Heartworm disease is a severe, potentially fatal condition transmitted by mosquitoes. Prevention is simple and inexpensive but treatment of established infections is lengthy, costly, and causes significant welfare harms.
Heartworm disease causes progressive, chronic welfare harms in affected dogs. Adult worms in the heart and pulmonary arteries impair circulation, causing chronic coughing, exercise intolerance, and respiratory distress. Dogs with advanced disease experience fatigue, difficulty breathing, and reduced quality of life over months to years before death or treatment. Treatment itself causes welfare harms: restricted activity for months as worms die and are resorbed, and risk of pulmonary embolism from dying worms. The straightforward welfare calculus strongly favours prevention over treatment — monthly preventive medication administered year-round in endemic areas prevents infection entirely at minimal cost and risk. Climate change is expanding heartworm range, making prevention relevant in areas previously considered low-risk.