I have 3 strickly indoor -never-step-outside-the-door cats. Do they need monthly heartworm/tick medicine? I have had them on 'Revolution' parasite control, but it is very pricy for the all 3 of them, I have to buy a 6 dose box, approx.$70 every 2 months. My vet says I should keep them on it 'in case of a flea getting into my apt from outside' but I'm not so sure. Any thoughts?