Antibody to the Dirofilaria immitis aspartyl protease inhibitor homologue is a diagnostic marker for feline heartworm infections

Feline heartworm disease, caused by the filarial nematode Dirofilaria immitis, has been diagnosed with increased frequency in areas endemic for canine heartworm infection. The routine methods for determining the infection status of dogs, such as identification of circulating microfilariae in blood or identification of circulating antigen in serum, plasma or blood, have proven inadequate for screening cats. The inadequacies are due to the likelihood of single-sex infections and clinical disease during prepatent infections. Current antibody detection methodologies rely on crude or partially purified worm antigen preparations that may result in poor specificity. This report describes the cloning, expression, and diagnostic utility of the D. immitis homologue (PDi33) of the Onchocerca volvulus aspartyl protease inhibitor (Ov33). PDi33 is present in all stages that occur in the mammalian host (microfilariae, L3, L4, adult males, and females) and is released by adults cultured in vitro. An indirect enzyme-linked immunosorbent assay (ELISA) using antibody to recombinant PDi33 as a diagnostic marker for infection in cats was very sensitive and was useful for identifying prepatent infections. Testing of sera from cats infected with common gastro-intestinal parasites also indicated excellent specificity. The same ELISA in dogs, although demonstrating reasonable sensitivity and specificity, appeared to be of less value as compared with the currently accepted antigen detection methodologies.