on wild creatures. As we besiege them, as we corner them, as we exterminate them and eat them, we’re getting their diseases.

how are we bringing these afflictions upon ourselves?