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Heartworm Disease in Dogs

By the Dog Health Handbook team · Updated · Fact-checked against current veterinary references (see sources)

Key takeaways

  • Heartworms are spread only by mosquitoes and live in the heart and lungs.
  • Found in all 50 states, most in the Southeast and Mississippi River Valley.
  • Early infections often have no signs; a cough and tiring come later.
  • Most dogs can be treated, but treatment takes months of strict rest.
  • Year-round prevention is safer and far cheaper.

Table of Contents

Research What it is Where How dogs get it Symptoms Testing Treatment Prevention Outlook FAQ Sources

Heartworm Quicklinks

Heartworm Symptoms How Dogs Get It Testing Treatment Prevention Other Worms

Heartworm disease in dogs is a serious and potentially fatal infection caused by Dirofilaria immitis. These are long, thin worms, up to about a foot long, that live in the heart, lungs and nearby blood vessels.1,2,3 Dogs get it from the bite of an infected mosquito, and it isn't spread directly from dog to dog.4,3 Heartworm has been found in all 50 US states and is most common in the Southeast, along the Gulf Coast and in the Mississippi River Valley.5,6,7 Early infections often cause no signs. Later signs include a mild, persistent cough, tiring easily, weight loss and, in severe cases, heart failure.1,3 Vets diagnose it with blood tests and treat it with a months-long protocol that ends with a series of injections, plus strict rest.5,8 Most infected dogs can be treated successfully.9 Prevention is far easier and cheaper: a year-round prescription preventive, started by 8 weeks of age.5,10

Emergency: go to a vet immediately if your dog has:

  • Sudden severe weakness or collapse with labored breathing1,5
  • Dark, red-brown urine along with weakness. This can be caval syndrome, which "is usually fatal within 2 days" without surgery.5,11
  • Trouble breathing, coughing up blood or a fever during heartworm treatment12

See your vet soon for a persistent cough, tiring on walks or weight loss, especially if your dog has missed heartworm prevention.1,4

What the Research and Guidelines Say

  • Updated guidelines. The American Heartworm Society (AHS) revised its canine guidelines in 2024, and the Companion Animal Parasite Council (CAPC) updated its recommendations in January 2026. Both call for year-round prevention and yearly testing.5,6
  • Shifting hot spots. In the 2025 AHS incidence survey, Texas ranked first for the first time. Most vets reported steady case numbers, but cases are spreading into new areas.13,14
  • Prevention costs far less. A 2026 study of 679 US clinics found treatment averaged $1,457 per dog, compared with about $183 a year for prevention.10
  • Lasting heart effects. Dogs treated for heartworm were more than twice as likely to be diagnosed with heart disease later.15

What Is Heartworm?

Adult heartworms live in the pulmonary arteries and right side of the heart. A dog can carry several hundred, and they can live 5 to 7 years.1,6 The worms damage the arteries and lungs, which can lead to lung disease and heart failure, and some damage can persist after the worms are gone.1,15 Dogs are the main host. Wild canids such as coyotes, foxes and wolves also carry it, and cats and ferrets can be infected.1,16

Where Heartworm Is Found

"Heartworm infection has been diagnosed in all 50 states and around the globe."5 The greatest numbers of cases are in the southeastern US and Mississippi River Valley. Cases are rising in western states such as California, Oregon, Washington and Colorado.6,17 More than 1 million US dogs are estimated to be infected.18,19 Heartworm is also spreading in Europe and exceeds 30% in some tropical hot spots.20,21

How Dogs Get Heartworms

A mosquito picks up baby worms (microfilariae) when it bites an infected dog or wild canid. The larvae develop inside the mosquito for about 10 to 14 days and are passed to the next dog it bites.1,16 The larvae take about 6 months to become adult worms.1,6 Indoor dogs can be infected too.2,22 See how dogs get heartworms.

Symptoms

  • Early: often no signs3,22
  • Mild to moderate: a persistent cough, tiring after exercise, reduced appetite and weight loss1,4
  • Severe: labored breathing, a swollen belly from fluid, fainting and heart failure5,3
  • Caval syndrome: sudden collapse, labored breathing and dark urine5,1

See heartworm symptoms.

Testing

Vets use a blood antigen test, which detects adult female worms, together with a test for microfilariae in the blood. Because about 20% of infected dogs have no microfilariae, both tests are recommended.5,6 Tests can't detect infections until about 5 to 6 months after a mosquito bite.5 AHS and CAPC recommend yearly testing for all dogs over 7 months of age, even those on prevention.5,6 See heartworm testing.

Treatment

After a positive test is confirmed, dogs are stabilized if needed. Treatment then follows the AHS protocol:8,5

  • A heartworm preventive and a 4-week course of a tetracycline-class antibiotic
  • Three injections of an arsenic-based adulticide, the only FDA-approved drug that kills adult worms, about 2 to 3 months after diagnosis
  • Steroids and pain control as needed
  • Strict rest from diagnosis until 6 to 8 weeks after the last injection

The three-injection protocol kills about 99% of worms.5 See heartworm treatment.

Prevention

Preventives in the macrocyclic lactone class are given as monthly chews or topicals, or as an injection that lasts 6 or 12 months. They require a prescription.23,4 AHS recommends starting puppies by 8 weeks of age and giving prevention year-round.5 AVMA says heartworm is "almost 100% preventable" when preventives are given on time.22 See heartworm prevention.

