The 5 Stages of Heartworm Disease in Pets

Katy Nelson, DVM
By Katy Nelson, DVM on Feb. 27, 2015

Heartworm Disease – All Pets Are At Risk

By Dr. Katy Nelson


Heartworm disease is a serious and potentially fatal condition of dogs, cats, and ferrets, caused by infestation of a parasite known as Dirofilaria immitus. Heartworms are transmitted by mosquitoes to animals, and are most commonly found in the Southern United States. However, Heartworm disease has been diagnosed in all 50 states and risk factors are impossible to predict.  Even indoor only pets are at risk of infection.


For that reason, the American Heartworm Society recommends that you “Think 12: (1) Get your pet tested every 12 months for heartworm, and (2) Give your pet heartworm preventive 12 months a year.”

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Asymptomatic – No Signs

In the early stages of heartworm infection, no outward signs are seen. However, within days of the parasite arriving in the circulatory system, the damage to the arteries begins. Immune cells are called in to the area, but the worm is far too large for the cells to destroy, so the ensuing inflammatory process begins. The longer the infection persists, the more likely symptoms are to develop.

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Chronic Soft Coughing, Exercise Intolerance

Coughing and exercise intolerance result as areas of the lung are unable to fully oxygenate blood. Nose bleeds may occur due to abnormal clotting in the lungs. And a form of non-infectious pneumonia (pulmonary eosinophilic granulomatosis) can result from excessive inflammation in the lungs due to the parasite infiltration.

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Chronic Immune Stimulation

When a pet goes without heartworm treatment for a long period of time, chronic immune stimulation occurs. Antibodies serve a key role in the body’s response to immune challenges. However, antibodies also produce inflammatory proteins, and when chronically stimulated, they produce large quantities of these proteins all the time. These proteins can cause major tissue damage in delicate areas like the eyes, kidneys, blood vessels, and joints.

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Congestive Heart Failure

In a normal heart, blood is pumped with ease through the vessels of the heart and lungs. However, in arteries plugged with inflammation and worms, the heart has to work harder and harder to push blood through. The right side of the heart must drastically improve its efficiency, though it may not be able to. If the right side of the heart is rendered too weak to keep up, fluid may accumulate in the chest and abdominal cavities, as well as disruption of normal electrical impulses leading to arrhythmias. When arrhythmia is a possibility, so is sudden death.

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Signs of Heartworm Disease in Cats

The cat’s immune system is extremely reactive to the inflammatory proteins produced by the presence of heartworms. Cats develop more signs of lung disease, complete with respiratory distress and chronic coughing and/or vomiting. Feline heartworm disease is often misdiagnosed as feline asthma. Sudden death may occur just as in dogs. The effects of this widespread inflammation can reach far beyond the lung and circulatory system. The kidneys, GI tract, and even nervous system can also be affected.

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Preventing Heartworm Disease

This is a potentially fatal, but ultimately highly preventable disease. There is no excuse for an owned pet to suffer and die from heartworm disease. Heartworm preventives are readily available, highly effective, extremely safe, and reasonably priced. Talk with your veterinarian today about getting your cats, dogs, and ferrets on heartworm preventive year round.