This newsletter reprints information from the June, 1988 Antech News in preparation for the upcoming heartworm season. In the last 12 months, the Antech East Coast Laboratory on Long Island tested an average of 1,044 cats per month for feline heartworm disease, with the peak testing months being March-October, 1998. The highest number of cats screened was in July (1,460) and August (1,311), 1998.

Diagnostic tests available for feline heartworm disease include those which detect antibodies against heartworm larvae or adults, and tests for antigen released by adult female heartworms. Heartworm antibody tests typically become positive 2-3 months after infection with heartworm larvae, whereas antigen tests detect current infections with adult female heartworms.

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 an assessment by McCall et al of data on feline heartworm testing 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) 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*.

Of the 42 preinfection samples, all were negative with each of the three test labs. 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 (24-30%), indicating increased sensitivity for early exposure. None of the samples from cats infected with GI parasites was positive at lab C, however, 1 of 23 samples from cats with GI parasites was positive at both labs A and B. The six samples from cats with lungworm infection were negative at all 3 labs.

With respect to heartworm antigen testing, samples were evaluated at labs A and B only. Sixty-three percent of cats with adult heartworms present at necropsy were antigen test-positive 6-9 months post infection at lab A, whereas 100% were antigen test-positive at lab B. For cats with no heartworms at necropsy, all 40 samples were antigen test-negative at lab A (specificity 100%), whereas 4 of 42 were antigen test-positive at lab B (specificity 90%). None of the pre-infection, GI parasites or lungworm infected cat samples were positive for heartworm antigen.

*Lab A, Heska Corp., Ft. Collins, CO;
Lab B, Animal Diagnostics, Inc., St. Louis, MO;
Lab C, Antech Diagnostics, using reagents from Hansen Immunologics.

Reference: McCall et al, Triennial Meeting, American Heartworm Society, Tampa, FL, May 1-3, 1998, abstr.

Antibody Tests

• Detection of heartworm antibodies is highly sensitive for 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, and a positive heartworm antibody test does not necessarily mean that the cat has adult heartworms.
  
• Positive antibody test in a cat with pertinent clinical signs (vomiting, cough, respiratory distress) supports the diagnosis of heartworm disease.
  
• Positive heartworm antibody test in a cat without clinical signs indicates past exposure or asymptomatic infection.
  
• Negative heartworm antibody test excludes the diagnosis of heartworm disease, although it does not detect a recent (<30 days) exposure.
  
• There is no cross-reactivity of Antech's feline hearworm antibody test with GI parasites or lungworms.

Antigen Tests

• Positive heartworm antigen test is diagnostic for the presence of adult heartworms.
  
• False negative heartworm antigen results can occur with low worm burdens or single sex infections. 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.