I hope these are indoor cats. If the cats go outside, they can get new fleas and bring them in, so I don't know what you do in that situation.

A friend of mine used to allow her cats to go outside because they longed for 'the freedom.' Then a gang of old college buddies came for a visit and started counting the cats hit by cars, killed by feline leukeimia, or simply gone missing. They started teasing my friend about having 'disposable cats.' She took it hard.

Now she keeps her cats indoors and all is well.