Preventative. Though you can get an injectable heart worm prevention.
And if you're really worried about it, you can give it to cats. Revolution is one example of HW prevention for cats.
Thing about cats is, one, they're less likely over all to get it. Two, if they do get it, there's few ways we can tell since our antigen tests don't work with cats and they're relatively asymptommatic. Three, there's no easy way to diagnose it in a cat other than 'Oh man, my cat suddenly died! Let's do a necropsy!"
For the most part, it's a non-issue in the feline variety. Heart worms in cats is a relatively new phenomenon (like we just saw it first happen in the last 40 years or so).