Canine Heartworm Symptoms
Canine heartworm symptoms 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.

Heartworm symptoms in dogs are caused by the parasite heartworm (Dirofilaria immitis). They are known as “heartworms” because the adult worms live in the right side of a host’s heart.

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

Mosquitoes are the culprits that transmit heartworms to dogs. Since mosquitoes exist almost everywhere, this disease can occur in dogs all over the world.

This page focuses on canine heartworm symptoms. Visit this page to learn more about the life cycle of the heartworm. Also visit our page on Canine Heartworm Prevention to see how we can prevent heartworm infections in dogs.

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. But 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 (2 large veins of the heart) 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, if a dog has only a few worms in the body and if he doesn’t exercise strenuously, he 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 at all. As the infection becomes more severe, the dog slowly show more symptoms.

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 serious and obvious. The dog will also have difficulty breathing, panting, and have severe coughing after exercise to the point of fainting.

Additionally, the dog will lose weight. The ribs will become prominent and the chest will start to bulge.

Using a stethoscope, we can hear a heart murmur 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. This leads to liver failure with jaundice, fluid accumulation in the abdominal cavity, spontaneous bleeding, and anemia.

In two to three days, the dog will collapse and die.

Canine Heartworm Diagnosis

A vet uses a number of blood tests and other means such as X-rays to make a diagnosis.

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.

Also, most tests can only detect infections with one or more mature female heartworms that are at least seven or eight months old. 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.

While a positive test definitely indicates heartworm infection, a negative test does not necessarily mean there is no infection. The reason is, 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.