Heart worms only come from the bite of a mosquitoe that is a carrier. If there are mosquitoes, your dog is in danger of heartworms. A few pennies spent on Imervection each month takes care of the problem. As others have said, Imervection doesn't take care of all other worms, but gets most of them. A couple of wormings each year with conventional wormers and Fido is a happy dog. Other wormers also don't treat heart worms. If you don't know what you are doing, you can kill the dog along with the worms. Suddenly killing all the heart worms can cause a blockage of dead worms in the heart, killing the dog. Untreated dogs don't live to a ripe old age either. Prevention is MUCH cheaper than treatment.