Wednesday, March 6, 2013

Spillover reviewed

The purpose of this book is not to make you more worried. The purpose of this book is to make you more smart.

—David Quammen, Spillover: Animal Infections and the Next Human Pandemic

I've been looking forward to this one for a while now, and Spillover was not a disappointment. My wife commented, "I can't believe I got you a disease book for Valentine's Day." I responded that Spillover isn't a disease book, it's an ecology book, and I recommend it be read in that light. One of my favourite science writers (Song of the Dodo, Monsters of God, and a couple of essay collections from his "Natural Acts" column), Quammen establishes a few central recurring themes in the first two pages:
  • "The subject of animal disease and the subject of human disease are, as we'll see, strands of one braided cord."
  • "...Humanity is a kind of animal, inextricably connected with other animals: in origin and in descent, in sickness and in health."
  • "...Everything, including pestilence, comes from somewhere."
Throughout the book, he delivers well-researched facts leavened with tales from the field and his dry humour, often in the form of asides to the reader. There are fairly extensive endnotes, but Quammen (or possibly the publisher) has chosen not to clutter the text with superscript which could interfere with the flow of the story—and there is a flow, for Quammen is a good storyteller. (He's published several works of fiction as well, though I haven't read these.)

The first section, "Pale Horse", is about Hendra, a relatively obscure virus originating in fruit bats and affecting horses and, to a lesser extent, humans. Quammen uses Hendra to introduce the overall topic of zoonosis as well as to define related concepts such as reservoir hosts and amplifier hosts, spillover and emergence, and to make the case that "they [outbreaks of zoonotic disease] are not simply happening to us, they represent the unintended consequence of things we are doing."

The tension increases with "Thirteen Gorillas", starring the much better-known (if not much better understood) Ebola. This one may also come from fruit bats and, as devastating as it can be on humans, its rapid lethality has kept it from making the leap from epidemic to pandemic. However, it may be affecting our close relatives, gorillas, on the population level. (If anyone wants to see foreshadowing here, I won't discourage them.) This section concludes with a key quote: "People and gorilla, horses and duikers and pigs, monkeys and chimps and bats and viruses: We're all in this together."

Quammen touches on the mathematics of contagion in "Everything Comes From Somewhere"—that phrase again—employing the vehicle of malaria and, mercifully, making us sit through very little mathematics. One of the topics addressed is the differential effects of even closely related diseases, for malaria is several. (Differential effects will come into play later, in the chapter on HIV.) "Dinner at the Rat Farm" examines SARS as a lucky escape, an epidemic that could have been so much worse than it was (and one that may yet get another bite at the apple).

Bacterial zoonoses such as psittacosis, Q fever, and Lyme disease get their turn in "The Deer, the Parrot, and the Kid Next Door". I particularly like the section on Lyme disease, which makes it clear that even those who do appreciate disease outbreaks as an ecological phenomenon often fail to grasp the ecological particulars. Although Lyme disease is closely linked in the public mind with white-tailed deer, the rise and fall of deer populations is almost irrelevant to the incidence of Lyme disease—it's the population of certain small mammals that make the difference, and the disparities between intact and fragmented ecosystems. ("The unintended consequences of things we are doing.")

In "Going Viral", however, we return to the group of pathogens likeliest to cause The Next Big One. The chapter begins with herpes B in macaques, goes from there to the larger topic of viral biology in general, and then to RNA viruses more particularly: "they jump species a lot". The succeeding chapter, "Celestial Hosts", explores the ongoing research into why so many viruses—including Hendra, Nipah, SARS, Marburg, and probably Ebola—seem to be linked to bats.

The longest chapter, "The Chimp and the River", is about the history and origins of the current Big One, HIV. It includes an extended imagining of a crucial spillover event; the passage dealing with Quammen's hypothetical Voyager is a bit out of character with the rest of the book, but it deals with an important dichotomy. The biogeography and genetic diversity of the human immunodeficiency viruses indicate that there have been at least a dozen separate spillovers from primate hosts into human ones, but the global AIDS pandemic (HIV-1 type M) results from a single event—a single event recently pinpointed in both time and space with a surprising degree of accuracy, though we're probably at the limit of resolution.

As with malaria, HIV is a family of pathogens, some more transmissible and more virulent than others, rather than a single entity. HIV-2 has never really expanded much beyond west Africa; most of the HIV-1 types are even more limited in scope. But for that one spillover event, HIV-1 type M remains, at least for a time, a virus of chimpanzees, and AIDS an unrecognized condition, barely distinguishable from the background noise of disease in central and western Africa.

The book closes with "It Depends", in which Quammen tells a tale of tent caterpillars taking over his Montana town, devastating the town's trees, and just as abruptly vanishing, victims of a virus. It is in this chapter, with Quammen noting an important ecological similarity between the tent caterpillars and humankind, that he writes "this book is to make you more smart", for there is an important ecological dissimilarity between the caterpillars and us as well. Can humanity's behavioural flexibility prevent, or at least postpone, humanity's own meltdown? As the chapter title depends.

Highly recommended, especially as a companion/followup to Laurie Garrett's wider-ranging The Coming Plague (1995).

Related post: "Stellaluna and Wilbur will kill us all"

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