Accuracy of Feline Heartworm Tests

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.

Feline Heartworm Tests

Test codes

Feline Heartworm Tests: Diagnostic Summary

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.

Fructosamine for Monitoring Diabetics

Serum fructosamine levels can be used to monitor glycemic control in dogs and cats with diabetes mellitus. The term "fructosamine" refers to the albumin and other plasma proteins which have been linked to a sugar (usually glucose) by the nonenzymatic chemical reaction of glycosylation. Fructosamine concentration is proportional to blood glucose concentration over the lifespan of the glycosylated protein being measured (e.g. about 2-3 weeks for human, and 1-2 weeks for feline and canine albumin). Therefore, serum fructosamine concentration provides an assessment of an individual's average blood glucose concentration over the preceding few weeks. Serum fructosamine can be measured quickly, easily, and economically for evaluating dogs and cats with diabetes. Recently completed studies show that fructosamine concentration reflects metabolic control of diabetics more objectively than sporadic blood glucose measurements. Reference fructosamine ranges in the cat and dog are from

175-400 µmol/L and 258-343 µmol/L, respectively. Measurement of serum fructosamine should be included as part of the initial diagnostic workup in the diabetic animal and should be monitored periodically during follow-up evaluations. This test is especially helpful in the cat because fructosamine is not affected by the stress hyperglycemia which commonly occurs during blood collection. Well-regulated diabetic cats and dogs usually have serum fructosamine values below 450 µmol/L, while poorly regulated diabetics usually have fructosamine concentrations above 600 µmol/L.

Serum Fructosamine

LAB TIPS

Use of Recombinant Human Erythropoietin

Erythropoietin (r-HuEPO) was administered to 6 client-owned dogs and 11 client-owned cats with naturally developing chronic renal failure. This drug stimulated erythropoiesis in both species. Variable improvement in the well-being of the patients was observed with increasing hematocrits. Adverse side-effects included seizures in patients without seizure history prior to administration; hypersensitivity reactions at injection sites; anti-r-HuEPO antibody production with resulting anemias; systemic hypertension; and iron deficiency. Clinicians should weigh the pros and cons of r-Hu-EPO before administering this drug to canine and feline patients (Cowgill et al, JAVMA 212: 521-528, 1988).

Steroid-Induced Alkaline Phosphatase

When serum alkaline phosphatase (ALP) is increased on routine panels, veterinarians often request the steroid-induced (SI) ALP measurement. Frequently, this SIALP level is elevated and the lab recommends ruling out Cushing's disease or exogenous corticosteroid administration. However, if further diagnostic testing fails to confirm Cushing's disease and no exogenous steroids have been given, clinicians may become confused. According to Feldman (Textbook of Vet Int. Med., W.B. Saunders, 1995, p. 1551) "A major concern with an abnormal SIALP is the inability to distinguish three disorders commonly confused with naturally occurring hyperadrenocorticism; iatrogenic Cushing's, diabetes mellitus, and hepatopathies. The conclusion reached by most groups has been that finding no SIALP in the serum may have diagnostic value in ruling out the diagnosis of hyperadrenocorticism but that an increase in SIALP can be caused by a variety of disorders and must be considered nonspecific."