I give all 5 of mine Heartgard for cats each month in the summer (mostly because they travel with me, and I take them for walks outside on the leash). Getting them to eat it does require bribing with tuna fish.

However if you are concerned about yours, Revolution is a topical liquid applied to the skin on the back of the neck once a month, it kills fleas, and roundworms, prevents heartworm AND treats earmites!! For an outside cat especially it's a great product. The only reason I don't use it, is that my cats are exposed to ticks and it's cheaper for me (at our hospital anyways) to use the Frontline Plus AND Heartgard rather than Revolution.