Both German and US scientists developed a poison about ten years ago that would wipe the Rat population out in 10 to 15 years, it was a liquid solution, the germans tried it on female rats and it brought their litters down from around 10 to 3 or 4, then they decided to try it on males, and mated them with the females, of which none had any litters, the poison made the males sterile, the plan then was that they could introduce it into all the sewers in the main cities all over the world, of course it was then that all the animal rights groups got involved, ones wanting to protect the Rats, but there were lots of other groups pointing out that killing the Rat population off would affect lots of other wildlife like Foxes, Badgers, birds of prey, animals that use rats as their main food source, plans to try and kill them off was then shelved, a friend of my sons bought a property on the outskirts of Belfast about 5 or 6 years ago, house needed a lot of work done to it, which they did, nearest neighbours were 100 yards either way, so a quiet peaceful life ,which they happily enjoyed for a few years, then a main sewer pipe that ran alongside their house burst, despite the department of environment working to fix the pipes, the rats were getting out and it wasnt long before they found themselves a new cosy home, he was sitting watching racing on the tv one day, when he heard his wife scream the place down, he ran out to find her outside their utility room while she was trying to do some washing, two rats were playing about the room, they bit their way through from outside where the water pipes were, they ended up killing well over a hundred in only a few months, paid money to ferret keepers, men with rat killing dogs, put all sorts of poison down, killed lots more, but in the end they just became overrun with them, his mrs and kids had moved in with relatives some time before he then gave up and took up the advice given to him by almost everyone, he had to admit defeat in end, they were just breeding like wildfire, the whole thing got him fame on the local news channels, then a half hour documentary slot on BBC N-Ireland.