Short answer: Heartworm (Dirofilaria immitis) is transmitted by mosquitoes – a single bite from an infected mosquito injects larvae that mature into adult worms in the dog’s heart and pulmonary arteries over 6 months. Dogs cannot infect each other directly. Year-round monthly prevention is now AAHA-recommended in all 50 states.
What you should actually do
- Life cycle: mosquito takes a blood meal from an infected dog, picks up microfilariae, the microfilariae mature inside the mosquito for 10-14 days, then are injected into a new dog at the next bite.
- Larvae take 6 months to grow into adult heartworms; adults live 5-7 years in the heart and pulmonary arteries.
- Annual heartworm test (4Dx SNAP) recommended even for dogs on prevention – missed doses, vomiting, and resistance can cause breakthrough infections.
- Treatment for adult heartworm disease is harsh: 3 doses of melarsomine (Immiticide) over 60 days + strict cage rest + doxycycline + ivermectin. Cost ~$1,000-2,000.
- Prevention drugs (ivermectin, milbemycin, moxidectin, selamectin) all kill larvae before they mature – costs $5-15/month and is dramatically cheaper than treatment.
Heartworm is no longer a southern-state problem. The American Heartworm Society’s 2022 incidence map shows endemic transmission in all 50 states, driven by movement of rescue dogs and climate-driven mosquito range expansion. The classic ‘summer-only’ protocol is outdated.
Heartworm-positive dogs need staging: 3-view chest X-rays and ideally an echocardiogram, plus modified Knott’s microfilaria count. Caval syndrome (a heavy worm burden occluding the right atrium) is a surgical emergency and requires mechanical extraction. Class 1-2 disease can be treated with the Immiticide protocol on a strict 60-day exercise restriction.
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⚕️ Medical Disclaimer
The information on this page is intended for educational purposes only and does not replace a hands-on veterinary examination. Drug doses depend on your dog’s complete clinical picture, concurrent medications, and the exact product formulation. Always confirm dosing with your veterinarian before administering any medication, and contact a 24-hour veterinary emergency service or animal poison control immediately if you suspect a medication overdose or adverse reaction. Editorial standards: every drug dose published on PuppaDogs is cross-checked against multiple authoritative veterinary references and reviewed by PuppaDogs Veterinary Editorial Team before publication.
