Rats do carry many things - the most common ones are parasites (worms), fleas and ticks, but your cat is already exposed to those since she is allowed outdoors. The biggest thing you have to worry about is poisoning. Many people put out baits to poison rodents. The problem is that when another animal, or bird of prey, eats the poisoned mouse or rat they too become poisoned, and die without getting treatment. I'd be scooping up the carcass, and take it to the vet to determined if it was poisoned or not, so that if it was your cat could get the emergency treatment she would need. Of course you are going to have to pay for the testing on the rat - but that is the price you pay for having a cat that is allowed outside.