Clinical Presentation

In humans:

            Since humans are a dead-end host for Dirofilaria immitis, most symptoms of an infection are due to the presence of an adult worm. Pulmonary dirofilariasis is the most common outcome of infection.  In this disease, a nodule forms in the lung tissue when the worm begins to degenerate, resulting in the characteristic “coin lesion” that can be observed in an x-ray.  Because the disease is usually asymptomatic, the lesion is first evidence of the disease in an individual.  If symptoms occur, they usually include eosinophilic pneumonitis weeks to months before the discovery of the lesion.  Fever, cough, malaise, chest discomfort and sometimes hemoptysis have been reported in some cases.

Less common are infections in the arterial, cardiac, ocular and abdominal regions by the worm.  Only in limited cases have microfiliariae been found inside a human host, requirements of which have been infection more than a single sex of the worm and the rare development of the third-stage larvae.

Figure 3: Noncalcified nodules in right upper lobe shown in chest radiograph.  24

In dogs:

            As primary hosts of D. immitis, dogs suffer from symptoms of both adult worms and microfilariae.  The worms are typically found in the lower caudal pulmonary arteries – with high enough numbers they can infect the right chamber of the heart.  A thickening of the heart and lungs results from infection, which can lead to respiratory insufficiency and ultimately death.  The severity and onset of the disease is determined by the number of worms, which range from 1 to 250, the length of the infection, and the activity level of the dog.  An occult heartworm infection does not cause any problems for the host, and results from either a single sex infection, host immune responses to microfilariae, or administration of heartworm prevention medicine.

  4 Stages of Clinical Signs

Figure 4: Clinical signs associated with canine heartworm.  7

Figure 5: Adult heartworm in dog.  15