Outlook

AHS says "most infected dogs can be successfully treated," especially dogs with no or mild signs.9 In recent studies, 99% to 100% of treated dogs that were retested months later were negative, and deaths during treatment were about 1%.24,25 Dogs with severe disease face more complications, and some heart and lung damage can be permanent.9,15 Without treatment, the disease gets worse over time and "eventually kills the host."26

Frequently Asked Questions

Can heartworms in dogs be cured?

Yes, in most dogs. The AHS treatment protocol kills about 99% of adult worms.

Can a dog survive heartworm?

Most dogs survive treatment, especially when it's caught early.

Do indoor dogs need heartworm prevention?

Yes. Mosquitoes get indoors, and indoor dogs can be infected.

Is heartworm contagious between dogs?

No. It needs a mosquito to spread.

How common is heartworm in dogs?

More than a million US dogs are estimated to be infected, most in the Southeast.

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Sources and further reading

This article was rewritten and fact-checked on October 24, 2026 against current veterinary references and specialist guidelines:

  1. American Heartworm Society. Heartworm Basics. American Heartworm Society.
  2. Atkins CE. Heartworm Disease in Dogs. Merck Veterinary Manual, Pet Owner Version, updated September 2024.
  3. Barnes C, Williams K, Weir M, Barnette C, Ward E. Heartworm Disease in Dogs. VCA Animal Hospitals, updated June 2026.
  4. US Food and Drug Administration. An Ounce of Prevention is Worth a Pound of Cure: Protect Your Pet from Heartworms Year-Round. FDA Consumer Update, 2023.
  5. Nelson CT, McCall JW, Moorhead A, Starkey L, Ames M. Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs. American Heartworm Society, revised 2024.
  6. Companion Animal Parasite Council. CAPC Guidelines: Heartworm. CAPC, updated January 2026.
  7. Castillo T. Heartworm hotspots, expanding risk areas identified in latest report. Veterinary Practice News, April 2026.
  8. American Heartworm Society. Canine Heartworm Guidelines: Highlights. American Heartworm Society, 2024.
  9. American Heartworm Society. Heartworm Positive Dogs. American Heartworm Society.
  10. Mwacalimba K, Ferguson B, Brennan C, Poulsen Nautrup B, Little J, Stiefelmeyer K, Moldavchuk M, Mosko A, et al. Treatment of heartworm (Dirofilaria immitis) disease in dogs in the USA: general findings and cost of disease. Parasites & Vectors, 2026.
  11. Texas A&M College of Veterinary Medicine and Biomedical Sciences. Heartworm Disease in Dogs. Pet Talk, 2016.
  12. Barnes C, Williams K, Barnette C, Ward E. Heartworm Disease in Dogs: Treatment. VCA Animal Hospitals, updated July 2026.
  13. Ames M. What Does the Latest AHS Incidence Map Show Us? American Heartworm Society, 2026.
  14. Coppock Crossley K. New heartworm incidence map shows shifting hot spots. dvm360, April 2026.
  15. Mwacalimba K, Morrison JA, Ly P, Spofford N, Yang M, Saito E, Sheehy J, Adolph C, Poulsen Nautrup B, Brennan C. Prevention and long-term outcomes of naturally occurring canine heartworm infection in primary care settings. Frontiers in Veterinary Science, 2024.
  16. Montoya-Alonso JA. Dirofilariosis: Clinical Update of an Emergent Disease. South European Veterinary Conference, Barcelona, 2014 (IVIS).
  17. Companion Animal Parasite Council. 2025 Annual Pet Parasite Forecasts. CAPC, 2025.
  18. Walsh S. The State of Heartworm Incidence in the United States. Today's Veterinary Practice, 2020.
  19. Larkin M. Heartworm incidence climbs despite preventive efforts. AVMA News, October 2024.
  20. Morchon R, Montoya-Alonso JA, Rodriguez-Escolar I, Carreton E. What Has Happened to Heartworm Disease in Europe in the Last 10 Years? Pathogens, 2022.
  21. Dantas-Torres F, Ketzis J, Perez Tort G, Mihalca AD, Baneth G, Otranto D, et al. Heartworm adulticide treatment: a tropical perspective. Parasites & Vectors, 2023.
  22. American Veterinary Medical Association. Heartworm disease. AVMA.
  23. American Heartworm Society. Heartworm Prevention for Dogs. American Heartworm Society.
  24. Maerz I, Rutjes S, Genz S, et al. Adulticide treatment with an arsenic-based adulticide: outcome in 283 heartworm-positive dogs in Germany. Parasites & Vectors, 2026.
  25. Still MB, Tedesco D, Hawkins C, Putnam H. Improving access to arsenic-based adulticide therapy: treating canine heartworm infection in a high-volume, outpatient community clinic setting. Parasites & Vectors, 2024.
  26. Montoya-Alonso JA, Carreton E. Heartworm Disease: Novel Clinical Aspects in Pathogenesis, Diagnostic and Treatment. South European Veterinary Conference, Barcelona, 2015 (IVIS).

About this article

Last updated : written using the 2024 American Heartworm Society canine guidelines, 2026 Companion Animal Parasite Council recommendations, the FDA, the Merck Veterinary Manual, VCA and the 2025 AHS incidence survey, plus International Veterinary Information Service (IVIS) proceedings and research published 2022 to 2026.

This page is for general information and is not a substitute for an exam by your veterinarian.