Quantitative fluorescent immunoassay for measurement of antibody to Dirofilaria immitis in dogs.

An automated fluorescent immunoassay technique (FIAX; International Diagnostic Technology, Santa Clara, Calif.) has been developed for the quantitation of circulating antibodies in dogs infected with Dirofilaria immitis. Two groups of sera, group I consisting of 77 samples and group II consisting of 126 samples, were obtained from experimentally infected microfilaremic dogs, known negative controls, and clinically diagnosed cases of occult dirofilariasis. Antibody against a partially purified trichloroacetic acid-soluble extract of a soluble somatic extract of D. immitis was measured by FIAX and by an enzyme-linked immunosorbent assay (ELISA). A standard curve was drawn from four samples with known FIAX titers and fluorescent signal unit values. The standard curve was used to determine titers of unknown samples. The correlation coefficients determined in the analysis of log10 ELISA and log10 FIAX values were r = 0.9057 and r = 0.8976 in groups I and II, respectively. Eighty-three percent of the titer values calculated by FIAX in group I were within one dilution, and 95% were within two dilutions, of those titers obtained by ELISA. In group II, 79 and 96% of the values obtained by FIAX were within one and two dilutions, respectively, of those obtained by ELISA. FIAX proved to be a reproducible and convenient assay for the measurement of serum antibody in dogs experimentally infected with Dirofilaria.