The Risks of Global Worming
June 8, 2012

For decades, the overuse of antibiotics has encouraged the evolution of drug-resistant bacteria, which, though they have never broken out and caused an epidemic in the way that was once feared, have nevertheless been responsible for many deaths that might otherwise have been avoided. Now something similar seems to be happening in agriculture. The overuse of drugs against parasitic worms, which infest stock animals means that these, too, are becoming drug-resistant. That is bad for the animals' health and welfare, and equally bad for farmers' profits. 
This, at least, is the conclusion drawn by Ray Kaplan, Ph.D., a parasitologist at the University of Georgia who has just published a review of research on the problem. His results, which appear in Veterinary Parasitology, make grim reading. 
Sheep and goats are the worst affected. Studies in Australia, Brazil and the United States suggest that animals in half or more of farms in many parts of these countries are infested with drug-resistant worms. In some cases, the parasites are resistant to every drug that can be thrown at them. 
Cattle, too, are afflicted. Kaplan cites work done in Argentina, Brazil and New Zealand. And horses suffer as well, with resistant worms turning up in both America and Europe. 
The root of the problem is what Kaplan refers to as "global worming"--giving drugs prophylactically to all livestock rather than reserving them for use as a treatment when an animal actually becomes infested. It is common sense, of course, to try to prevent infestation rather than merely treating it once it has arisen. Unfortunately, such promiscuous use of drugs is the best way to put selection pressure on the worms and encourage the evolution of resistant strains. 
What is needed, says Kaplan, is more selective drug use and better management. Worms are not evenly distributed. Instead, a minority of animals play host to most of them. Aiming treatment at those animals would reduce the likelihood of resistance emerging without harming a farmer's ability to control infestations. Better husbandry might help, too. Not grazing so many animals on a given patch of land would discourage transmission. 
No one farmer is to blame. This is a tragedy of the commons, in which sensible individual decisions have led to a collective difficulty. But it might behove farmers to think more about how they use anti-worm drugs. If they do not, they may find that those drugs have become useless. 
Reprinted in part from The Economist