OXFORDSHIRE has been infested by a new breed of mutant rat resistant to all legal poisons – and the problem seems to be getting worse.

The mutant gene which first emerged on a farm north of Newbury has spread as far north as Wallingford, according to a new study.

Colin Prescott of Reading University co-authored the study by the Rodenticide Resistance Action Group, revealing the spread of this gene, which is making rats resistant to traditional poisons.

The study found that eight out of nine rats sampled from Wallingford and the surrounding area were fully resistant.

Oxfordshire pest controller Rob Eckton said he is getting more calls to deal with rats than ever before. He mostly deals with rats using poison but sometimes shoots them.

He said “There are currently more resistant rats.

“I was called to one Oxfordshire farm where the rats managed to eat 30kg of poisoned bait and we still shot over 200 – and I’m talking about big rats.

“This problem is definitely getting bigger. Last year, as a private contractor I laid 500kg of bait. Previous years have been nowhere near that.

“Especially in the last three weeks as we come into rat season, it’s been crazy.”

In the UK, there are only about eight poisons which can be used outside, because of the risk of secondary poisoning to birds of prey which eat rats.

Indoors, licensed pest controllers like Mr Eckton can use the more powerful brodifacoum, which is four times as effective, or deadly contact dust, 1,000 times more powerful than bait poisons.

Now the Health and Safety Executive is looking at allowing outdoor use of the more potent poisons.

Mr Prescott said: “Rats are geared up for massive explosive growth, and that is the whole problem with the spread of this gene.

“By poisoning the rats you might kill off 50 per cent of the population, none of which have the gene for resistance.

“The population will quickly return to full size but then all the rats will have the gene for resistance.”

The gene first mutated on a farm north of Newbury in the 1990s. Mr Prescott was called in for his expertise in rodent control, and was flummoxed by the new gene.

“One time, we cut open a 400g rat and found by using bait markers that he had eaten 450g of poisoned bait.”

He estimated that by using brodifacoum the mutant gene could be wiped out in six to eight weeks.

Alan Buckle, Mr Prescott’s co-author on the paper, said: “People keep using these poisons and it is piling up in the food chain, which is damaging for the environment, but you can use pretty small quantities of stronger poisons more usefully.”

The HSE is hoping to publish the results of its decision on whether to allow the use of stronger poisons out of doors early next year.

A 2010-11 survey by the British Pest Control Association revealed that the South Oxfordshire district has a human population of 131,000, and an estimated 786 rats, six for every 1,000 people.