Heartworm (Dirofilaria immitis) is a serious, potentially fatal parasitic infection in dogs transmitted by mosquitoes. Prevention is safe and inexpensive; treatment of established infection is costly, risky, and painful.
Heartworm causes progressively worsening welfare as the worm burden grows. Early infection may be subclinical, but as adult worm numbers increase, coughing, exercise intolerance, and right-sided heart failure develop. Advanced disease causes severe respiratory distress and abdominal fluid accumulation. Treatment is effective but involves significant welfare burden: the melarsomine injections are painful, and the 6-8 weeks of strict cage rest required to prevent embolic crises impose additional psychological stress on active dogs. Preventable suffering is the defining feature of heartworm welfare: monthly preventive medications essentially eliminate the risk, yet millions of dogs in endemic areas remain unprotected due to owner unawareness or cost. Prevention is not just cheaper than treatment — it avoids months of suffering entirely.