Heartworm disease — caused by Dirofilaria immitis, transmitted by mosquitoes — is a serious, often fatal parasitic disease prevalent in many parts of the world. In endemic regions, prevention is a fundamental welfare measure; in non-endemic regions, international travel creates import risk.
Adult heartworms live in the pulmonary arteries and right heart, causing progressive pulmonary arterial damage, right-sided heart failure, and eventually severe multi-organ disease. The disease progresses over months to years without treatment. Clinical signs range from asymptomatic in early infection to exercise intolerance, coughing, collapse, and death in advanced disease. The welfare impact of established heartworm disease is severe and prolonged — chronic dyspnoea, weakness, and cardiac compromise cause months to years of suffering before death without treatment.
Heartworm is endemic across the Americas (North, Central, and South), Mediterranean Europe, parts of Africa, and Asia. In the UK, heartworm is absent from the indigenous mosquito population, but dogs travelling to southern Europe (Spain, Italy, Portugal, France) are at significant risk. International rescue dogs imported to the UK from endemic regions represent another exposure route. Any dog that has lived in or visited endemic areas should be tested and potentially treated before returning to or entering the UK.
In endemic regions, monthly preventive treatment is the standard welfare intervention. Options include: oral ivermectin (Heartgard-type products), milbemycin oxime, moxidectin (injectable, 6-monthly, or topical), and selamectin (topical monthly). These drugs kill larval stages (microfilariae and early larvae) before they develop to adults. Prevention must begin before first mosquito-season exposure, ideally from 8 weeks of age in endemic areas. Consistent monthly administration is critical — gaps in prevention allow larval development to progress beyond drug-susceptible stages.
Treatment of established heartworm infection uses melarsomine (an arsenic-based drug) to kill adult worms. Treatment is expensive, potentially risky (dead worms can cause pulmonary emboli), and requires strict exercise restriction during the treatment protocol. Prevention is dramatically simpler, safer, and more welfare-protective than treatment.
UK veterinarians increasingly see heartworm-positive dogs through rescue importation and returning travellers. Veterinary guidance for all dogs arriving from endemic regions recommends heartworm testing and appropriate treatment or preventive protocols. Raising awareness among owners of dogs travelling abroad supports appropriate preventive care decisions.
← Back to Animal Welfare Hub