Super Rat - big, smart and immune to poison - alarms pest experts

A Plague of "super-rats", immune to traditional poisons and capable of eating their way through sewer pipes, is posing a serious health risk.

Rising in numbers each year, the streetwise rats are unfazed by human beings. They are also believed to carry more disease than previously thought.

A surge in the rat population, which is up by almost 40 per cent on 1970 figures according to the latest Government research, means there are now about 60 to 70 million in the UK.

Pest control experts blame the failure to develop new poisons and widespread installation of cheaper, inefficient drainage pipes which rats can chew through. Some also blame poor co-ordination between local councils and the water companies, compared with the closer working relationship in pre-privatisation days. Water companies complain that councils don't notify them about infestations; councils in turn allege the utilities are failing to bait sewers regularly.

In addition, warmer winters and the rising number of fast food outlets - generating more food waste in the streets - are making it easier for rats to survive

Tony Stephens of Rentokil has warned that new poisons are not being developed quickly enough and that there is a real danger that so-called super-rats could become the norm, rather than the exception.

"Rats are definitely picking up a natural resistance to some of the traditional poisons and they will continue to unless new rodenticides are developed."

Sue Dow, zoologist and rat expert, believes the number of rats that actually become immune to poison is low. But, she says, "rats have got a very crafty way of dealing with poison, so they can be quite hard to kill. A wild rat will taste a small amount of food and then go away for a while. If it's sick then it won't go back to the food, but if after a while it is still ok, it will go and eat the rest."

One of the most dangerous diseases communicable from rats is Weil's disease which can cause jaundice, renal failure, even death. Rats can also carry salmonella, listeria, and Lyme disease.

Stephen Battersby, of the Robens Centre for Public and Environmental Health, believes the threat to human health is now serious particularly from infestations above ground, which he believes are caused by inadequate underground drainage and the use of plastic piping.

If that were not alarming enough, in her research into rural rats, Dr Joanne Webster of Oxford University has found some fatal diseases previously considered to be extremely rare were occurring much more frequently than expected, including Q fever and Hantaan fever.