Coughing, especially a soft, persistent cough
Fatigue or reduced stamina during walks or play
Reluctance to exercise
Weight loss
Decreased appetite
Swollen belly from fluid buildup
Labored or rapid breathing
Weakness or fainting
Pale gums
Signs of heart failure in advanced cases
No symptoms in early stages
Positive heartworm test from a veterinarian
Abnormal chest X-rays or ultrasound findings
Blood tests showing heartworm infection or related changes