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The cats wouldn't tolerate a younger cat in the household. I've tried encouraging a neighbour's young cat to the shed area and my cats go berserk when they see him.
VHG, in these austere times, I'm afraid my local council has suspended the rodent extermination service indefinitely. I understand that even council tenants in my area now have to pay private companies out of their own pockets to get rid of rats as councils are no longer obliged to carry out this service.
As far as harm is concerned, apart from the natural revulsion to rats that most people have and their scavenging around in my bins, probably not. However, I am concerned that a young rat could bite one of the elderly cats if cornered with the risk of Weil's disease etc. Besides, given that they can apparently have half a dozen litters per year, I don't fancy being overrun with rats in a very short space of time. One of the cats a few years ago brought a rat into the house through the cat flap early one morning and the lounge looked like The Texas Chain Saw Massacre had occurred there when I got up - I'd really not like to see that again.