Heat Treatment for Improving Heartworm Testing in Cats

Detection of Dirofilaria immitis antigen has traditionally been the most sensitive and specific way to diagnose heartworm disease in dogs. Heartworm disease in cats is harder to diagnose for several reasons: low circulating antigenemia resulting from lower worm numbers, higher likelihood of male-only infections, and stunted development of D immitis adults in an aberrant host. Pretreatment of samples with heat and/or EDTA before testing has been performed in early antigen-based assays for dogs to destroy immune complexes. Antigen–antibody complex formation has been recognized as a cause for poor antigen detection in some canine samples, but elimination of complexes to improve detection of D immitis has not been evaluated in cats.

In this study, 6 cats were experimentally infected with third-stage heartworm larvae via SC injection. Infection was confirmed by recovery of adult worms at necropsy or confirmation of histologic lesions (in 1 cat that did not have adult worms). Heat treatment of feline serum before antigen testing resulted in dramatic increase in detection of D immitis antigen. Only 1 of 6 samples from cats was antigen positive before heat treatment; 5 of 6 became positive after heat treatment. Antigen blocking resulted in false-negative results from most cats harboring both D immitis and circulating antigen in the present study and heat treatment seemed prudent for more accurate testing.

Although the sample size in this study was small, the data indicate that improved sensitivity of feline heartworm antigen assays by preheating the samples would support more accurate diagnosis of this infection. This must translate into a point-of-care test or otherwise easy application for the general practitioner but can greatly improve feline health management in areas where heartworm disease is overrepresented. It is currently common for practitioners to avoid heartworm testing in cats, as tests are almost always negative and do not justify the expense for the owner.—Heather Troyer, DVM, DABVP, CVA

Heat treatment prior to testing allows detection of antigen of Dirofilaria immitis in feline serum. Little SE, Raymond MR, Thomas JE, et al. PARASITE VECTOR 7:1, 2014.