Science: Last Licks

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In the legends of Eastern Europe, the vampire took many horrendous forms, but south of the Rio Grande vampiro means just one thing: a tiny bat that sucks the blood of humans and animals and carries rabies, the deadliest of infectious diseases. Despite its minuscule proportions—an adult may weigh as little as one-half ounce and seldom more than 1½ ounces—the common vampire has made it economically impractical to raise cattle or horses in large areas from central Mexico to central Argentina. Efforts to destroy Desmodus rotundus by such crude methods as dynamiting or using flamethrowers in his cave roosts have proved too costly, inefficient, and disastrous for neighboring populations of beneficial, insect-eating bats.

Now, after years of dangerous work at inhospitable field stations, investigators from the U.S. and Mexico have developed techniques that promise to vanquish the vampire. With ghoulish justice, the little beast that lives by blood will be made to die by it.

Two quirks of nature promise to be the vampires' undoing. One is the fact that bats, like rats, are more sensitive than most mammals to the hemorrhagic properties of anticoagulants. These are the chemicals used medicinally to protect human victims of heart attacks and strokes against the recurrence of dangerous blood clotting; overdoses can cause fatal internal bleeding. The best known anticoagulant, warfarin, is used in calculated overdoses as a rat poison. In 1968 a two-nation team began work at the National Livestock Research Institute in Mexico City and the U.S. Department of the Interior's Wildlife Research Center in Denver to try to kill bats with an anticoagulant. Choosing the poison—diphenadione—was one thing. But how to get the stuff into the vampires?

One way would be to inject the anticoagulant into the stomachs of cattle, from which it would pass into their bloodstreams. The dose would not be enough to harm the large animals, but any blood-sucking vampire would get enough to kill it.

Then a second quirk of nature suggested another method. Like cats, vampires lick themselves to clean their reddish-brown fur, and they are as clubby as monkeys, eagerly grooming each other. One researcher reasoned that it would be effective to catch a few vampires, daub them with diphenadione, then release them to return home and bleed to death—and incidentally poison their grooming partners.

To catch the vampires for their fatal treatment, the bat killers suspend, above the fence of a cattle corral, a Japanese nylon mist net, as fine as a lady's "invisible" hair net. The net is invisible in the dark when the bats sortie and, more important, its fine threads give back no detectable echo for the vampire's sonar system. When a bat is netted, a technician wearing tough leather gloves carefully removes it from the net and rubs its back with half a teaspoonful of petroleum jelly laced with 50 milligrams of diphenadione.

Well Groomed. Shocked, the bat returns to his roost in a cave, hollow tree or old building, and licks as much of the goo off his back as he can. In the process he poisons himself fatally. Other vampires come to help groom him, and so poison themselves. A single smeared bat has been found to cause, on the average, the death of 20 others, sometimes as many as 30.