Dougal's right on the toxo risk ....but rather that risk than the rats. To date we have managed to have feral rescued farm cats and sheep with no problems, partly because I believe the risk is highest with ....um.....kittens and ours came at 12 months. So if you have sheep you might want to rehome the babies once weaned and keep Mum, but if no sheep then hurrah!
The only thing is, if you do keep them, please do neuter/spay them, otherwise the numbers will get out of hand very quickly! Cats Protection may be able to help with cheap neutering.