Thanks to yskat for sending this. One of the misconceptions about cats is that they need to hunt and kill rats whereas from what I've observed, the presence of cats is enough to be a deterrent. Of course, there are some areas where there are cats and rats in the same vicinity. Desmond Morris mentioned in his seminal book, Catwatching that the rats need to be below a certain size for cats to be able to hunt - or I suspect, even intimidate them.
Eliminate the cats - and rats are likely going to follow.