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.