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7 Heartworm Symptoms in Dogs

Heartworm symptoms can take months to appear. Here are some of the common signs that a dog may have heartworms.

warning signs your dog has heartworms
Heartworm symptoms may take more than 6 months to appear. By: Josh

Heartworm disease in dogs is a silent killer that is often overlooked until it’s too late. This preventable condition starts with a mosquito bite and ends with either a long, expensive treatment or death.

Heartworm symptoms may not appear until the worm reaches maturity (usually after around 6 months).

Heartworm Symptoms in Dogs

1. A soft, dry cough

The parasites make their way to the lungs and start multiplying in the lungs and surrounding veins. Coughing may be most noticeable after exercise and may end with fainting. Even light exercise can cause fainting.

2. Inactivity or lethargy

Your pet suddenly seems tired more often, doesn’t want to go outside, or avoids all physical activity. Dogs with heartworm infection are weakened and find it difficult to be active even when performing small tasks.

3. Weight loss or anorexia

Even minor physical activities, such as eating, can become difficult and exhausting chores.

4. Rapid or difficult breathing

Along with coughing, breathing problems occur when the worms inhabit the lungs and surrounding veins. Fluid can also build around the blood vessels in the lungs, making it difficult for the lungs to oxygenate the blood.

5. Bulging chest

The ribs may seem to protrude, and the chest has a bulging appearance as a result of adult heartworm infection. This symptom can result from weight loss and anorexia caused by heartworms. This can also be caused by fluid buildup in response to the parasite’s presence.

6. Allergic reaction

Although allergic reaction is more common in cats, it is possible for dogs to show symptoms similar to an allergic reaction or asthmatic symptoms in response to the heartworms or their offspring.

7. Collapse

Large numbers of heartworms invade the heart and cause blockage of blood flow (known as caval or vena cava syndrome). Collapse is usually accompanied by shock and red blood cell destruction. Death can follow within days.

Other heartworm symptoms are possible, too:

  • Nosebleeds
  • Secondary pneumonia
  • Increased blood pressure
  • Excessive sleeping
  • Seizures
  • Blindness
  • Lameness

Seizures, lameness and blindness occur when the parasites get lost and end up in places other than the heart or lungs. They can end up in the brain or eyes, although this is rare.

Some of the heartworm symptoms above can also be signs of other conditions, which makes it more difficult to detect heartworm infection. There are other tools used by vets to detect the condition more accurately.

Learn More About Heartworm Symptoms

 “Thank goodness I found this book!” readers say. “This book is a blueprint to keeping your animals free of heartworm and to begin healing any existing infections safely.” Find out more on Amazon: Heartworm Survival Guide: Every Pet Owner’s Guide to Understanding, Preventing and Treating Heartworm Disease.

Bottle with heartworm, extracted during surgery. By: mvatrabu
Bottle with heartworm, extracted during surgery. By: mvatrabu

Diagnosis and Treatment of Heartworm in Dogs

Blood tests to detect the various stages of infection can include viewing a blood sample under a microscope and checking for antigens of adult heartworms (a protein the heartworms produce and also the most commonly used test, according to VCA Animal Hospitals).

Other blood tests can check for abnormalities in complete blood counts and evaluate the level of function of the internal organs.

  • Radiographs (X-rays) may detect inflammation, enlargement, or swelling of the heart, lungs and the large artery leading to the lungs from the heart (pulmonary artery).
  • Abnormal heart rhythms and enlarged heart chambers can be detected by an electrocardiogram.
  • An ultrasound of the heart (echocardiography) can help determine the health of the heart for treatment and offer a visual of any existing heartworms.

These tests are also used to determine the health status of the internal organs. This is required to evaluate whether the dog is healthy enough for treatment.

In the video below, Dr. Courtney Campbell, DVM, explains even more about heartworm symptoms in dogs:

Treatment begins by eliminating the adult roundworms and takes about 1 month to complete. The dead adult worms are then absorbed by the body.

After this stage, the next treatment gets rid of the younger parasites and offspring. In extreme cases, surgical removal of heartworms from the internal organs may be necessary.

This condition is preventable. If your pet is not taking a heartworm preventive, please talk with your vet about getting started.

Credit: American Heartworm Society
Protect your pet all year long, and test every 12 months. By: American Heartworm Society

Additional Resources


This pet health content on dog heartworm symptoms was reviewed for accuracy by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed and updated Aug. 14, 2018.

Petful Veterinary Team

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Over the past nearly 10 years, the Petful® veterinary team of writers has included a number of experts, such as veterinarians Dr. Pippa Elliott, BVMS, MRCVS; Dr. Debora Lichtenberg, VMD; Dr. Kenya Crawford, DVM; and Cate Burnette, RVT, among others. Providing accurate, trustworthy information is our utmost concern, so all of our pet health content is regularly reviewed, updated and edited by veterinary professionals.

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