Heartworm disease is caused by a parasitic worm spread by mosquitoes. It can cause lasting damage to the heart, lungs and arteries — and it's far easier to prevent than to treat.
How dogs get heartworm
An infected mosquito deposits larvae when it bites. Over roughly six months, those larvae mature into adult worms that live in the heart and lung vessels. Heartworm has been reported across all 50 U.S. states and in many other countries.
Symptoms
Early infections often show no signs. As disease progresses, you may see:
- A mild, persistent cough
- Tiring easily on walks
- Reduced appetite and weight loss
- A swollen belly in advanced cases
Prevention and testing
Preventives come as monthly chews, topical treatments or longer-lasting injections prescribed by your vet. The American Heartworm Society recommends year-round prevention and annual testing, because missed doses happen and seasons are unpredictable.
A dog should be tested before starting prevention, since giving preventives to an infected dog can cause reactions.
Frequently asked questions
Do indoor dogs need heartworm prevention?
Yes. Mosquitoes get indoors, and a single bite is enough to transmit infection.
Can heartworm spread directly from dog to dog?
No. It requires a mosquito to carry the larvae between animals.
Key takeaways
- Heartworm is spread by mosquitoes and is preventable.
- Year-round prevention and yearly testing are recommended.
- Treatment for an established infection is long and costly.
This guide is for general information and doesn't replace advice from your veterinarian. If your dog seems unwell, contact your vet. Read our disclaimer.
Sources
Pawly Editorial Team
Pet care writers & editors
Pawly's articles are written and edited by our editorial team of experienced dog owners and pet-care writers. Every guide is built from published veterinary guidance — from organizations such as the AVMA, AAHA, WSAVA and the ASPCA — and is reviewed for accuracy before publishing and regularly after.
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