Heartworm (Dirofilaria immitis) is transmitted by mosquitoes. One infected dog can serve as a reservoir for thousands of microfilariae. Prevention is safe, effective, and affordable; treatment is expensive, prolonged, and carries risk. Use the Parasitology Specialist and Drug Formulary for guidance.
Mosquito bites infected dog → ingests microfilariae → L3 larvae develop in mosquito → mosquito bites susceptible dog → L3 enter tissue → molt to L4, L5 → adult worms in pulmonary arteries and heart (~6 months). Adults produce microfilariae.
Cats are atypical hosts; fewer worms develop, but disease can be severe. Heartworm-associated respiratory disease (HARD) occurs even with low worm burdens.
Monthly preventives (ivermectin, milbemycin, moxidectin, selamectin) kill L3/L4 larvae before they mature. Given year-round, they prevent establishment. Options: oral (chewables), topical (spot-on).
Annual testing (antigen test for dogs; antigen + antibody for cats) detects infection. Test before starting prevention (if >7 months old) and annually thereafter. Missed doses require testing and potential delay in restart.
Year-round prevention is recommended even in northern climates. Mosquito season is unpredictable; travel spreads infection; compliance improves with routine.
Adulticide treatment (melarsomine) involves deep IM injections, exercise restriction, and risk of thromboembolism when worms die. Pre-treatment with doxycycline (Wolbachia) and macrocyclic lactones reduces worm mass. Treatment is costly and prolonged (several months).
Warning: No FDA-approved adulticide for cats. Supportive care only. Prevention is critical—once infected, options are limited.
- Heartworm: mosquito-borne; prevention is safe and effective.
- Monthly preventives kill larvae; year-round recommended.
- Annual testing before and during prevention.
- Treatment in dogs: melarsomine, doxycycline, strict rest.
- No adulticide for cats—prevention is essential.