New Strategies for Johne's Disease Testing
Can I use culture to diagnose Johne's disease?
For many years the gold standard for detection of Mycobacterium avium spp paratuberculosis (MAP), the agent of Johne's disease, has been culture either of feces or tissues such as ileal lymph node or rectal biopsies. Culture has well known limitations in sensitivity, especially in subclinically infected cattle, and turnaround time, positive results usually taking a minimum of ~8-9 weeks to be reported, and cultures being reported as negative after 13 weeks. Thus, serology currently plays a major role in the control strategies outlined in the USDA's Uniform Methods and Rules (http://www.aphis.usda.gov/animal_health/animal_diseases/ johnes/downloads/johnes-umr.pdf) or other published control programs (Collins MT, et al. Consensus recommendations on diagnostic testing for the detection of paratuberculosis in cattle in the United States. JAVMA, 229(12):1912-1919, 2006). Nonetheless, culture – or some other method to detect MAP directly - still has a place at certain stages of a Johne's disease control program.
Is PCR as good as or better than culture?
WADDL now recommends that MAP testing of feces be done by polymerase chain reaction (PCR) directly from feces rather than traditional culture. In the past, due to problems with removing test inhibitors from fecal samples, the PCR test was less sensitive than culture. However, based on more recent studies, the performance of our laboratory and other MAP-testing laboratories on the NVSL Johne's Disease Proficiency Test, and our own internal validation, have shown PCR to be at least as sensitive as culture. PCR is probably not significantly more sensitive than culture. A major advantage of the PCR test is that results will usually be available in a week or less compared to months for culture.
What if PCR is not recommended for use in recommended control programs?
Most of the published control strategies indicate culture, not PCR, as the method to detect MAP in feces. But with improvement of PCR test performance, PCR detection can now be used in lieu of culture without any significant change in sensitivity and specificity when compared to culture. Please keep mind that the same interpretive problems associated with negative cultures will also apply to negative PCR tests.
What samples do I need for PCR?
Fecal samples for PCR should be collected and shipped the same way as they were for culture. Because PCR has not been validated on tissues, biopsy or necropsy tissues will continue to be cultured.
What about testing in sheep?
Johne's disease can be a problem in sheep (http://www.aphis.usda.gov/publications/ Johnes disease in Sheep). Culture testing in sheep is more problematic than cattle because the ovine strains are harder to grown and take much longer – negative results are reported for sheep only after 6 months. Therefore, the direct fecal PCR test has significant advantages over culture for identification of MAP in sheep.
What about testing in goats?
Johne's disease can be a problem in goats. The MAP strains in goats may be either the cattle-type strains or the sheep-like strains. When testing strategies include detection of the organism in feces, PCR testing will work as well as, but not better than, culture. Sheep and goats tend to shed less organisms than cattle, so that even greater care is needed when interpreting negative culture or PCR tests.