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Heartworm in Dogs: Symptoms and Tests

Diagnosis

The signs of heartworm disease depend on the number of worms and the size of the dog. Dogs with a light infestation involving only a few worms may remain asymptomatic.

The typical early signs of heartworm infestation are tiring easily, exercise intolerance, and a soft, deep cough. As the disease progresses these symptoms become more severe and the dog loses weight, breathes more rapidly, and may cough after exercise to the point of fainting. The ribs become prominent and the chest starts to bulge. Acute vena cava syndrome or episode of thromboembolism can lead to collapse and death.

A number of blood tests are available to diagnose heartworms. The most accurate is the heartwormantigen test, which identifies an antigen produced by the adult female heartworm. False negatives occur in dogs with early infections (before the appearance of mature worms), in light infections with fewer than five adult worms, and in infections in which only males are present. False positives are rare.

Another important heartworm test is the microfilarial concentration test, in which parasites in a sample of blood are identified under the microscope. Although a positive test definitely indicates heartworms, a negative test does not rule out the diagnosis because typically 10 to 25 percent of infected dogs do not have microfilariae circulating in the peripheral blood.

A chest X-ray is the best test for determining the severity of the infection. Dogs with a heavy burden of worms in the pulmonary artery have X-rays that show enlargement of the right ventricle and/or pulmonary arteries.

An ECG may show right ventricular enlargement and cardiac arrhythmias. An echocardiogram may show worms in the main pulmonary artery or the right ventricle. In dogs with vena cavasyndrome, heartworms can be seen in the vena cava. Blood and urine samples are obtained to check for anemia and assess kidney and liver function.

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WebMD Veterinary Reference from "Dog Owner's Home Veterinary Handbook"

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