Symptoms of heartworms in dogs include a decrease in appetite, weight loss, fatigue after moderate activity, a reluctance to exercise and a mild, persistent cough, according to the American Heartworm Society. As heartworm disease progresses, some dogs develop heart failure and a swollen belly due to excessive fluid in the abdomen.
During the early stages of heartworm disease, some dogs show little or no symptoms at all, according to the American Heartworm Society. However, symptoms are more likely to develop the longer the infection persists. Dogs with a large amount of heart worms are at risk for a life-threatening cardiovascular collapse, called caval syndrome, which causes sudden blockages of blood flow within the heart. Symptoms of this syndrome include labored breathing, pale gums and dark-colored urine. Immediate surgery to remove the heartworm blockages are essential to ensure a dog's survival, warns the American Heartworm Society.
Heartworm disease is a potentially fatal disease caused by footlong worms that live in the heart, lungs and associated blood vessels of affected animals, according to the American Heartworm Society. Heartworm disease cause severe lung disease, heart failure and damage to other organs in the body. Many dogs contract heartworms from infected mosquitoes. The American Heartworm Society recommends the use of monthly preventative heartworm medication for dogs.