Heartworm testing is required before any prevention program can be started, except for those pups under 6 months of age. Adult heartworms in dogs can be killed with drug therapy. However, this therapy can be dangerous. Pieces of the dead adult worms, leaving the heart, may clog major arteries resulting in death. When caught early, before many adult worms are in the heart, the circulating microfilaria can be treated without serious risk.
Prevention is the answer.
Simple monthly medication will keep your dog free from heartworm disease. The preventive works by blocking the development of the microfilaria from one larvae stage to the next. Adult heartworms do not develop. Your dog should be heartworm tested before starting on the preventive medication. They could experience a severe adverse reaction to the medication if given with an existing active heartworm infection.