Validation of the fluorescence polarization assay for detection of milk antibody to Brucella abortus

A fluorescence polarization assay (FPA) for detection of antibody to Brucella abortus in individual milk samples was developed and validated. Samples from 190 cattle from which B. abortus was isolated; milk samples from cattle in herds infected with B. abortus (n = 1086) and positive in the milk ring test (MRT), as well as milk samples from Canadian cattle (with no evidence of brucellosis, n = 2974) were tested by the indirect enzyme immunoassay (IELISA) and the FPA. The sensitivity (based on samples from culture positive cattle) and specificity (based on Canadian milk samples) of the IELISA and the FPA were 100%. The relative sensitivity value obtained with milk from cattle of infected herds and the specificity values of the IELISA were 98.5 and 99.9%, respectively. The relative sensitivity and specificity of the FPA with the same samples were 82.2 and 99.4% using a cutoff value of 90 millipolarization units (mP). The low relative sensitivity value of the FPA was shown, by competitive enzyme immunoassay (CELISA), to be due to vaccinal antibody (assumed as vaccinal antibody against B. abortus S19 is excluded by the FPA and CELISA but not by the MRT and the IELISA), present in some of the milk samples. The FPA is a homogeneous assay which, unlike the MRT and the IELISA, may be used for testing in the field.