Spillover

by David Quammen

  That’s the salubrious thing about zoonotic diseases: They remind us, as St. Francis did, that we humans are inseparable from the natural world. In fact, there is no “natural world,” it’s a bad and artificial phrase. There is only the world. Humankind is part of that world, as are the ebolaviruses, as are the influenzas and the HIVs, as are Nipah and Hendra and SARS, as are chimpanzees and bats and palm civets and bar-headed geese, as is the next murderous virus—the one we haven’t yet detected.

  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.

  Greg Dwyer came around to this point during our talk in Chicago. He had studied all the famous mathematical models proposed to explain disease outbreaks in humans—Anderson and May, Kermack and McKendrick, George MacDonald, John Brownlee, and the others. He had noted the crucial effect of individual behavior on rate of transmission. He had recognized that what people do as individuals, what moths do as individuals, has a large effect on R0. The transmission of HIV, for instance, Dwyer said, “depends on human behavior.” Who could argue? It has been proven. Consult the changes in rate of transmission among American gay men, among the general populace of Uganda, or among sex workers in Thailand. The transmission of SARS, Dwyer said, seems to depend much on superspreaders—and their behavior, not to mention the behavior of people around them, can be various. The mathematical ecologist’s term for variousness of behavior is “heterogeneity,” and Dwyer’s models have shown that heterogeneity of behavior, even among forest insects, let alone among humans, can be very important in damping the spread of infectious disease.

  “If you hold mean transmission rate constant,” he told me, “just adding heterogeneity by itself will tend to reduce the overall infection rate.” That sounds dry. What it means is that individual effort, individual discernment, individual choice can have huge effects in averting the catastrophes that might otherwise sweep through a herd. An individual gypsy moth may inherit a slightly superior ability to avoid smears of NPV as it grazes on a leaf. An individual human may choose not to drink the palm sap, not to eat the chimpanzee, not to pen the pig beneath mango trees, not to clear the horse’s windpipe with his bare hand, not to have unprotected sex with the prostitute, not to share the needle in a shooting gallery, not to cough without covering her mouth, not to board a plane while feeling ill, or not to coop his chickens along with his ducks. “Any tiny little thing that people do,” Dwyer said, if it makes them different from one another, from the idealized standard of herd behavior, “is going to reduce infection rates.” This was after I had asked him to consider The Analogy and he had pushed his brain against it for half an hour.

  “There’s only so many ways gypsy moths can differ,” he said finally. “But the number of ways that humans can differ is really, really huge. And especially in their behavior. Right. Which gets back to your question, which is, How much does it matter that humans are smart? And so, I guess I’m actually going to say that it matters a whole lot. Now that I stop to think about it carefully. I think it will matter a great deal.”

  Then he took me into the basement of the building and gave me a glimpse of the experimental side of his work. He unlocked a door to what he called “the dirty room,” opened an incubator, took out a plastic container, and showed me gypsy moth caterpillars infected with NPV. I saw what it looks like to go splat on a leaf.

  115

  Of the two giant elm trees that stood before my neighbor Susan’s house, only one remains. The other died about four years ago, senescent, drought stricken, and harried by aphids. A contract arborist came with his crew and his truck and took it down, limb by limb, section by section. That was a sad day for Susan—for me too, having lived in the shade of that majestic hardwood for almost three decades. Then even the stump, big enough to serve as a coffee table, vanished. It had been ground down with a stump grinder and covered with grass. The tree is now gone but not forgotten. The neighborhood is less graceful for its loss. But there was no choice.

  The other big elm is still here, arching grandly over our little street. Circling the tree’s grayish brown bark, at waist level, is a stain—a dark band of discoloration, evidently indelible against weather and time, marking where it was defended with toxic goo against the tent caterpillars, twenty years ago. The caterpillars are long departed, just another outbreak population that crashed, but this mark is like their fossil record.

  When I’m home in Montana, I walk past that tree every day. Usually I notice the dark band. Usually I remember the caterpillars, how they came in such numbers and then disappeared. Conditions had been good for them. But something happened. Maybe luck was the crucial element. Maybe circumstance. Maybe their sheer density. Maybe genetics. Maybe behavior. Often nowadays, when I see the mark on the tree, I recall what Greg Dwyer told me: It all depends.

