In cases of heavy infections or in chronic cases, dogs may exhibit a mild, persistent cough, reluctance to move or exercise, fatigue, reduced appetite and weight loss. (pettrust.com)
However, active dogs and those with heavier infections may show the classic signs of heartworm disease. (en.wikipedia.org)
[2] Occasionally, adult heartworms migrate to the right heart and even the great veins in heavy infections. (en.wikipedia.org)