Heartworm disease — caused by Dirofilaria immitis, a parasitic roundworm transmitted by mosquitoes — is a serious, potentially fatal condition in dogs. While historically confined to tropical and subtropical regions, increasing mosquito range expansion and global travel have made heartworm awareness relevant to more dog owners. Prevention is far simpler and safer than treatment.
Life Cycle and Pathogenesis
Infected mosquitoes transmit heartworm larvae (L3) when feeding. Larvae migrate through subcutaneous tissue over 45–65 days before entering pulmonary arteries and the right ventricle of the heart. Adult worms (males 15–20cm, females up to 31cm) live in pulmonary vessels for 5–7 years. Heavy worm burdens cause pulmonary hypertension, right heart failure, and potentially caval syndrome (obstruction of blood flow by worms in the right atrium and vena cava).
Clinical Signs
Severity correlates with worm burden and duration of infection:
- Class 1 (mild): Asymptomatic or mild occasional cough; no clinical signs at rest
- Class 2 (moderate): Soft cough, exercise intolerance, abnormal lung sounds
- Class 3 (severe): Dyspnoea, weight loss, right heart failure signs (ascites, jugular distension), syncope
- Class 4 (caval syndrome): Sudden cardiovascular collapse; requires emergency surgical worm removal
Diagnosis
Antigen tests (detecting adult female heartworm antigens) are the primary diagnostic tool — accurate from 5–6 months post-infection. Microfilariae detection by modified Knott's test or filter test confirms active infection. Radiography and echocardiography assess cardiopulmonary damage and guide treatment decisions.
Treatment
Treatment (melarsomine dihydrochloride — an arsenical compound given intramuscularly) carries significant risk — dying worms embolise to pulmonary vessels causing potentially fatal pulmonary thromboembolism. Strict exercise restriction (4–6 weeks of cage rest) is essential during treatment to reduce embolic complications. Pre-treatment doxycycline (eliminating Wolbachia endosymbiont reducing adult worm viability) and anti-inflammatory treatment reduce treatment risks.
Prevention
Monthly preventive products (ivermectin, milbemycin, moxidectin) prevent larval development and are highly safe and effective. In endemic regions, annual testing followed by year-round preventive treatment is the standard of care. Dogs returning from endemic regions should be tested 6 months after return. Prevention welfare is unambiguous — the safety and efficacy of preventive products vastly outweighs treatment risk and animal suffering from established infection.