The April 1997 issue of Antech News summarized current views on the diagnosis of feline heartworm disease. Feline heartworm antibody and antigen testing are both available, although the indications, specificity and sensitivity of these tests vary with the case presentation and different test kits used. Filarial antibody tests typically become positive 2-3 months after infection, whereas antigen tests detect current infections with adult heartworms and not larval or previous infections. Most cats that test antigen positive have occult infections with significant pathology in the lungs and heart. A negative antibody test strongly suggests that no heartworms are present in the cat's heart, lungs or pulmonary vessels. If the cat is heartworm antigen negative and has symptoms of heartworm disease, additional testing with heartworm antibody, radiographs, blood profile and ultrasonography may be indicated.

Accurate diagnostic testing is especially important given the low prevalence of heartworm disease in cats, frequent lack of symptoms, or variable nonspecific clinical signs of pulmonary, neurologic or other disease. Results of a recently completed assessment of results from 3 laboratories are summarized below:

Sensitivity and specificity of several feline heartworm serologic tests were compared using serum samples collected serially from 42 laboratory cats experimentally infected with heartworms (n = 33), gastrointestinal hematodes (n = 6) or lungworms (n = 3). The 33 cats with Dirofilaria immitis infection were sampled before and 1, 2, 3, 5 and 6-9 months afterwards. The 6 cats with GI parasites (tapeworms, roundworms and hookworms in a staggered infection) were tested beforehand and 4 times afterwards, whereas the 3 cats infected with lungworms were assessed prior to and 1 and 2 months afterwards. Three sets of split serum samples from 233 bleedings of these cats were submitted for in-house feline heartworm antibody and antigen testing to laboratories A, B and C*.

All three laboratories detected heartworm antibody with a high degree of sensitivity (97-100% at three months post-infection, and 100% thereafter). At two months post-infection, lower sensitivity results were found. [85% (lab C), 64% (lab B) and 27% (lab A)]. Only the tests measured at labs C and B detected infection at one month, and the rates were low range, (24-30%). Of the 42 preinfection samples, all were negative with each of the three test labs, indicating 100% specificity for these antibody tests in the absence of parasite infection. None of the samples from cats infected with GI parasites or lungworms were positive at lab C, and none of the six lungworm infected samples were positive at labs A and B. However, 1 of 23 samples from cats with GI parasites was positive at both labs A and B, indicating decreased sensitivity.

With respect to heartworm antigen, antibody positive samples were measured for antigen at labs A and B only. Results were influenced greatly by the presence or absence of heartworms at necropsy. Detection rates for cats with heartworms at necropsy varied from 0, 0, 5 and 63% at the last four post-infection collection points for lab A, and fluctuated from 19, 86, 43, 52 and 100% for lab B. For cats with no heartworms at necropsy, none of the 40 samples tested at lab A was antigen-positive, whereas 4 of 42 were positive at lab B. Also, none of the pre-infection, GI parasites or lungworm infected cat samples were positive for heartworm antigen.

Antibody Tests

• Detection of heartworm antibodies is diagnostic of exposure to heartworm larvae or adults. The test can be positive as early as 30 days post-infection, and virtually all infected cats become positive by 3 months. However, not all larval-infected cats will develop adult heartworms. Adult worms are most often found in the pulmonary vasculature rather than the heart.
  
• Positive test in a cat with pertinent clinical signs (vomiting, cough, respiratory distress) strongly supports the diagnosis.
  
• Positive test in a cat without clinical signs indicates past exposure or occult infection.
  
• Negative test does not support the diagnosis, although false negatives following exposure may occur < 30 days post-infection.

Antigen Tests

• Positive test is diagnostic for the presence of adult heartworms.
  
• False negative results can occur with low worm burden or single sex infection.