American Scientist

SPILLOVER: Animal Infections and the Next Human Pandemic. David Quammen. 587 pp. W. W. Norton and Company, 2012. $28.95.

In his first book since the 2008 essay collection Natural Acts: A Sidelong View of Science and Nature, David Quammen looks at the natural world from yet another angle: the search for the next human pandemic, what epidemiologists call “the next big one.” His quest leads him around the world to study a variety of suspect zoonoses—animal-hosted pathogens that infect humans. Quammen interweaves his narrative with facts about zoonotic spillover—when a pathogen moves from its host species into another species—discussing a wide range of viruses (Hendra, Ebola, SARS, herpes B, HIV, Nipah and Marburg), bacteria (Q fever, Lyme disease and psittacosis) and malarial protists (a varied cast of Plasmodium species). Such diseases are difficult to study, he emphasizes, because of their complex ecological and evolutionary contexts and the mystery and misinformation that surround them. Quammen hunts for the generalities connecting high-profile zoonoses, which biologists have identified in order to better predict what the next big one will be like.

Like many readers, I have long enjoyed Quammen’s work, but I picked up this book with trepidation: The cover, which features a blurry photograph of a primate with red teeth bared, suggested the contents might contribute to the already excessive hype that surrounds many of these pathogens. But the image is misleading: Spillover is not sensationalist. Quammen notes:

I don’t say these things about the ineradicability of zoonoses to render you hopeless and depressed. Nor am I trying to be scary for the sake of scariness. The purpose of this book is not to make you more worried. The purpose of this book is to make you more smart. That’s what most distinguishes humans from, say, tent caterpillars and gypsy moths. Unlike them, we can be pretty smart.

In addition to the difficulty of describing human and animal suffering, or perhaps because of it, public perception of zoonotic diseases is fraught with misunderstanding. Quammen recognizes this problem; he spends pages debunking myths about Ebola that were most notoriously popularized by Richard Preston’s The Hot Zone. (Quammen confesses to gobbling up this popular book when it came out in 1994—as did I.)

Although the zoonoses covered in Spillover are gruesome, the number of deaths they cause is generally small in comparison to that of other human diseases. Quammen acknowledges this difference but makes a compelling case for the need to study these “anomalies”:

Given the global scorecard of morbidity and mortality caused by old-fashioned and nonzoonotic infectious diseases . . . why divert attention to these boutique infections, these anomalies, that spill out of bats or monkeys or who knows where to claim a few dozen or a few hundred people now and then? Why? Isn’t it misguided to summon concern over a few scientifically intriguing diseases, some of them new but of relatively small impact, while boring old diseases continue to punish humanity? . . . It’s a fair question but there are good answers. . . . The bluntest is this: AIDS.

A book that covers AIDS, Ebola, and other notoriously disturbing ailments is not for the faint of heart. Quammen gets down to business in the first chapter, describing “horse heads lying around, severed limbs, blood and other fluids flowing down the gutter, suspect organs and tissues going into bags.” Those who aren’t comfortable reading anecdotes that regularly mention vomit and diarrhea, or mass killings of animals harboring zoonotic infections, might do better to read one of Quammen’s earlier books instead. But if you can set aside your queasiness, you’ll be rewarded by Quammen’s prose.

He stresses that understanding ecology is essential to understanding enigmatic and puzzling new epidemics: “Ecological circumstance provides opportunity for spillover. Evolution seizes opportunity, explores possibilities, and helps convert spillovers to pandemics.” Thankfully, at the mention of ecology, Quammen does not resort to circle-of-life campiness; instead he details the ecological complexities that influence the incidence of outbreaks, including forest fragmentation, biodiversity loss, and increased contact between forest systems and high densities of people and livestock. In one of my favorite parts of the book, he reports a conversation with Rick Ostfeld, an expert on Lyme disease from the Cary Institute of Ecosystem Studies:

Some people take “All life is connected” to be the central truth of ecology, Ostfeld added. It’s not. It’s just a vague truism. The real point of the science is understanding which creatures are more intimately connected than others, and how, and to what result when change or disturbance occurs.

The narratives included in Spillover are not simply disturbing or scary. They often incorporate the fun and quirkiness of scientific research, not to mention the passion and effort behind it. The book includes plenty of lines like this one: “Picture two guys in a dark stone room, wearing headlamps, high-fiving in nitrile gloves.” Studying potential epidemics is exciting but risky work, and Quammen follows researchers into caves harboring cobras, onto roofs decked out with mist nets for catching bats, deep into African jungles to tranquilize gorillas, onto Bangladeshi temple grounds full of semi-tame macaques and onto rat farms in China (where he samples the fare). Paraphrasing an unnamed American field biologist, Quammen writes, “If you take too many risks, you don’t get home. If you take too few, you don’t get the data.” In the face of dicey field excursions, understanding mathematical models and crunching statistics are the least of these researchers’ worries. In addition to stories of fieldwork, though, Quammen tackles some of the most difficult concepts to understand in infectious disease biology. With verve and clarity, he details mathematical epidemiological models, the life cycle of the malaria-causing Plasmodium and the relation between transmission and virulence. An example: “The first rule of a successful parasite is slightly more complicated than Don’t kill your host. It’s more complicated even than Don’t burn your bridges until after you’ve crossed them. The first rule of a successful parasite is βN / (a + b + n).” Although he doesn’t spell out the meaning of each variable, he translates the formula into plain English, so readers will come away knowing this model says that the infection rate is based on changes in population density, as well as transmission rate and virulence. In addition to new diseases, Quammen touches on many of the examples often included in standard disease ecology or epidemiology courses—for example, myxomatosis in nonnative rabbits of Australia, and mutation rates in RNA versus DNA viruses. I highly recommend this book to readers who are interested in biology, and as a source of supporting discussion material for a course on infectious disease.

Quammen concludes with an answer to the question of when, where and what the next big disease will be:“It depends.” More specifically, as he puts it in a discussion of work by ecologists Roy Anderson and Robert May, “It depends on the specifics of the linkage between transmission and virulence. . . . It depends on ecology and evolution.” That makes a pretty good answer to any question about the future of humankind.

Katie Burke is associate editor at American Scientist. She received her Ph.D. in 2011 from the University of Virginia, where she studied the disease ecology of American chestnut and chestnut blight. She blogs about North American forests at www.the-understory.com.