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Canine Heartworm Symptoms

Heartworm symptoms in dogs include coughing, exercise intolerance, and in later stages panting and breathing difficulty. Organs commonly affected by heartworms include the heart, the lungs, the liver, and the kidneys. Read on for more information on dog heartworm symptoms and diagnosis.


Canine heartworm symptoms are caused by the parasite heartworm (Dirofilaria immitis), so called because the adult worms live in the right side of a host's heart.

Dogs are the natural hosts of heartworms although cats can occasionally be infected as well.

Heartworms are transmitted by mosquitoes, and therefore this disease can occur in dogs all over the world.

This page focuses on canine heartworm symptoms. The life cycle of the heartworm is described in our page on Heartworm Life Cycle. Please also visit our page on Canine Heartworm Prevention.

Canine Heartworm Symptoms - Organs Affected

In most dogs, the major organs that are affected by heartworms include the heart, the lungs, the liver, and the kidneys.

Generally speaking, when there are fewer than 50 adult heartworms in the dog, the worms live mostly in the pulmonary arteries and the right ventricle of the heart. However, when the numbers are over 75, the worms extend to the right atrium. With an even heavier infestation, the worms may migrate into the superior and inferior vena cavae and the veins of the liver.

Canine Heartworm Symptoms and Signs

Dog heartworm signs and symptoms depend on the number of worms in the dog, and the size and health condition of the dog.

Very often, dogs with only a few worms in the body and are not exercised strenuously may never show any symptoms of heartworm.

Although some dogs infected by heartworms may show acute signs, in most cases, the disease begins with little or no symptoms and slowly progresses as the infection becomes more severe.

Below is a brief outline of the canine heartworm symptoms according to the severity of the disease.
  • Early Infection: In most cases, early infection of heartworm in dogs shows no symptoms.
  • Mild to Moderate Infection: The typical signs include tiring easily, exercise intolerance, and a soft, deep cough.
  • Severe Infection: As the disease progresses, the above symptoms (cough, exercise intolerance, tiring) become more pronounced. In addition, the dog will have difficulty breathing, panting, and have severe coughing after exercise to the point of fainting.

    The dog will also lose weight. The ribs will become prominent and the chest will start to bulge. Using a stethoscope, a heart murmur can be heard over the right side of the chest.

    The worms in the pulmonary arteries can obstruct blood flow and cause the vessels to clot, leading to a condition known as pulmonary thromboembolism. Chronic pulmonary thromboembolism causes loss of lung tissue and right-sided congestive heart failure. The dog may cough up blood sputum.

    When the worms migrate and live in the vena cava or hepatic veins, they can cause a condition called vena cava syndrome, which leads to liver failure with jaundice, fluid accumulation in the abdominal cavity (ascites), spontaneous bleeding, and anemia. In two to three days, the dog will collapse and die.

Canine Heartworm Diagnosis

Diagnosis is made using a number of blood tests and other means such as X-rays.
  • Antigen Test: The heartworm antigen test is the most accurate test although there are limitations to the test. This test identifies an antigen produced by the adult female heartworm but cannot detect infections in which only males are present. In addition, most tests can only detect infections with one or more mature female heartworms that are at least seven or eight months old, but they cannot detect infections of worms that are less than five months old.
  • Microfilarial Concentration Test: The test examines a sample of blood under the microscope to identify the presence of microfilaria. Note that while a positive test definitely indicates heartworm infection, a negative test does not necessarily mean there is no infection - about 10 to 25 percent of infected dogs do not have microfilaria in their blood circulation.
  • Microfilarial Differentiation Test: There is another type of microfilaria called Dipetalonema which can be detected in a dog's blood tested for heartworm infection. Dipetalonema is the offspring of a harmless worm that matures in the tissues beneath the skin of dogs. This type of microfilaria can be differentiated from those produced by heartworms through microscopic examination evaluating size, shape and their movement.
  • Chest X-Ray: If a dog is tested positive for heartworm, a chest X-ray is usually taken to determine the severity of the infection. X-rays of dogs with heavy infestations will show enlargement of the right ventricle and/or pulmonary arteries.
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