Heartworm disease causes damage to the pulmonary arteries which eventually leads to heart failure. If you have any acquaintances with chronic heart failure, ask them how much joy there is in their lives. Heart failure robs the dog of its ability to play comfortably. Eventually it becomes difficult to just participate in normal activities. Serious damage begins to occur in other organ systems affected by the heart failure. Treatment of the symptoms alone fails to resolve the problem over the long term and the dog eventually dies -- after having been miserable for some time. Some dogs do manage to live almost normal lifespans despite infection with heartworms but they are very lucky. A rough estimate from our practice experience: 80% of dogs affected with heartworms probably die from the heartworm disease before something else causes them to die and 90% eventually show some or all of the symptoms of the disease.