Science: Get ready for Son of Super Rat: The National Rodent Survey is about to be released, and the news is not good. Charles Walton explains

Something crept into my bedroom the other night. It rustled a discarded crisp packet and sent my wife scurrying for the safety of the children's beds. They, at least, were two feet off the floor. I slept on uneasily, conscious that only three inches of futon separated me from the scavenging mouse.

Ours was an all-too-frequent visitor to British homes. Roughly one in 20 households was believed to harbour mice when the last comprehensive survey was undertaken in the Seventies. More recent reports suggest that rodents, particularly rats, are on the increase. A definitive answer will soon be with us, courtesy of the National Rodent Survey. Fifteen months of peering into the nation's larders came to an end last December, and the results are expected this month.

The environmental health officers who are conducting the survey will be looking for exactly the kind of evidence that we found the morning after our nocturnal visit. Mice leave about 80-odd droppings a day. These rice grain-sized black spindles were spread around the house, often accompanied by the sweet and depressing smell that signals a mouse's territory.

Rats and mice owe their success to an astonishing fecundity - a breeding pair of mice can generate 2,000 offspring in a year - and to man's propensity to dump refuse on his own doorstep. Not until the introduction of efficient public sanitation were the plagues of rats that besieged earlier generations eradicated. Ironically, it is those same sewers that now harbour the rodent population. Constant vigilance is required to keep numbers in check, yet the water authorities who are responsible are regularly accused of cutting funds for sewer baiting.

Rodents present a serious public health risk. Rats are still recognised carriers of bubonic plague and were responsible for an outbreak during the Vietnam war. The last British case of plague was in the early 1900s. Another potentially fatal and increasingly common health problem is Weil's disease. Water sports enthusiasts are particularly at risk since the bacteria responsible find their way into water via rat urine.

Rats and mice carry other diseases such as murine typhus, are major destroyers and contaminators of food and have been known to attack children. They also take great pride in their teeth, gnawing continually to keep them short and sharp. When gas pipes and electric cables are involved the consequences can be disastrous. The arguments for reducing rodent numbers are strong.

When all else fails we resort to chemicals. Rodent pesticides are not permitted to cause pain or suffering, so the chemical industry has adapted to delivering death with a smile. The most common of these gentle killers are anticoagulant poisons. The first generation of anticoagulants worked well until the targets became resistant to them.

This development resulted in a host of newspaper headlines proclaiming the arrival of Super Rat and the demise of civilisation. In the event, civilisation scraped through, but resistance became so widespread that a second generation of anticoagulants was introduced. Can we expect to see resistance developing again? Peter Bateman, of Rentokil, believes so. 'Though still rare, Super Rat Mark II is already with us,' he claims.

The onward march of rodent populations has been well documented by the press. In 1989, the Institution of Environmental Health Officers published a report claiming that rat numbers in England and Wales had increased by 20 per cent. Various explanations were offered: a succession of mild winters, cutbacks in the funding for environmental health departments, changes in agricultural methods and increased levels of urban refuse and litter.

Two groups came in for particular criticism: the water authorities for giving a low priority to sewer baiting, and British Rail for an unwillingness to clean up its property or co-operate with local pest control officers.

So what is happening to rodent numbers in the Nineties? The co-ordinator of the National Rodent Survey, Dr Adrian Meyer, of the government's Central Science Laboratory, says: 'Scare stories appear periodically, but nobody knows what the real situation is.'

Rodent surveys that rely purely on reported infestations are known to be unreliable in London since fewer than 10 per cent of mouse and less than 30 per cent of rat infestations are reported to local authorities. The National Rodent Survey looks instead at households selected at random from council tax registers. This allows a direct comparison with a similar survey conducted during the Seventies. In addition, the survey will identify differing treatment methods and highlight regional variations in the allocation of responsibility for rodent control.

The headline numbers may not mean much in themselves. Rodent populations rise and fall under the influence of many factors, the greatest of which is the weather. Cold winters reduce the population, mild ones bring rapid increases.

Another round of shock newspaper stories will at least concentrate minds on rodents. If that unlocks funding for prevention and persuades the nation to change its habits, the survey will have done a worthy job. Jackie Marsh, of the Institution of Environmental Health Officers, says: 'Everyone has a double responsibility: to take care not to encourage rodents and to report

infestations immediately.'

(Photograph omitted)