In dogs with large heartworm infections, symptoms include a persistent cough, fatigue and loss of appetite, according to the American Heartworm Society. However, most dogs show no noticeable symptoms in the earlier stages of infection.
The cough associated with heartworm disease is generally dry and mild but persistent. It does not respond to conventional cough treatments. Dogs may also have shortness of breath, especially after exercise. They tend to lose stamina and become tired quickly. Dogs may exhibit nervousness or restlessness. Muscle weakness also may occur. Infected dogs become listless and lethargic, reports VCA Animal Hospitals.
In very advanced cases, dogs may suffer from congestive heart failure, and owners may notice swelling in their abdomens and legs. This occurs due to fluid buildup from the heart problems, according to VCA Animal Hospitals. Lack of appetite and poor health can also cause weight loss and muscle wasting. In advanced stages, the dog may die suddenly during exercise or times of stress.
In worst case scenarios, a dog with heartworm disease can develop caval syndrome, a dangerous form of cardiovascular collapse in which blood flow to the heart is blocked. Without surgical intervention, few dogs survive caval syndrome.
The most reliable way to diagnose heartworm disease is to have a veterinarian test the dog's blood for evidence of the parasites, according to the American Heartworm Society. This should be done regularly, and dogs should be given preventative medication. In dogs with more advanced infections, veterinarians may also notice abnormal sounds from the heart and lungs during routine exams. A dog should be tested annually for heartworms, even if it is already on preventative medication. Puppies should start heartworm prevention as early as they can, with the dosage determined by their weight
If a dog has been infected, a backup test is immediately ordered to verify the lab's findings. While the results are processing, the dog's exercise should be curtailed; physical activity can increase the damage heartworms cause to the heart and lungs. A dog with mild or no symptoms has a high chance of survival, but as the disease grows more severe, the possibility of complications rises.