Heartworm disease (dirofilariasis) is a potentially fatal parasitic infection transmitted by mosquitoes that causes serious heart and lung disease in dogs, with prevention being far superior to treatment in welfare terms.
Heartworm disease causes progressive, serious suffering. Dogs with established infection experience chronic respiratory distress from pulmonary hypertension, exercise intolerance, coughing, and declining cardiac function. In caval syndrome — where massive worm burdens physically obstruct blood flow — dogs collapse and may die within hours without emergency surgery. The treatment process for established infection is itself a welfare challenge: dogs must be strictly rest-confined for weeks during worm die-off to prevent potentially fatal emboli from dead worms. The acute contrast between the simplicity of prevention (a monthly tablet) and the severity of established disease makes heartworm one of the clearest examples of the welfare benefits of proactive preventive care.