Dogs usually do not show symptoms until the heartworms have already done extensive damage to the heart and lungs. As the heartworms do more damage, your dog will start showing symptoms such as:
- Difficulty breathing
- Exercise intolerance
- Coughing
- Unusual tiredness or lethargy
As the damage gets worse, your dog will eventually start to show more severe and immediate symptoms. These include Congestive Heart Failure or Pulmonary Failure. The final stage is death.