Spillover and The Silence

“Spillover” by David Quammen examines the ecology and evolution of diseases that spill over from animals into humans.

Advisory: If your husband catches an ebolavirus, give him food and water and love and maybe prayers but keep your distance, wait patiently, hope for the best—and, if he dies, don’t clean out his bowels by hand. Better to step back, blow a kiss, and burn the hut. (from Spillover by David Quammen)

I’m a latecomer to Doctor Who, only gradually catching up on the rebooted TV series beginning last year. Friday night, I watched the episodes introducing The Silence, the creepy alien species that interfered with human culture over the millennia, while erasing memories of their presence whenever they were spotted.

Shortly after, I finished reading Spillover by David Quammen, a magnificent omnibus of history, ecology, and evolution of zoonoses: diseases that “spill over” from animal reservoirs into humans. (The first “s” is hard in “zoonoses” and the two o’s  are pronounced as separate syllables. Sometimes you’ll see it spelled “zoönoses” by old-fashioned or excessively Tolkien-influenced people.) Many important and frankly scary diseases are zoonoses, from the Black Death to AIDS to potentially emergent pandemics such as the H5N1 “bird flu” and SARS. Rabies, influenza, and Ebola are similarly zoonoses.

This is not what people usually mean by “zoonoses”.

Of course, a lot of less catastrophic diseases are also zoonotic. However, spillover can lead to a lot of deaths, especially if the disease can be transmitted between humans as well as from animal to person. A virus like H5N1 carries a high fatality rate if a human gets it, but so far, it doesn’t seem to be contagious between people: one must catch it from an infected bird, at least in its current incarnation. Ebola, on the other hand, travels easily between infected humans, but its very quick effects and high fatality rate mean that it hasn’t spread far. It’s a horrible disease—make no mistake on that!—but influenza, rabies, and cholera each kill a lot more people annually.

Quammen tells the story of zoonoses and the people who study them: epidemiologists, virologists, veterinarians, and microbiologists. Many of the chapters, such as the one on the origins of HIV/AIDS, read like the most gripping detective story. (In fact, I think this book would be good to spread among the students who have watched CSI or NCIS on TV and want to be forensic investigators; research into zoonoses would satisfy many of the same impulses, and be of greater service to humanity.) Quammen is also an adventurous traveler, so his book mingles travel narrative with the science—a fact I particularly appreciate.

Zoonoses can be nasty precisely because of spillover: the virus can “hide” in reservoir species, which are by definition not strongly affected by it. These species either pass the virus to humans directly, or through an intermediary species, including domesticated or wild food animals. The few attempts to successfully eradicate disease—smallpox, and hopefully polio in the near future—are possible because those viruses aren’t zoonotic.

The Silence from Doctor Who.
The Silence from Doctor Who.

Like The Silence in Doctor Who, zoonoses are always present, affecting human history through impressive spillover events. A total understanding of the ecology and evolution of the diseases is necessary if we want to avoid pandemic. As Quammen eloquently points out, this understanding includes grasping our role in exacerbating zoonoses thanks to environmental destruction, agricultural and game-hunting practices, and other population pressures on reservoir or vector species.

One response to “Spillover and The Silence”

  1. The question I would pose is how our diseases, including those that have migrated to us from animals, have affected other animals. Have we bought an ‘ebola’ virus to some poor species of bird? Do our coughs and sniffles occasionally wipe out entire populations of rodents? Are we a huge vector for some horrid disease that plagues frogs?

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