It depends on where you live. It is not extremely rare where I live, even though I live in a cold region. It's humid here and we get mosquitoes and cats get heartworm. Revolution for cats, a topical flea treatment sold by prescrption is also used as a heartworm preventative/treatment in cats.
I live in Florida so I am kind of concerned about it, but again, I know a few vet techs that say not to worry. They have actually given me samples of the revolution, which I may just get for fleas. He seems a lot more comfortable while he is on it. Not a single scratch.