Spillover: Animal Infections and the Next Human Pandemic
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Nearly all zoonotic diseases result from infection by one of six kinds of pathogen: viruses, bacteria, fungi, protists (a group of small, complex creatures such as amoebae, formerly but misleadingly known as protozoans), prions, and worms.
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Economically, it is the most important disease of animals in the world,” according to one authority, who reports that “an FMD outbreak in the US could cost $27 billion in lost trade and markets.”
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One experimental study found that pig breath carried thirty times as much FMD virus as the breath of an infected cow or sheep, and that once airborne it could spread for miles. That’s why pigs are considered an amplifier host of this virus.
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Make no mistake, they are connected, these disease outbreaks coming one after another. And they are not simply happening to us; they represent the unintended results of things we are doing. They reflect the convergence of two forms of crisis on our planet. The first crisis is ecological, the second is medical.
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Karl Johnson
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The Hot Zone, Richard Preston’s account of a 1989 outbreak of an Ebola-like virus among captive Asian monkeys at a lab-animal quarantine facility in suburban Reston, just across the Potomac from Washington, DC.
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Zoonoses by definition involve events beyond the ordinary, and the scope of their consequences can be extraordinary too. Every spillover is like a sweepstakes ticket, bought by the pathogen, for the prize of a new and more grandiose existence. It’s a long-shot chance to transcend the dead end. To go where it hasn’t gone and be what it hasn’t been. Sometimes the bettor wins big.
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Every year and a half came a big measles wave. The logic of such cycles, Hamer suspected, was that an outbreak declined whenever there weren’t enough susceptible (nonimmune) people left in the population to fuel it, and that another outbreak began as soon as new births had supplied a sufficient number of new victims.
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Wagner-Juaregg solved that problem after noting that Treponema pallidum didn’t survive in a test tube at temperatures much above 98.6 degrees Fahrenheit. Raise the blood temperature of the infected person a few degrees, he realized, and you might cook the bacterium to death. So he began inoculating patients with Plasmodium vivax.
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One thought that turns up in the latter sort is that “humankind has had a lucky escape.” The scenario could have been very much worse. SARS in 2003 was an outbreak, not a global pandemic. Eight thousand cases are relatively few, for such an explosive infection; 774 people died, not 7 million.
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The much darker story remains to be told, probably not about this virus but about another. When the Next Big One comes, we can guess, it will likely conform to the same perverse pattern, high infectivity preceding notable symptoms. That will help it to move through cities and airports like an angel of death.
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Acarologists (tick biologists) have a wonderfully high-flown term for the behavior by which a tick seeks its next attachment, climbing to the top of a grass stem or out to the edge of a leaf, front legs extended, sniffing the signals, positioned to grab a new host; the word is “questing.”
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Four kinds of small mammal fueled nine-tenths of the disease-bearing ticks.
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the first operational definition of viruses: infectious but “filterable,” meaning so small they would pass through where bacteria wouldn’t.
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Viruses, from the beginning of virology, have been defined in the negative (not captured by a filter, not cultivable in chemical nutrients, not quite alive), and the most fundamental negative axiom is that a virion is not a cell.
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RNA viruses therefore evolve quicker than perhaps any other class of organism on Earth. It’s what makes them so volatile, unpredictable, and pesky.
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Evolution lowers virulence, tending toward that “more perfect mutual tolerance” between pathogen and host.
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Such viruses try to outrace the immune system of each host, taking what they need and moving onward before a body’s defenses can defeat them.
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RNA viruses, thus constrained, compensate for their error-prone replication by producing huge populations and achieving transmission early and often.
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Habitat disturbance, bushmeat hunting, the exposure of humans to unfamiliar viruses that lurk in animal hosts—that’s ecology. Those things happen between humans and other kinds of organism, and are viewed in the moment. Rates of replication and mutation of an RNA virus, differential success for different strains of the virus, adaptation of the virus to a new host—that’s evolution. It happens within a population of some organism, as the population responds to its environment over time. Among the most important things to remember about evolution—and about its primary mechanism, natural ...more
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Bats come in many, many forms. The order Chiroptera (the “hand-wing” creatures) encompasses 1,116 species, which amounts to 25 percent of all the recognized species of mammals. To say again: One in every four species of mammal is a bat.
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Bats probably meet the critical community size standard more consistently than most other mammals. Their communities are often huge and usually large, offering a steady supply of susceptible newborns to become infected and maintain the viral presence.
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So those large aggregations—comprising bats that are more sedentary, more urban, less needful of flying long distances in search of wild food—tend to reinfect one another less frequently? And in the interim they accumulate more susceptible individuals? So when the virus does arrive, the spread of new infections is more sudden and intense? The virus is more prevalent and abundant? “Exactly.
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That the AIDS pandemic is traceable to a single contingent event. That this event involved a bloody interaction between one chimpanzee and one human. That it occurred in southeastern Cameroon, around the year 1908, give or take. That it led to the proliferation of one strain of virus, now known as HIV-1 group M. That this virus was probably lethal in chimpanzees before the spillover occurred, and that it was certainly lethal in humans afterward. That from southeastern Cameroon it must have traveled downriver, along the Sangha and then the Congo, to Brazzaville and Léopoldville. That from those ...more