The fix for Britain’s giant rat problem? Rodent birth control

With rat poison and glue traps on their way out, rodent contraception looks set to be the next weapon against rats on UK streets.

Several drugs that stop rats from giving birth are in development and could be in use within 10 years.

Britain has a growing rat problem, thanks to increased food waste on the streets and milder winters caused by climate change. But killing the disease-ridden pests is getting tougher, as British rats are increasingly resistant to the poisons usually used to kill them.

In Birmingham, where rubbish collection has been reduced for a year due to ongoing strikes, residents report seeing rats as big as rabbits roaming the streets.

And a giant rat measuring 22 inches from nose to tail, the size of a small cat, was found in a Yorkshire home last year.

The number of these poison-resistant “super rats” is highest in the south and north-west of England, and in Glasgow and Edinburgh, according to the trade body Rodenticide Resistance Action Committee.

Even where rats are susceptible, the poison is dangerous to use around pets or children, and there are concerns that it then kills birds of prey. “Endangered species like barn owls and red kites can eat the bodies of poisoned rats, and die themselves,” said Richard Parr, director of the Center for Wild Animal Welfare.

A build-up of rat poison contributed to the death of Flaco, an eagle owl that escaped from a New York zoo and briefly lived in Central Park in 2024. “Everybody is worried about poisoning owls,” said Professor Giovanna Massei, European director of the Botstiber Institute for Wildlife Fertility Control at the University of York.

Physical traps, designed to snap down on the animal’s back, can break people’s fingers, and so are also dangerous around pets and children.

Glue traps, which work by trapping the rodent on a board, are also restricted on cruelty grounds, as the trapped rodent may take days to die. They have been banned in Wales, and a ban is due to come into effect in July in Scotland. In England, homeowners cannot use them; only licensed pest controllers can, and they are supposed to check the traps frequently.

There is also growing “behavioural resistance” to traps, where rats learn to avoid them, said Neill Gallagher of the British Pest Control Association. “Some will actively run over bait boxes,” he said. “They’ll just run straight over the top.”

Some pest controllers are even using terriers to kill rats in places where other methods would be impractical.

Using contraception in the war on rats might seem far-fetched, but scientists at the 10th International Conference on Wildlife Fertility Control, held in Barcelona next month, will hear about several promising research avenues.

One such product, based on hormones similar to the contraceptive pill for humans, has been shown to work in maize farms in Tanzania, where it reduced rodent numbers long-term.

Field trials showed that spreading the contraceptive bait, called EP-1, worked as well as poisoned bait in terms of reducing rat numbers and maize loss, said researcher Professor Steven Belmain, an ecologist at the University of Greenwich.

Hormones in rivers

However, while the product has been approved for use in Tanzania, it is unclear if it would get the go-ahead in the UK and Europe because of increasing concerns that hormones in the environment could affect wildlife and human health.

There are already concerns about sex hormones getting into rivers and lakes due to their presence in urine from women on the pill, said Belmain.

An alternative contraception involves vaccinating animals so that their immune cells attack parts of their reproductive system, for instance, proteins on sperm or eggs or a reproductive hormone called GnRH.

GnRH immunisation is already used to control wild horse populations in the US. However, the horses are injected with the vaccine individually, which would be impractical with rats. So, a method has been developed to vaccinate rodents using bait containing a genetically altered virus that triggers an immune response.

The reaction only happens in rats that eat the bait – it does not spread between animals like an infection. “This could overcome all these problems with non-target poisoning and worries about the environment,” said Belmain.

Triggering menopause in rats

Another product being discussed at the conference is called Ratbegone, which causes female rats to go into early menopause and become permanently sterile.

The developer, Professor Rudi Marquez-Mazlin at the University of Canterbury in New Zealand, is waiting until the compound’s patents have been issued before he releases the name of the chemical, but has said it degrades quickly in the environment.

In trials so far, all of the rats treated with it either could not get pregnant or had pups that died soon after birth.

“We’re really confident that it will stop fertility and create sterilisation in possums and rats,” he told New Zealand publication Farmers Weekly last year.

If effective rodent contraceptives do succeed, they could have wider benefits, including in the long-standing UK campaign to reduce grey squirrels, so that red squirrels can return to more areas.

The UK Government’s Animal and Plant Health Agency is trying to develop one such squirrel contraceptive, which could be put in bait boxes that only the heavier greys could access.

But whatever happens with rodent contraception, we will still need options for killing rats that have found their way into a home or restaurant, said Massei. “Even one rat is in your house is one too many,” she said.