Canine Heartworm Disease with Pulmonary Embolism

History. You examine a 6-ycar-old male beagle that has been a hunting companion of his owner for several years. The owner reports that the dog tires more easily than usual and has developed a cough that is worse during exercise.

Clinical Examination. On physical examination of the dog, you notice the cough reported by the owner and an apparent, modest accumulation of Iluid in the abdominal cavity (ascites). You also note a systolic murmur that is heard the loudest over the left third and fourth intercostal spaces. The chest radiograph and electrocardiogram show evidence of right ventricular enlargement. In addition, the pulmonary vessels are more prominent than normal on the radiograph and are tortuous (twisted). You suspect canine heartworm disease. You obtain a blood sample, some of which you submit for an enzyme-linked immunosorbent assay (ElJSA) to test for heartworm antigen. Additionally, you use a pipette to apply a sample of the buffy coat (from the centrifuge tube) onto a glass slide for microscopic examination. You see microfilaria of the type shed by adult canine heart worms (Dirofilaria immitis), and the ELISA is positive for the presence of D. immitis antigen. You diagnose canine heartworm parasitism.

Comment. Mosquitoes transfer the microfilaria from the bloodstream of an infected dog to the bloodstream of a non­infected dog. The microfilaria develop into adult worms, which grow to a length of 10 to 20 cm while clinging to the walls of the pulmonary artery and its major branches. Heartworm infestation typically causes pulmonary arterial vessels to become enlarged and tortuous.

Treatment. You advise the owner that the dog should be treated with an arsenic-containing medication that kills adult worms over several days. You also warn the owner that the treatment of severely infested dogs is risky. Dead adult worms break away from the right ventricle and pulmonary artery and lodge in smaller pulmonary vessels. These vascular occlusions (pulmonary emboli) restrict pulmonary blood flow and reduce cardiac output. J herefore, it is necessary to keep the dog in a quiet, unstressed state for 8 to 10 days after beginning treat­ment. In addition to restricting pulmonary blood flow, the emboli are likely to cause inflammation and blood clots in the lungs. Pulmonary edema is expected. Pulmonary blood vessels may break down, allowing blood to enter the airways of the lungs. Respiratory failure is possible. Antiinflammatory drugs are sometimes administered to reduce these complications.

With the owner’s consent, you keep the dog at your clinic for 2 days (to allow him to become accustomed to the sur­roundings) and then begin treatment. During the next week, the dog becomes even more lethargic than before and begins to cough up blood. The dog has a low-grade fever (102^o-103^o F), and his ascites becomes worse. However, his systolic murmur begins to fade. After I week, all the clinical signs have greatly improved. The dog is sent home for a prolonged period of recuperation. The long-term prognosis is good.