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Heartworm Disease in Dogs

By: Dr. Nancy Laste

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Canine heartworm disease is caused by infection with Dirofilaria immitis (D.immitis), a filarial worm living in the pulmonary arteries. Dogs who are not taking heartworm preventative medications become infected with heartworms when an infected mosquito carrying an immature form of D. Immitis (larvae) injects those larvae into the bloodstream of the dog. A portion of the larvae mature and migrate through the bloodstream and heart to arrive in the pulmonary (lung) arteries. In a period of 4 months after injection, the young form of the adult heartworm is present. It takes another few months before the worms are sexually mature and produce baby worms (microfilaria) that begin circulating in the bloodstream of infected dogs. Additional mosquitoes become infected by ingesting the microfilaria with the blood of the dog. The microfilaria are infectious to mosquitoes, not to other dogs. The microfilaria undergo a few changes in the body of the mosquito before they reach the larval form infective to the dog, completing the life cycle of the heartworm.

There are no known predispositions for heartworm disease other than living in a warm, moist environment favorable to the mosquitoes, spending the majority of the day and evening outside and the major predisposing factor of failing to receive heartworm preventative medication. Although some reports suggest male dogs are more likely to get heartworm disease, one study found spayed female dogs to be at higher risk. The modern heartworm preventative medications are highly effective and, if religiously administered, should prevent heartworm infection.

The severity of heartworm disease is variable depending on the number of heartworms present, the intensity of the reaction of the body to the presence of the heartworms, the duration of the infection, the age of the dog and the presence of other medical conditions. Heartworm disease is frequently detected in dogs without symptoms which are presented to the veterinarian for routine care. Left untreated, many dogs will display clinical signs. Clinical signs may include coughing, hemoptysis (the coughing up of blood), shortness of breath, unwillingness or inability to exercise or signs of right-sided congestive heart failure (RCHF). Signs of RCHF include accumulation of fluid in the belly (ascites), accumulation of fluid around the lungs (pleural effusion) and pulsation of the jugular veins located in the neck. Dogs with RCHF will appear fat or bloated, but are actually usually very thin and have difficulty breathing. Dogs who suffer a blood clot to the lung (pulmonary thromboembolism, PTE) will show signs of coughing, fast breathing and lethargy. In severe cases, PTE may be fatal.

A less common syndrome seen with HWD is "caval syndrome." This syndrome occurs when the heartworms are in such large numbers that they are present in the right heart chambers and jugular veins. In this position, the worms form a mesh that causes some of the red blood cells in the heart to break up (hemolysis). The end result of the hemolysis is anemia (low red blood cell count) and jaundice (yellow color to the eye whites and gums caused by byproducts released by red blood cell breakdown in the bloodstream). Caval syndrome is very serious and requires mechanical removal of the heartworms from the heart using special instruments that are advanced into the heart through the jugular vein in the neck. This removal process is dangerous and sometimes results in death of the patient.

Disease which might mimic signs of heartworm disease would be other diseases resulting in coughing (bronchitis, tracheal collapse, pneumonia), other diseases resulting in right-sided congestive heart failure such as:

  • Pericardial disease (diseases affecting the sac covering the heart)
  • Dilated cardiomyopathy (disease of the heart muscle)
  • Diseases affecting the right-sided heart valves (tricuspid or pulmonic valve disease)
  • Other diseases resulting in pulmonary thromboembolism (Cushing's disease, etc.)
  • Other diseases causing red blood cell breakdown (immune-mediated hemolytic anemia, zinc toxicity, onion toxicity).

    Because there is a test for heartworm disease, diagnosis is not difficult. However, in a dog who is ill and positive for heartworm, one needs to determine if the signs the dog is showing are related to the heartworm disease or not.

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