Real-time PCR, compared to liquid and solid culture media and ELISA, for the detection of Mycobacterium avium ssp. paratuberculosis.

A goal of Johne's disease control programs is to accurately detect Mycobacterium avium ssp. paratuberculosis (MAP) infected cattle as quickly as possible to reduce disease transmission. A newly introduced real-time PCR provides results rapidly, but its accuracy in the field has not been evaluated. Fecal and serum samples collected from dairy cows in northern Indiana were used to estimate the sensitivity and specificity of a newly licensed real-time PCR test for direct fecal detection of Mycobacterium avium ssp. paratuberculosis (MAP). Results of the real-time PCR were evaluated in parallel with solid and liquid media culture systems and a serum ELISA for detection of MAP antibodies to determine the accuracy of the real-time PCR and the tests' potential usefulness in the field. A total of 143 samples were tested by all four methods. Using prior published estimates for sensitivity and specificity of each of the tests and Bayesian methodology, the sensitivity and specificity of the real-time PCR test was estimated to be 0.60 and 0.97, respectively. The accuracy of real-time PCR (0.90) was comparable to both solid (0.91) and liquid (0.93) culture. Because real-time PCR accuracy is comparable to standard culture methods, it is a useful new test. In addition, test results are obtained as rapidly as an ELISA, but are more accurate than the ELISA (0.82). This makes real-time PCR an attractive test and should shorten the quarantine period required for new purchases of unknown MAP-status animals into herds participating in an MAP control program.