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Driven out of sewers by summer floods and an urban building boom, then nurtured by warmer winters and the leftovers of fast food, rats have been moving into homes, gardens and even cars around Britain.
Rat-catchers and companies selling poison and traps are reporting a boom in business, with urban housing estates among the worst affected areas.
Long-term growth in rodent populations is also blamed on a decline in "sewer-baiting", the practice of laying down poison twice a year to prevent rat numbers building up underground.
Because rats breed on average five times a year, with seven or eight in each litter, growth can be rapid. The recent surge in numbers has been linked to a boom in urban development - not least the preparations for the 2012 London Olympics - and last (northern) summer's floods, which drove rats out from underground, through holes and cracks in pipes and drains.
Rentokil, the UK's largest pest control company, said demand rose by more than a quarter last year as hits on its website trebled. Killgerm, the UK's biggest seller of rat poison, said sales rose by a quarter last year.
Rentokil estimates there are 65 million to 80 million rats in Britain, eating their way through 210 tonnes of food a year. This compares with an estimated 45 million to 50 million a decade ago, a rise of nearly 40 per cent, though the company admits such calculations are not an exact science.
The biggest increases appear to have been in the south of England, western Scotland and Northern Ireland; only East Anglia and the south Midlands reported a fall.
"It's a bit like crime statistics: it's difficult to tell whether the number of incidents has gone up, or if the reporting is more prevalent," said Rentokil's UK managing director, Jed Kenrick. But calls about rodents were significantly up on 12 months ago.
People for the Ethical Treatment of Animals says that rather than call in pest control it is more humane to avoid attracting rats in the first place by keeping food and buildings sealed.
"Rats are highly intelligent, social animals who excel at learning," said Poorva Joshipura, director of Peta Europe. "They do not want to die trying to gnaw their leg off in traps or slowly suffering from poison."
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