Diagnostic potential of the PMS-phage assay and PMS-culture to detect Mycobacterium avium subsp. paratuberculosis in bovine milk samples

Controlling the spread of Johne’s disease, caused by Mycobacterium avium subsp. paratuberculosis (MAP), in domestic livestock is challenging. Current diagnostic methods lack sufficient sensitivity to detect sub-clinically infected animals, and thus better diagnostic methods are needed. This study was carried out to investigate the diagnostic potential of two novel peptide-mediated magnetic separation (PMS)-based tests - a PMS-phage assay and PMS-culture – both of which have been developed and optimised to detect viable MAP cells in bovine milk. Individual milk samples (50 ml) were obtained from 105 ‘non-infected’ and 40 ‘MAP-infected’ animals (classified as such on the basis of prior faecal culture and serum-ELISA results) in three dairy herds, and tested in parallel by the PMS-phage assay and PMS-culture. Diagnostic sensitivity (DSe) and specificity (DSp) of the PMS-phage and PMS-culture methods were determined relative to the MAP infection status of the animal contributing the milk sample. The PMS-based tests applied individually showed moderate DSe (PMS-culture 0.250 and PMS-phage assay 0.325) and high DSp (0.962 and 1.000, respectively). When results of the two PMS-based tests were combined, DSe increased substantially to 0.525 and the DSp was calculated to be 0.962. It was concluded that combined application of the PMS-phage assay and PMS-culture provided the most complete picture regarding the presence of viable MAP in bovine milk samples. A comprehensive validation of the PMS-based assays relative to currently used diagnostic methods (faecal culture and serum-ELISA) would be the next step in assessment of the diagnostic potential of these novel PMS-based methods.