Heartworm disease causes severe cardiovascular damage in dogs. Prevention is safe, effective, and far more welfare-positive than treatment of established infection.
Heartworm disease causes progressive and serious suffering. Mature infections involve worms living within the heart and major vessels, causing cardiovascular compromise, coughing, exercise intolerance, and eventual heart failure. Treatment itself is an intensely welfare-significant process requiring strict cage rest for months to prevent fatal pulmonary embolism as worms die. The contrast between the simple welfare cost of monthly prevention and the severe welfare cost of established infection makes heartworm one of the clearest examples of prevention being dramatically superior to treatment for animal welfare.