While medical researchers fret about disease-causing microbes unbowed by antibiotics, a less-celebrated plague of resistance is spreading across the English countryside. An emerging population of rats can eat outdoor rodent pesticides with no ill effects--except perhaps on the hapless foxes, owls, or other predators that snack on them. Environmental biologist Robert Smith of the University of Leicester describes the hardy rats as "Parcels of poison with four legs and a tail."
The rats are unaffected by "second-generation" toxins, anticoagulants that kill by causing internal bleeding, says Smith. Rats worldwide evolved tolerance to an earlier anticoagulant, warfarin, during the 1960s. Helen MacVicker, a graduate student working with Smith, found that rats in two southern English counties can store five times as much of the newer toxins in their tissues as their susceptible relatives elsewhere. "On one chicken farm, I went out toward sunset one day and the ground was covered in rats," says Smith.
"I have not heard any verifiable reports of rat resistance to second-generation compounds in the United States, but I wouldn't be at all surprised to hear from somebody who did find this," says ecologist and rat expert William Jackson of Bowling Green State University in Ohio. "Sooner or later, it's going to come."