Spillover: Animal Infections and the Next Human Pandemic Read Online Free PDF Page B

of human infection to be detected by Selvey’s study. Not a single antibody-positive person out of 128 carers. What did that tell you, I asked her, about the nature of this virus?
    “That it needed some sort of amplifier,” she said. She was alluding to the horse.
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    L et’s think about foot-and-mouth disease for a moment. Everybody has heard of it. Everybody has seen Hud. Most people aren’t aware that, at least tenuously, it’s a zoonosis. The virus that causes foot-and-mouth disease (FMD) belongs to the picornaviruses, the same group that includes poliovirus and some viruses similar to those that cause the common cold. But infection with FMD virus is a rare misfortune in humans, seldom causing worse than a rash on the hands, the feet, or the mouth lining. More frequently and consequentially, it afflicts cloven-hoofed domestic animals such as cattle, sheep, goats, and pigs. (Cloven-hoofed wildlife such as deer, elk, and antelope are also susceptible.) The main clinical signs are fever, lameness, and vesicles (little blisters) in the mouth, on the snout, on the feet. In a lactating female, the teats sometimes become blistered and then, as the blisters break, ulcerated. Bad for the mother, bad for the calf. Lethality from FMD is relatively low but the morbidity (incidence of the disease within a population) tends to be high, meaning that the disease is very contagious, making livestock ill, putting them off their feed, and causing losses of productivity that, in big-volume operations with narrow profit margins, are considered disastrous. Because of such losses, plus the swiftness of contagion, it’s often treated as a terminal condition in commercial terms: Infected herds are slaughtered to prevent the virus from getting around. Nobody wants to buy stock that might be carriers, and the export trade drops to zilch. Cows, sheep, and pigs become worthless—less than worthless, an expensive liability. “ 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.” The virus spreads through direct contact, and in feces, and in milk, and is even capable of transmission by aerosol. It can travel from one farm to another on a humid breeze.
    Impacts of FMD differ from one kind of animal to another. Sheep tend to carry the infection without showing symptoms. Cattle suffer openly and pass the virus to one another by direct contact (say, muzzle to muzzle) or vertically (cow to calf) by suckling. Pigs are special: They excrete far more of the virus than other livestock, and over a longer period of time, broadcasting it prodigiously in their respiratory exhalations. They sneeze it, they chuff it, they oink it, they wheeze it and burp it and cough it into the air. 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.
    An amplifier host is a creature in which a virus or other pathogen replicates—and from which it spews—with extraordinary abundance. Some aspect of the host’s physiology, or its immune system, or its particular history of interaction with the bug, or who knows what, accounts for this especially hospitable role. The amplifier host becomes an intermediate link between a reservoir host and some other unfortunate animal, some other sort of victim—a victim requiring higher doses or closer contact before the infection can take hold. You can understand this in terms of thresholds. The amplifier host has a relatively low threshold for becoming infected, yet it produces a vast output of virus, vast enough to overcome the higher threshold in another animal.
    Not every zoonotic pathogen requires an amplifier host for successful infection of humans, but some evidently do. Which ones, and how does