  NOTES

  I. Pale Horse

  24. “Viruses have no locomotion”: Morse (1993), ix.

  28. “He remained deeply unconscious”: O’Sullivan et al. (1997), 93.

  29. “It seems,” McCormack’s group concluded, “that very close contact”: McCormack et al. (1999), 23.

  35. “Economically, it is the most important”: Brown (2001), 239.

  41. “If you look at the world from the point of view”: William H. McNeill, in Morse (1993), 33–34.

  44. “Furthermore, 71.8% of these zoonotic”: Jones-Engel et al. (2008), 990.

  II. Thirteen Gorillas

  54. “The chimpanzee seems to have been the index case”: Georges et al. (1999), S70.

  70. “Only limited ecological investigations”: Johnson et al. (1978), 272.

  71. “No more dramatic or potentially explosive epidemic”: Johnson et al. (1978), 288.

  72. “No evidence of Ebola virus infection”: Breman et al. (1999), S139.

  77. “Contact with nature is intimate”: Heymann et al. (1980), 372–73.

  84. “Viruses of each species have genomes that”: Towner et al. (2008), 1.

  87. “bad human-like spirits that cause illness”: Hewlett and Hewlett

  (2008), 6.

  88. a final “love touch” of the deceased: Hewlett and Amola (2003), 1245.

  90. “This illness is killing everyone”: Hewlett and Hewlett (2008), 75.

  91. “Sorcery does not kill without reason”: Hewlett and Hewlett (2008), 75.

  92. “jumped from bed to bed, killing patients left and right”: Preston (1994), 68.

  92. “transforms virtually every part of the body”: Preston (1994), 72.

  92. “suddenly deteriorates,” its internal organs deliquescing: Preston (1994), 75.

  92. “essentially melts down with Marburg”: Preston (1994), 293.

  92. comatose, motionless, and “bleeding out”: Preston (1994), 184.

  93. “Droplets of blood stand out on the eyelids”: Preston (1994), 73.

  99. “It is difficult to describe working with a horse infected with Ebola”: Yaderny Kontrol (Nuclear Control) Digest, No. 11, Center for Policy Studies in Russia, Summer 1999.

  119. “Taken together, our results clearly point”: Walsh et al. (2005), 1950.

  120. “Thus, Ebola outbreaks probably do not occur as”: Leroy et al. (2004), 390.

  III. Everything Comes from Somewhere

  132. some interesting points about “smouldering” epidemics: Hamer (1906), 733–35.

  132. This idea became known as the “mass action principle”: Fine (1979), 348.

  133. “the acquisition by an organism of a high grade of infectivity”: Brownlee (1907), 516.

  133. “the condition of the germ”: Brownlee (1907), 517.

  133. “extirpated once and forever”: Ross (1910), 313.

  133. a “theory of h
appenings”: Ross (1916), 206.

  134. “so little mathematical work should have been done”: Ross (1916), 204–5.

  141. “This indicates,” they wrote confidently, “that human P. falciparum”: Liu et al. (2010), 424.

  141. “a monophyletic lineage within the gorilla P. falciparum radiation”: Liu et al. (2010), 423.

  143. “One of the most important problems in epidemiology”: Kermack and McKendrick (1927), 701.

  144. “Small increases of the infectivity rate”: Kermack and McKendrick (1927), 721.

  146. “very small changes in the essential transmission factors”: MacDonald (1953), 880.

  146. “the number of infections distributed in a community”: MacDonald (1956), 375.

  147. “It all but destroyed malariology”: Harrison (1978), 258.

  151. “The effect was remarkable”: Desowitz (1993), 129.

  152. “This occurrence,” wrote a quartet of the doctors involved: Chin et al. (1965), 865.

  161. “it is possible that we are setting the stage for a switch”: Cox-Singh and Singh (2008), 408.

  IV. Dinner at the Rat Farm

  169. “hospitalized for treatment of severe, acute respiratory syndrome”: World Health Organization (2006), 257.

  169. “During the past week,” it said, “WHO has received reports”: World Health Organization (2006), 259–60.

