Heartworm Disease


Clinical Signs

Clinical presentation of heartworm infection depends on whether the patient is a dog or a cat and, particular in dogs, the duration of disease and the size and position of the worm burden. The clinical signs can be roughly grouped according to severity into categories of mild, moderate and severe. Not all dogs will necessarily show all signs indicated for the respective category or will exhibit mixed signs. Small dogs, with smaller hearts and blood vessels will tend to show more severe signs with a smaller worm burden than larger breeds.

Mild or sub-clinical stage:

  • normal condition of the dog
  • few worms tolerated without obvious clinical signs

Moderate stage:

  • fair or good condition of the dog
  • a degree of exercise intolerance
  • occasional coughing during exercise
  • eventually some pallor indicative of anemia

Severe stage:

  • possible loss of weight and condition.
  • intolerance of exercise
  • regular coughing
  • increased respiratory rate at rest
  • eventually pale with severe anemia (PCV < 20%)

The clinical evolution of heartworm disease in dogs is usually chronic. Most infected dogs do not show any symptoms of the disease for a long time, depending on the worm burden, individual reactivity and exercise. Signs of the disease develop gradually and may begin with a chronic cough, maybe followed by dyspnea, from moderate to severe, weakness, and sometimes faint after exercise or excitement. Later, when right cardiac congestive heart failure is developing, swelling of the abdomen and sometimes the legs, anorexia, weight loss and dehydration are usually noted. At this stage, cardiac murmur due to tricuspid valve insufficiency and abnormal cardiac rhythm due to atrial fibrillation are common findings. Sudden death rarely occurs and usually happens following respiratory distress and cachexia.

Caval syndrome (Vena cava syndrome)

After severe sponanteous thromboembolism following the natural death of many heartworms, dogs may show acute life-threatening dyspnea and hemoptysis. In addition, even living worms can move out of the pulmonary arteries into the right ventricle and atrium, and rarely into the vena cava, due to pulmonary hypertension and sudden fall in right cardiac output. This caval syndrome is commonly noted in small sized dogs. The worms may move backwards and forwards through the tricuspid valve causing valvular incompetence. Fatal outcome is usual. The following signs may be observed:

  • anorexia, weakness and dyspnea
  • red urine, anemia, pale mucus membranes
  • heart failure
  • right-sided systolic murmur
  • jugular pulse and weak femoral pulse
  • liver and kidney dysfunction
  • vomiting

Human symptoms

The filarial species of the carnivores are zoonotic pathogens. Occasionally D. immitis is transmitted via mosquitoes to humans. In some individuals, worm migration and development occurs, with or without clinical signs. The lungs are the most common location for worm settlement (pulmonal dirofilariosis). There they can cause a "coin lesion" seen on radiography and often being mistaken for a tumor. This may be asymptomatic or may be associated with symptoms including coughing or chest pain. Aberrant migration may also occur.


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