Effects of Heartworms – From the Dog’s Point of View
Heartworms irritate the sensitive lining of blood vessels and heart chambers by direct physical contact. They obstruct blood flow through the pulmonary arteries and cause allergic reactions, vascular inflammation and fluid accumulation (edema). Up to one-half of dogs infected with heartworms eventually develop right-sided congestive heart failure. Kidney, lung and liver damage are also common. Heartworms that block critical large vessels can cause “caval syndrome,” a serious condition involving liver failure, anemia, jaundice, internal bleeding, blood-clotting disorders and fluid retention. Death can occur in a matter of days.
Canine heartworm disease is classified as follows:
Class 1: Very few or no signs of illness. Can reflect early infection, infection by only one gender of worms or administration of heartworm preventatives that kill microfilaria but don’t kill adults.
Class 2: Moderate respiratory and cardiac signs.
Class 3: Severe respiratory and cardiac signs.
Class 4: Caval syndrome – extremely severe respiratory and cardiac signs; guarded to grave prognosis.
The symptoms of heartworm disease are directly related to the location, number and size of living and dead adult worms inside the dog. Most infected dogs are in Class 1 and show no signs of illness. Heartworms frequently are first detected during routine blood testing. Affected animals most commonly suffer from coughing and lethargy.
Symptoms of Heartworms – What the Owner Sees
Owners of infected dogs may notice:
- Exercise intolerance
- Tires easily
- Soft, deep cough
- Difficulty breathing (dyspnea)
- Rapid breathing (tachypnea)
- Weight loss (prominent ribs and chest)
- Abdominal distension (“pot-bellied” appearance; enlarged liver and spleen)
- Jugular vein pulsation
- Red-tinged urine
Dogs at Increased Risk
Dogs in endemic areas that spend substantial time outdoors have an increased risk of developing heartworm disease. Sporting breeds are predisposed. Middle-aged, outdoor males are almost 4 times more likely to be affected by heartworms than are other dogs, especially in hot, mosquito-friendly climates during the spring and summer hunting months. Dogs living at high altitudes are less prone to infection. Certain breeds are extremely sensitive to ivermectin, a drug contained in many heartworm preventative medications; their reaction to ivermectin can be fatal. A simple cheek-swab test for this hereditary trait is available through the Washington State University Veterinary Laboratory of Clinical Pharmacology. Collies, Border Collies, Australian Shepherds, Shetland Sheepdogs, Old English Sheepdogs, longhaired Whippets and crosses of those breeds probably should be tested. Owners of those breeds should discuss potential ivermectin sensitivity with their veterinarian before starting a heartworm prevention program.