Diagnosis can be difficult, because several other diseases can mimic the signs of heartworm disease. Disorders that exhibit similar symptoms include asthma; cardiomyopathy, or abnormality of the heart muscle; lungworm infection; lung fluke infection; and pleural effusion disorders, which cause fluid build-up in the space surrounding the lungs.
Routine bloodwork results may indicate a parasitic infection, but cannot positively identify heartworms. Thoracic radiographs, or chest x-rays, may help support the suspicion of heartworm infection, although abnormalities in the heart can be difficult to detect through this method. Definitive diagnosis of heartworm infection usually depends on the demonstration of adult heartworms by echocardiography, an ultrasound of the heart and vessels. Other techniques for positively identifying heartworms include pulmonary arteriography, a dye injection to visualize the lung arteries through x-rays, or detection of adult heartworm antigens via blood serology tests that detect adult heartworm antigens.
Echocardiograms cannot rule out heartworm disease definitively if there is a low worm count, but these tests are beneficial in ruling out other possible heart diseases. Cats need to be referred to a veterinary cardiologist for an echocardiogram or pulmonary arteriography.
Tests for the presence of heartworm offspring are available, but a negative test in no way rules out heartworm infection. Most feline heartworm cases have a low number of microfilaria, or offspring, that are present for a very limited time.
The most useful blood tests are for adult worm antigens, and moderately sensitive test kits are now are available in many veterinary practices. The antigen detected is believed to come from the reproductive tract of female adult worms. If less than three females are present in a cat, the test results may be falsely negative, even though adult worms are present. However, a positive heartworm antigen test gives the most definitive evidence of feline heartworm infection.
Some laboratories also can perform an enzyme-linked immunosorbent assay, or ELISA, test for antibodies in the cat's blood sample. This test is better at picking up infections, but a positive test result does not indicate whether the worms are still living.
