Detection of Mycobacterium avium subsp. paratuberculosis in bulk tank milk by combined phage-PCR assay: Evidence that plaque number is a good predictor of MAP

Conventional culture and a rapid phage-PCR method were used to detect Mycobacterium avium subsp. paratuberculosis (MAP) in bulk tank milk (BTM) samples. Only two of 225 samples (0.9%) were found to contain MAP by culture whereas 50 (22%) MAP-positive samples were identified using the phage-PCR assay, including both samples that were MAP-culture positive. Results using the phage-based method for independently tested duplicate samples indicated that the assay is very reproducible (r2=0.897), especially when low levels of mycobacteria are present. A relationship was established between plaque number and the presence of MAP in a sample. A cut-off value was determined allowing identification of MAP-positive samples based on plaque number alone (90% sensitivity, 99% specificity; area under the curve=0.976). These results indicate that the assay is a robust method for screening BTM, providing results within 24h. •We evaluate a rapid, phage-PCR detection of Mycobacterium avium subsp. paratuberculosis in milk.•The assay is more sensitive than culture and BTM results were gained within 48h.•A correlation was identified between plaque number and the presence of MAP.•A cut off value was determined to simplify the test (Sn=90%; Sp=99%).•This test could be used to screen BTM and reduce human exposure to MAP.