Johne’s disease, a chronic intestinal disease of ruminants caused by Mycobacterium avium ssp. paratuberculosis (MAP), can be difficult to diagnose due to a delay of years between exposure and the development of clinical signs. Delayed antibody production and intermittent shedding in feces compound diagnostic difficulties. ADDL is now recommending PCR over culture as a means to detect MAP shedding in feces based on recent studies* and the performance of our laboratory and other MAP testing laboratories on the National Veterinary Service Laboratory’s (NVSL) Johne’s Disease Proficiency Test which have demonstrated that PCR is at least as sensitive as culture. In the past, culture was considered the gold standard due to inhibitors in fecal samples that interfered with the PCR tests, but new extraction methods have overcome the problem of PCR inhibitors. The benefit to our clients is the quick turn-around time of 48 hours for PCR as compared to the 7 weeks that it takes for culture. Culture will no longer be less expensive, as we have raised the price of Johne’s culture to $35 per sample to meet the rising costs of test materials and procedures, which, unbeknownst to our clients, also used PCR to confirm any suspect cultures. Keep in mind that the same interpretive problems associated with negative cultures also apply to negative PCR tests. A culture or PCR test on feces may be negative because the animal is not infected, or because the animal is infected but was not shedding the organism in feces at the time of sample collection. The difficulties of interpreting negative fecal samples for sheep and goats is amplified by the fact that small ruminants tend to shed less organism than cattle. Johne’s culture is still available by request, and serologic testing remains the preferred method for herd screening. Johne’s PCR will be run in the ADDL on Wednesdays and Fridays of every week except official holidays at Purdue University to accommodate everyone. Cost for each PCR test is $35 per sample or $40 for pooled samples.

*Alinovi. “Real-time PCR, compared to liquid and solid culture media and ELISA for the detection of Mycobacterium avium ssp. paratuberculosis.” Veterinary Microbiology. 2009

*Logar. “Evaluation of combined high-efficiency DNA extraction and real-time PCR for detection of Mycobacterium avium ssp. parartuberculosis in subclinically infected dairy cattle: comparison with fecal culture, milk real-time PCR and milk ELISA.” BMC Veterinary Research. 2012.