Host Pathology

Usually no signs of infection are present in dogs until 8 to 9 months postinfection. Symptoms are generally related to the intensity of infection. The presence of 25 worms may be tolerated by the dog with no signs of disease. Symptoms such as reduced exercise tolerance and coughing increase significantly when the nematode burden doubles and serious symptoms, such as dsypnea (difficulty in breathing), hepatomegaly (enlargement of the liver), syncope (temporary loss of consciousness), and ascites (fluid accumulation in the abdomen) appear. At this level, death may occur. Pathogenesis is related primarily to inflammation of pulmonary arteries and lungs induced by the adult stage of D. immitis. This chronic inflammatory process in combination with the physical obstruction by the nematodes of blood flow leads to pul monary hypertension and heart failure. Diagnosis depends on an accurate history, recognition of symptoms, and diagnostic procedures such as microfilarial detection, serology, clinical laboratory tests, radiology, ultrasonography, and angiography. Microfilaria are detected through concentration of the microfilaria from a small quantity of blood by the Knott's test or filtration. In some dog heartworm infections (occult heartworm infection), microfilaria are not detectable and diagnosis relies on serology and other diagnostic tools.

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