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Heartworm Symptoms and Stages in Dogs

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

Key takeaways

  • Many infected dogs have no signs early on.
  • A mild, persistent cough and tiring easily are usually first.
  • Advanced disease causes labored breathing, belly swelling and fainting.
  • Caval syndrome is an emergency that needs prompt surgery.
  • Heartworms aren't seen in poop; blood tests are needed.

Table of Contents

Research Early signs Advanced signs Stages Caval syndrome Look-alikes Poop FAQ Sources

Heartworm Quicklinks

Heartworm Symptoms How Dogs Get It Testing Treatment Prevention Other Worms

Heartworm symptoms in dogs often don't appear until the infection is well established. Early on, most dogs show no signs at all.1,2,3 The first signs are usually a mild, persistent cough, tiring more easily on walks and reluctance to exercise. Reduced appetite and weight loss follow.4,5,6 As the disease advances, dogs may have labored breathing, a swollen belly from fluid, pale or bluish gums and fainting. These are signs of severe disease and heart failure.7,1 The most severe form, caval syndrome, causes sudden collapse, labored breathing and dark red-brown urine. It is an emergency and is usually fatal within about 2 days without surgery.7,4 Vets grade heartworm disease in classes from mild to severe.7 Heartworms can't be seen in poop. They live in the heart and lung arteries, and vets find them with blood tests, not stool tests.4,7

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

  • Sudden severe weakness, collapse or labored breathing4,7
  • Dark, red-brown or coffee-colored urine with weakness7,6
  • Pale or bluish gums, or a rapidly swelling belly1

What the Research and Guidelines Say

  • Cough is the most common sign. The Merck Veterinary Manual says that "when clinical signs are present, cough due to pneumonitis is the most common complaint."3
  • Many dogs look healthy. In a 2026 study of 283 heartworm-positive dogs, 53% had no signs when diagnosed.8
  • Blood tests track severity. A 2026 study of 35 dogs found that liver, kidney and clotting values worsened as the disease class increased.9
  • Lungworms can look similar. A 2026 review notes that heartworm signs "must be differentiated from those associated with lungworm infections."10

Early Signs

  • No signs at all in many dogs1,2
  • A "mild persistent cough"4
  • Tiring after mild to moderate activity, or reluctance to exercise5,4
  • Decreased appetite and weight loss5,4

Advanced Signs

  • Shortness of breath and labored breathing1,7
  • A swollen belly from fluid buildup, a sign of right-sided heart failure1,6
  • Pale or bluish gums1
  • Fainting7
  • Weakness and weight loss1,4

Stages (Classes) of Heartworm Disease

The AHS guidelines group heartworm disease into classes:7

  • Class 1 (mild): no signs, or an occasional cough
  • Class 2 (moderate): cough, exercise intolerance and abnormal lung sounds
  • Class 3 (severe): labored breathing, an enlarged liver, belly fluid and fainting
  • Class 4 (caval syndrome): "sudden onset of severe lethargy and weakness" with red blood cell destruction

The class helps vets judge treatment risk. Dogs with signs, especially class 3, need stabilizing before treatment, and class 4 needs emergency surgery.7,11,3

Caval Syndrome

Caval syndrome happens when a large number of worms block blood flow into the heart. Dogs suddenly collapse, struggle to breathe and pass dark urine from destroyed red blood cells.7,3,6 Surgical removal of the worms through a vein is "typically necessary." Without it, the condition "is usually fatal within 2 days."3,7 Even after surgery, complications such as kidney injury can occur.12

What Can Be Mistaken for Heartworm?

  • Lungworms, including Angiostrongylus vasorum (sometimes called French heartworm), which cause coughing, breathing trouble and exercise intolerance10
  • A harmless filarial worm whose microfilariae can look like heartworm on a blood smear13
  • Other heart and lung diseases that cause coughing and tiring, which is why testing is needed3,7

Can You See Heartworms in Poop?

No. Adult heartworms live in the heart and lung arteries, not the intestines.4 Diagnosis is by blood antigen and microfilaria tests.7 Worms seen in stool are intestinal worms such as roundworms. See worms in dogs.

Frequently Asked Questions

What are the first signs of heartworm in dogs?

A mild, persistent cough and tiring easily. Many dogs have no early signs.

How long before heartworm symptoms show?

Often many months to years after infection. Worms take about 6 months just to mature.

Is a dog with heartworms in pain?

Advanced disease causes breathing distress and weakness. Treatment injections can cause muscle soreness.

Does heartworm make dogs cough?

Yes. Cough is the most common sign when signs are present.

Can heartworm cause sudden death?

Yes, especially with caval syndrome or blood clots in the lungs.

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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. Barnes C, Williams K, Weir M, Barnette C, Ward E. Heartworm Disease in Dogs. VCA Animal Hospitals, updated June 2026.
  2. American Veterinary Medical Association. Heartworm disease. AVMA.
  3. Ames MK. Heartworm Disease in Dogs, Cats, and Ferrets. Merck Veterinary Manual, updated August 2026.
  4. American Heartworm Society. Heartworm Basics. American Heartworm Society.
  5. 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.
  6. Texas A&M College of Veterinary Medicine and Biomedical Sciences. Heartworm Disease in Dogs. Pet Talk, 2016.
  7. 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.
  8. 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.
  9. Kim M, Seo M, Cho J, Park C. Clinicopathologic variables according to disease severity in dogs with heartworm disease. BMC Veterinary Research, 2026.
  10. Jesudoss Chelladurai JRJ. Diagnosis and Treatment of Lungworms in Dogs. Today's Veterinary Practice, July/August 2026.
  11. American Heartworm Society. Heartworm Positive Dogs. American Heartworm Society.
  12. Jun J, Yoon J, Jung J. Case Report: Successful management of acute kidney injury following interventional heartworm extraction in a dog with caval syndrome. Frontiers in Veterinary Science, 2025.
  13. Negron V, Saleh MN, Sobotyk C, Luksovsky JL, Harvey TV, Verocai GG. Probe-based qPCR as an alternative to the modified Knott test when screening dogs for heartworm infection in combination with antigen detection tests. Parasites & Vectors, 2022.

About this article

Last updated : written using the 2024 American Heartworm Society canine guidelines, 2026 Companion Animal Parasite Council recommendations, the Merck Veterinary Manual, VCA, Texas A&M and 2025 to 2026 research, 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.