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.
Is PCR more expensive than culture?
A disadvantage of PCR is cost – the PCR tests will cost $35 each for 1-3
samples, and $25 for each additional sample. But culture will no longer be less
expensive, as we have raised the price of culture to $35 per sample to meet
rising costs of the test materials and test procedures, which, unbeknownst to
our clients, also used PCR to confirm any suspect colonies identified on 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.
Can I pool fecal samples for PCR testing to save costs?
There are validated methods to pool samples for PCR testing. However, this will
only be cost effective in certain situations, mainly related to prevalence
within a herd or testing cohort. Any positive PCR pool will have to have each
animal retested individually. Thus, this method is only cost effective when few
pools are predicted to be positive (low prevalence herds). Please contact the
laboratory for more information about pooled fecal testing.
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.
If there are questions about Johne's Disease biology, sampling, testing and
interpretation of results please contact:
Dr. Tom Besser