Science
A cunning technique
Bernard Dixon
The scientists who work for the US Bureau of Sport, Fisheries and Wildlife are an ingenious crowd. Their latest brainchild, a discovery of devilish cunning, is a method of killing vampire bats by injecting a particular poison into cattle, which are comparatively resistant to its ill-effects. The bats attack the cattle, the cattle-blood they drink gives them a lethal dose of poison, and the eventual loser (as well as the bats) is the rabies virus, a very nasty microbe indeed.
This novel development, tested recently at three ranches in the state of San Luis Potosi, Mexico, is the outcome of a research programme initiated in 1968 by the Bureau in collaboration with the government of Mexico and the US Agency for International Development. Vampire bats feed exclusively on the blood of live vertebrates, and they are the major scourge of animal health in Latin America. Losses attributed to the rabies virus they carry (which kills a million head of cattle each year), plus the malnutrition, loss of blood, and infestation by fly larvae which result from vampire bat attacks, cost the Latin American cattle industry up to 250 million dollars annually. Before 1968, the only methods employed to attack the vampire bat population were gassing, poisoning, shooting, netting, trapping, and dynamiting, but none of these were consistently successful. Hence the search for alternatives.
One of the first fruits of the research programme was a method of inducing bats to carry poison back to their roost, where they distributed it among other members of the colony. Pest control officers netted bats found near cattle and painted them with petroleum jelly containing a slowacting poison. Hours later, after the toxic jelly had been spread around and consumed during grooming, the poison took Its toll. Though effective, there were two objections to this technique. First, any physical contact between the bats and members of the control team is hazardous; bats can pass rabies to man as well as to Cattle. Second, on ecological and other grounds mass poisoning of entire populations is risky; on animal health grounds, it is unnecessary. The new technique is highly specific and much safer. The control team do not contact their victims directly and the only bats killed are those which attack cattle. An anticoagulant, which prevents blood from clotting and is lethal in large quantities, is used as the poison. Cattle are relatively insensitive to its toxic effects, but bats are very much more so.
First results with this cunning technique, reported recently in Science (vol 177, p 806) by Dr Dan Thompson and his colleagues in Denver and Palo Alto, show that after the toxic injections, the blood of beef cattle became poisonous to bats and remained so for at least three days without harming the animals themselves. The frequency of bat bites on cattle at the three ranches where the tests were conducted declined by 93 per cent, reflecting the deaths of the predatory bats. Moreover, there seems to be no danger that the poison may persist in the cattle, causing ill-effects later on. Examinations of beef calves slaughtered thirty days after being given large quantities of the poison showed that their livers — the main site of action of the poison — did not contain detectable quantities. Similarly; there was no risk of the poison being transmitted to calves through milk.
Specificity aside, the beauty of the new technique is that a known and effective antidote, vitamin K, is available to treat cases of accidental over-dosage. But can the word beauty really be used to describe such a thoroughly nasty business?
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