Heartworm disease is often asymptomatic, reports petMD, but as the worms cause damage, dogs can begin to suffer from shortness of breath, coughing, resistance and intolerance to exercise, and, in severe cases, fainting and heart failure. Early infestation does not always show up on blood tests and can go untreated.
The American Heartworm Society says heartworms are foot-long worms that live and multiply in the heart, lungs and vessels of infected dogs. Heartworm disease is classified based on the number of worms present in the body. The more worms present, the more severe the heartworm disease.
Class I heartworm disease often goes undiagnosed because the dog does not display any symptoms, explains petMD. Blood tests do not detect heartworm during this stage making early diagnosis very difficult. Class II heartworm disease starts when the worms mature and multiply in the body. The heartworm larvae start to enter the blood stream and cause damage to the arteries. To counteract the damage, the body induces endarteritis, or inflammation of the artery. During this stage, the dog exhibits symptoms, including coughing, shortness of breath and nosebleeds. The arteries begin to dilate and change. These changes are characteristic to heartworm disease and can often be detected in an X-ray, leading to diagnosis.
If allowed to mature, the heartworms continue to multiply in and around the heart and lungs leading to Class III heartworm disease, says petMD. Increased antibodies cause damage to the eye, kidney and joints. Damage to blood vessels causes hypertension to the right side of the heart and leads to heart failure.
Heartworm disease is an aggressive, often fatal disease. The American Heartworm Society recommends a monthly preventive medication and yearly tests to ensure a dog is not infected.