  171. described simply as “a local government official”: Abraham (2007), 30.

  171. labeling it “atypical pneumonia”: Abraham (2007), 34.

  172. “Population estimates of R0 can obscure”: Lloyd-Smith et al. (2005), 355.

  173. “Each time they began to insert the tube”: Abraham (2007), 37.

  182. alarming rumors about “a strange contagious disease”: World Health Organization (2006), 5.

  184. “The first thing going through our minds”: Normile (2003), 886.

  185. announcing this new coronavirus as “a possible cause”: Peiris (2003), 1319.

  186. “We were too cautious,” one of them said later: Enserink (2003), 294.

  187. “Southern Chinese have always noshed more widely”: Greenfeld (2006), 10.

  189. “The animals are packed in tiny spaces”: Lee et al. (2004), 12.

  191. “from another, as yet unknown, animal source”: Guan et al. (2003), 278.

  195. “An infectious consignment of bats”: Li et al. (2005), 678.

  206. “humankind has had a lucky escape”: Weiss and McLean (2004), 1139.

  V. The Deer, the Parrot, and the Kid Next Door

  211. known initially as “abattoir fever”: Sexton (1991), 93.

  212. an example of “public hysteria” commensurate with flagellation: The Washington Post, January 26, 1930, 1.

  214. “three died in agony”: Van Rooyen (1955), 4.

  214. “The year 1929 marked a turning point”: Van Rooyen (1955), 5.

  215. “tall with a gnarled Lincolnian face”: De Kruif (1932), 178.

  218. “If the young cockatoo, after capture”: Burnet and MacNamara (1936), 88.

  219. “a distinct clinical entity”: Derrick (1937), 281.

  219. “a filterable virus,” meaning an agent so small: Burnet and Freeman (1937), 299.

  220. “Most significant discoveries just grow on one”: Burnet (1967), 1067.

  220. “From that moment, there was no doubt”: Burnet (1967), 1068.

  220. “Problems of nomenclature arose”: Burnet (1967), 1068.

  221. “the Nine Mile agent”: McDade (1990), 12.

  221. “sharp pains in the eyeballs”: McDade (1990), 16.

  221. “There is no disease to match Q fever”: Burnet (1967), 1068.

  222. “One of the more bizarre episodes”: Burnet (1967), 1068.

  223. “there was no drop of rain”: Karagiannis et al. (2009), 1289.

  226. The other was a “hobby farm”: Karagiannis et al. (2009), 1286, 1288.

  228. “windborne transmission” as the most likely source: Karagiannis et al. (2009), 1292.

  231. “a filterable virus,” a microbe so tiny: Burnet (1940), 19.

  233. “I just don’t know if I can watch it”: Enserink (2010), 266.

  234. “were on the whole too busy to think of anything but”: Burnet (1940), 2–3.

  235. “Other workers with an appreciation of modern developments”: Burnet (1940), 3.

  235. “The parasitic mode of life is essentially similar”: Burnet (1940), 8.

  236. “It will be clear, however,” Burnet wrote: Burnet (1940), 12.

  236. “Like many other infectious diseases, psittacosis”: Burnet (1940), 19.

  237. “those cockatoos, left to a natural life in the wild”: Burnet (1940), 23.

  237. “It is a conflict between man and his parasites”: Burnet (1940), 23.

  238. such a thing as “chronic Lyme disease”: Feder et al. (2007), 1422.

  238. “No convincing biologic evidence exists”: IDSA News, Vol. 16, No. 3, Fall 2006, 2.

  239. “post-Lyme disease syndrome” was another matter: IDSA News, Vol. 16, No. 3, Fall 2006, 1.

  239. “by allowing individuals with financial interests”: Quoted in press release, Office of the Attorney General of Connecticut, May 1, 2008, 2.

  239. “no convincing evidence for the existence”: Quoted in press release, IDSA (Infectious Diseases Society of America), April 22, 2010, 2.

  241. began calling the syndrome “Lyme arthritis”: Steere et al. (1977a), 7.

  242. were now calling “Lyme disease”: Steere and Malawista (1979), 730.

  243. “a disease of the past,” no longer justifying: Burgdorfer (1986), 934.

