According to a university study, large swathes of Britain's rat population are now immune to commonly used poisons.
A scientist at the University of Huddersfield has warned that, in certain parts of England at least, 100% of rats that were tested have mutated genetically and are no longer affected by popular rodenticides such as bromadiolone and difenacoum.
"They do have a super resistance to these poisons," Dr. Dougie Clarke, head of biological sciences at the University of Huddersfield, told ABC News. "If you gave these poisons to humans, they'd die."
Cases of resistance in rats have been known since the 1950s but Clarke's study - which was funded by leading European pest control companies, the British Pest Control Organisation and the National Pest Technicians Association, and began around three years ago - intends to document where the highest concentration of resistant rats can be found. The study, due to be completed in spring of next year, has revealed that in the English counties of Berkshire and Hampshire, all of the 100 rats tested were shown to be resistant. It's a finding that Clarke admits he's been surprised by.
So far his team have tested around 200 rats and he concedes he will fall short of his target of examining 600 rats around mainland Britain.
For Clarke, the issue of poison resistance is not just confined to a certain species of rat in the U.K. either. Clarke says that although he's mostly looked at brown rats that dominate mainland Britain, there have been cases of resistant rats globally, including the United States.
The gestation period of rats is around 22 days and they reach sexual maturity by 8 to 12 weeks. That means resistant rats have the ability to multiply rapidly.
While the British Pest Control Association admits that the resistant rat population is spreading, they also have the tools available to handle these resilient rodents.
"We have the products to solve this problem," Richard Moseley, technical manager at the British Pest Control Association, told ABC News.
Some stronger rodenticides, like brodifacoum and flocoumafen, are already used by pest control companies. But the usage is limited to inside buildings only because of the potential harm these more potent poisons can have on wildlife.
Moseley accepts the dangers but feels that only using these effective poisons indoors is a waste as "rat infestations are rarely confined to buildings alone."
While admitting a balance needs to be struck, he says rats still pose a very serious threat to humans, not purely through the diseases they carry but by chewing through pipes and electricity cables.
Clarke agrees that if these more potent poisons are used responsibly as part of a concentrated "short, sharp, shock," then rat populations can be effectively controlled without causing damage to the environment.
Jeff Knott, senior species policies officer with the Royal Society for the Protection of Birds, disagrees. He argues that using stronger poisons "ends up in a toxic arms race we can never win."
According to Knott, birds of prey such as barn owls and red kites are most at risk. Through "secondary poisoning," Knott says that ingesting these rats has the ability to wipe out an entire nest.
While acknowledging that rat control is necessary, he says that "prevention is always better than a cure." In New York a "smoothie" substance is being tested, with funding from the National Institutes of Health, which acts as birth control by speeding up menopause in the females for the subway's vast rat population. As yet, its effectiveness has not been proven.