They could have found their way into the forest before the extermination took place," the defence secretary suggested.
"It's possible, although the previous attacks suggested they were confined to certain areas of the city," said Lehmann. The other possibility is that they were somehow unaffected by the ultrasonic sound waves we used to draw the rats from their nests into the gas enclosures, and fled afterwards when they realized the game was up.
Nowadays the machines are used to drive the vermin away, not draw them in; but either way, our experiments with them at the Ratkill laboratories show that the ultrasonics become ineffectual eventually; the rats adapt, learn to ignore them."
"I must point out here," said Howard, 'that tests are still in progress with these machines. I think we can develop one that will be extremely effective once we find the correct wavelength or indeed, wavelengths."
To do that, we'd need a mutant rat itself. Our own over reaction killed them all off four years ago apart from the few that obviously escaped. We'd have been wiser to have saved some for study."
"Surely," said the defence secretary, 'you can experiment on ordinary rats?"
We've been doing just that," the biologist replied. "Unfortunately, the giant Black is no ordinary rodent: it's a mutation, its genes are different. They're not just bigger and stronger, they also have a high degree of intelligence. They'd need it, to have remained hidden these past few years. Of course, the fact that rats are nocturnal has helped; but what puzzles me is why there's been no evidence of them until now. Even more puzzling and, I may say, more ominous: why now?
"My guess is that after the mass destruction of their breed, the survivors developed an even stronger fear of man, which was passed on to the following generations. We already know of their abnormal brain-power. I'd say this has advanced with the new generations, too.
They've kept out of sight, foraged in places safe to them, left little evidence of their presence."
"It could be that there is just a small number of them," Whitney-Evans suggested hopefully.
Yes," agreed Major Cormack. "A small group would be hard to detect in a forest full of wild animals."
"It's unlikely," said Lehmann. The life-span of a rat is from fifteen months to two-and-a-half years; the female can have five to eight litters a year with as many as twelve new-born in each litter. She's ready again for mating within hours of giving birth, and the young ones reach the reproductive stage after only three months. You can figure out for yourself just how many could be bred in the space of four years."
Fender could almost hear the clicking of mental arithmetic going on around the hall.
"I think there's plenty of them," Lehmann continued, 'but they've gone literally underground. I believe they're in the sewer network beneath the forest; that's where we'll look for them. The perverse thing is that the normal Black rat, or Ship rat as it's sometimes known, is arboreal it can climb trees, high buildings; the mutant has been forced to live below ground. It could explain why they dug up the corpse at the church: they've learned to be burrowers."
"But that's impossible," Milton began to say. "It would take decades for them to evolve ..."
"For any normal animal, yes," the biologist cut in. We're dealing with the abnormal."
Thornton spoke. "So your recommendation is to tackle them at their source: the sewers."
Lehmann nodded. "If they're there. We'll pump gas into the network, using a proprietary powder that produces hydrogen cyanide gas when it comes into contact with damp soil or damp air. Our main problem other than attack from the rats themselves will be to block all holes leading from the sewers."
"I'm afraid many of the sewers have overflowed into some of the streams," said Whitney-Evans. "We've complained to the local authorities often enough."
Those outlets will have to be plugged. We'll need the help of your forestry staff to locate them and any other outlets from the sewers."
"Perhaps we can help too," said Milton. "My staff at the Centre know the forest like the backs of their hands."
"Fine, the more, the merrier."
Why not use rodenticides?" the defence secretary asked.
That could be our biggest problem, I'm afraid," Lehmann said grimly.
There are two main types we could use. One is of the single dose variety: sodium fluoroacetate and fluoro-acetamide, which is normally used in sewers; zinc phosphide;
nor bromide which is harmless to most other animals; arsenious oxide, which is dangerous to most other animals; alpha-chloralose, normally used only against mice. The big disadvantage with these is that rats have a built-in instinct against anything strange to them. We call it neophobia new object avoidance. It makes it difficult to get them to accept new bait. They might try it after a while, but only in small amounts. If they feel any ill-effects at all, they leave it alone completely. A single dose poison might just kill a few, but even that would serve as a warning to the others."
"And the other type of poison?" the defence secretary asked.
The others are anticoagulants. They kill by their reaction on the rodent's blood system: they interfere with a substance called prothrombin which causes the blood to clot when vessels are broken. The rat suffers a haemorrhage at the slightest damage to blood capillaries: a tiny scratch can kill it. Females having litters are obviously very susceptible.
Three kinds are in current use: Warfarin, coumatetralyl and chlorophacinone. They're administered gradually, building up to a lethal dosage. The rat gets used to the bait, feeds on it regularly, then suffers the effects."
"And all this takes time," said Whitney-Evans.
"Yes, but the process can be speeded up. However, that isn't our problem. Over the past few years, rodents in this country have been building up a resistance against anticoagulants. It began in a couple of countries on the Continent, now it's spreading over here. Luke Fender, there, has just returned from the North where he's been investigating the matter. Luke?"
The resistance was first noted in Wales and the Midlands, but now it's spread as far up as Cheshire and down to the south-west coast," Fender told them. We've bred Warfarin resistant rats in our own laboratories, but these others have developed their own immunity. The point is this: the Outbreak rats had developed that same immunity before gas was used as the final solution. It seems likely that resistance will be inherent in those descended from the rats that escaped from London.
That's why I agree with Mike: gas, providing we can trap them in the sewers, has to be the answer. If the machines can't be relied on to lure them out, we have to keep them in and destroy them there."
"I think we're all agreed, then," said Thornton. "Gas it shall be.
Gentlemen?" he asked the room at large. A murmur of assent was given.
A councillor raised his hand. "What about disease from these rodents?
How will we combat that?"
"I don't think we need worry ourselves about that problem at the moment," Stephen Howard said smoothly. The disease caused by the vermin at the time of the Outbreak was a particularly hideous distortion of Leptospirosis or Spirochaetal Jaundice. Fever first, before jaundice set in. The victim became prostrate, blind, then all senses were lost. Coma, then the skin began to stretch and tear, and the victim died. The horrifying thing is that the whole process took only twenty-four hours. Fortunately, an anti-toxin was soon produced, so we needn't fear the disease any more. The other, more normal rodent diseases are too minor nowadays to worry about. No, the main danger it would seem is attack from the beast itself. Of course, everyone "out in the field" as it were will be wearing protective suits." Howard reached behind his chair and drew out a large, mounted photograph of a dead mutant Black rat. "At this stage, I think it might be an idea to remind ourselves just what our old enemy looks like." He stood, resting the photograph's base against the tabletop so everyone could see.