  244. “No longer did we hear, ‘get out’”: Burgdorfer (1986), 936.

  244. later jovially called the “lymelight”: Burgdorfer (1986), 936.

  245. “Dammin’s northeastern deer ixodid”: Ostfeld (2011), 26.

  246. “The notion that Lyme disease risk is closely tied”: Ostfeld (2011), 22.

  246. One journal article had called white-tailed deer: Both this article and the next, quoted in Ostfeld (2011), 22.

  247. “The higher the number of deer in an area”: The Dover-Sherborn Press, January 12, 2011.

  247. “Any infectious disease is inherently an ecological system”: Ostfeld (2011), 4.

  248. “Thus began my interest in Lyme disease ecology”: Ostfeld (2011), x.

  249. “a messy and challenging task”: Ostfeld (2011), 48.

  250. “exquisitely sensitive” to chemical and physical signals: Ostfeld (2011), 23.

  250. the word is “questing”: Ostfeld (2011), 23.

  251.Ostfeld and others call “reservoir competence”: Ostfeld (2011), 12.

  258. “We know that walking into a small woodlot”: Ostfeld (2011), 9.

  258. Some people take “All life is connected” to be: Ostfeld (2011), 6–7.

  258. a sort of cystlike stage known as a “round body”: Margulis et al. (2009), 52.

  VI. Going Viral

  265. “the sap of leaves infected with tobacco mosaic disease”: Levine (1992), 2.

  267. “encouraged by the study of the so-called ‘filterable virus’ agents”: Zinsser (1934), 63.

  267. “Here, as in bacterial disease”: Zinsser (1934), 64.

  268. “a piece of bad news wrapped up in a protein”: Quoted in Crawford (2000), 6.

  273. “pain, redness, and slight swelling” around the bite: Sabin and Wright (1934), 116.

  273. They called it simply “the B virus”: Sabin and Wright (1934), 133.

  278. “no case” of human infection with the virus: Engel et al. (2002), 792.

  288. “a virus in search of a disease”: Weiss (1988), 497.

  295. the most “efficient” parasite, in Pasteur’s view: Pasteur’s view as summarized and reaffirmed by Rene Dubos, quoted in Ewald (1994), 188–89.

  295. “a more perfect mutual tolerance”: Zinsser (1934), 61.

  295. “In general terms, where two organisms have developed”
: Burnet (1940), 37.

  296. “A disease organism that kills its host quickly”: McNeill (1976), 9.

  297. “started jumping up and down, biting other animals”: Quoted in ProMED-mail post, April 22, 2011.

  297. “He barked like a dog,” his wife recalled later: Quoted in ProMED-mail post, April 1, 2011.

  298. Austin was an “ardent acclimatizer”: Fenner and Ratcliffe (1965), 17.

  299. causing what was called a “spectacular epizootic”: Fenner and Ratcliffe (1965), 276.

  301. “Laboratory experiments showed that all field strains”: Fenner (1983), 265.

  304. “weave together” the two approaches: Anderson and May (1979), 361.

  304. “unsupported statements” in medical and ecological textbooks: Anderson and May (1982), 411.

  306. “Our major conclusion,” wrote Anderson and May: Anderson and May (1982), 424.

  VII. Celestial Hosts

  315. “Pigs are a common host for the virus”: New Straits Times, January 7, 1999.

  316. “It became known as a one-mile barking cough”: Hume Field was the expert, quoted in a 60 Minutes (of Australia) television interview.

  327. “touching dead animals” looked like it might be important: Montgomery et al. (2008), 1529, Table 2.

  328. “increases the risk for wider spread”: Gurley et al. (2007), 1036.

  331. “Owners viewed the fruit bats as a nuisance”: Luby et al. (2006), 1892.

  344. “the revenge of the rain forest”: Preston (1994), 289.

  350. Do bats have a different “set point”: Calisher et al. (2006), 536.

  351. “Emphasis, sometimes complete emphasis, on nucleotide sequence”: Calisher et al. (2006), 541.

  351. “we are simply waiting for the next”: Calisher et al. (2006), 540.

  351. “The natural reservoir hosts of these viruses have not yet been identified”: Calisher et al. (2006), 539.