More than 70 percent of rats in western England have developed “mutant” DNA, making them resistant to conventional poisons, research by University of Huddesfield’s School of Applied Sciences has found.

The study’s main objective was locating the areas of the country with the largest number of genetically altered rodents immune to the most commonly used poisons.

ScienceDaily reported, “The highest prevalence has so far been found in certain areas of South of England and West Country where greater than 70% [of] the animals tested are of the ‘super’ rat type.”

Lead researcher Dr. Dougie Clarke told BBC News, “Some pest control companies are still using the same rodenticides—Bromadiolone and Difenacoum—in areas where there are resistant rats and things need to change. We’re wiping out the normal susceptible rats and we’re going to be left with resistant rats only in this area so alternatives have to be found so we’re not using these poisons.”

The rats have developed this immunity due to incorrect, or insufficient, application of the poisons. If a correct lethal dose is not administered, rodents survive and develop immunity.

According to ScienceDaily, only one solution remains: stronger poisons.

“In order to combat the problem there are stronger poisons—such as brodifacoum and flocoumafen—that can be used and they have proved to be effective, even against the so-called super rats. But these rodenticides have to be used in strictly controlled conditions, under licence from the Health and Safety Executive.”