Heartworm disease is a serious, life-threatening parasitic infection of dogs caused by Dirofilaria immitis. Prevention is straightforward and inexpensive; treatment is complex, expensive, and carries significant welfare costs — making prevention the clear welfare priority.
Heartworm disease causes progressive cardiopulmonary disease that may be present without obvious signs for months to years before causing exercise intolerance, coughing, and eventually right heart failure. By the time clinical signs appear, significant and potentially irreversible cardiac and pulmonary damage has occurred. Treatment, while effective in most cases, is itself a welfare event: deep intramuscular melarsomine injections are painful; the exercise restriction required to prevent thromboembolic complications from dying worms is stressful for active dogs; and the risk of pulmonary thromboembolism creates an acute welfare risk during the treatment period. The compelling simplicity of the welfare argument for prevention is almost unique in veterinary medicine: a once-monthly tablet or topical treatment, costing very little, prevents all of this suffering reliably. Dogs in heartworm-endemic areas not receiving preventive treatment are at meaningful risk of a serious, welfare-compromising, and expensive disease for which prevention is nearly perfectly effective.