How Heartworms are Diagnosed

Many dogs with heartworm infection show no obvious signs of illness, especially early in the course of their condition. Heartworms are usually diagnosed during a routine blood screening test conducted for an unrelated reason. Initial testing for dogs with a chronic cough or other possible signs of heartworm disease includes a complete blood count, a serum chemistry profile and a urinalysis. Depending upon the results of these tests, the attending veterinarian may recommend an electrocardiograph and/or chest radiographs (x-rays). Thoracic radiographs are one of the best ways to determine the severity of heartworm infection, as dogs with especially heavy worm burdens will have visibly enlarged right heart chambers and/or pulmonary arteries.

The most accurate screening for heartworm disease is a simple blood test that can detect an antigen produced by adult female heartworms in infected dogs. This antigen test, or ELISA (enzyme-linked immunosorbent assay and immunochromatographic test), is extremely sensitive and specific for heartworm infection. However, it is possible to get false negative test results if the infection is in its early stages, especially if only adult males are present or if there are very low numbers of adult female worms in the affected dog. False positive results are very uncommon.

If an antigen test is positive, another blood test is available to identify the concentration of microfilaria (if any) in circulation. This involves examining a blood sample carefully under a microscope. If this test is positive, the dog definitely has heartworm infection with adult male and female worms present. If this test is negative, the animal still may be infected. Typically, up to 25% of dogs with heartworm disease do not have identifiable microfilaria in their blood. Again, microfilaria are the immature stage of canine heartworms.

An electrocardiogram and/or echocardiogram can also be used to assess moderate to severe cases. Adult heartworms can be seen in the vena cava of dogs with caval syndrome using these more advanced diagnostic protocols.

Special Notes

Heartworm disease is not particularly difficult to diagnose. It is, however, difficult to treat successfully.