While many dogs do not display symptoms of heartworm disease in its initial stages, as the disease progresses, dogs often begin to display a mild cough, according to the American Heartworm Society. Other early symptoms include fatigue and a reluctance to exercise, as well as appetite and weight loss.
Heartworm disease in dogs can be difficult to diagnose because they can initially appear to be healthy, despite possibly displaying a mild cough. Even a blood test in the early stages may not reveal the presence of heartworm, according to PetCareRx. As the disease develops, the mild cough may become more persistent, and the dog may begin showing signs of fatigue. It is at this stage that the presence of heartworm in the blood may be detected.
As the number of heartworms increases, the dog's belly may begin to swell, due to fluid build-xup in the abdomen, as stated by the American Heartworm Society. Some dogs do not begin to display symptoms until these late stages, according to the ASPCA.
Toward the final stages of the disease, abnormal lung sounds may be detected, and the dog may suffer from a loss of blood to the brain, explains WebMD. The large numbers of worms may lead to a blockage in the blood vessels, called caval syndrome, which leads to cardiovascular collapse, according to the American Heartworm Society. The dog is likely to have difficulty breathing, pale gums and dark urine. Without prompt treatment, the dog is in danger of dying.