Download How Do Johne’s Disease Tests Work?

How Do Johne’s Disease Tests Work?
by Rebecca Calder and Julie Smith, DVM, Ph.D.
> ELISA stands for EnzymeLinked Immunosorbent
Assay. Commonly,
ELISAs are designed to
detect antibody (proteins)
>
>
generated by the immune
system in response to an
infectious disease agent.
> Fecal culture can be
done on solid or in liquid
media. “Decontamination”
steps prevent most other
bacteria from growing.
Specific temperature
and oxygen conditions
are needed to grow the
organism.
> PCR stands for Polymerase
Chain Reaction. PCR tests
use a DNA probe to detect
a specific matching DNA
sequence in the organism
of interest. Matching
sequences are “amplified”
in the test procedure so
initially tiny quantities
can be detected.
> Cattle in more advanced
stages of infection are
more likely to be detected
by tests. This is because
they are shedding larger
numbers of MAP and
have developed more
antibodies against the
organism.4
There are antibody detection tests and organism detection tests.3 The ELISA is an antibody
detection test. Fecal culture and fecal PCR are organism detection tests. Test sensitivity and costs
vary by laboratory; the relative differences shown below may need adjusted for your situation.
TEST
SAMPLE
DETECTS
SENSITIVITY
EASE OF SAMPLE
COLLECTION
COST
TURN AROUND
TIME
ELISA
BLOOD
SERUM
OR MILK
ANTIBODIES
***
****
$
FAST < 1 WEEK
SLOW
3 TO 8 WEEKS
FAST < 1 WEEK
FECAL
CULTURE
FECES
FECAL
PCR
FECES
MAP
*****
**
$$$
CAN POOL
SAMPLES
TO REDUCE
COST
MAP DNA
******
**
$$
HOW DO ELISA RESULTS RELATE TO TRUE STATUS?
Values below cut-off are
called negative and
considered non-diseased
CRITICAL CUT-OFF
Values above cut-off are
called positive and
considered diseased
TRUE NEGATIVE
TRUE POSITIVE
FALSE NEGATIVE
FALSE POSITIVE
There will always be some false negative and false positive results because test
values for truly negative and truly positive animals overlap the cut-off value.
KEY POINTS TO KEEP IN MIND AS YOU DEVELOP AND IMPLEMENT A PLAN
• Testing strategies should always be goal-driven.
• Consider the short and long-term goals of your farm and what you plan to do with the
test results.
• Available tests do not have 100% sensitivity; that is, they do not detect all infected cattle.
• As more information is gained about the prevalence of Johne’s disease in a herd,
test interpretation improves.
THIS MATERIAL WAS MADE POSSIBLE, IN PART, BY THE GEORGE WALKER MILK FUND
DECEMBER 2010
UVM Extension helps individuals and communities put research-based knowledge to work. Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in
cooperation with the United States Department of Agriculture. University of Vermont Extension, Burlington, Vermont. The University of Vermont Extension, and U.S. Department of
Agriculture, cooperating, offer education and employment to everyone without regard to race, color, national origin,gender, religion, age, disability, political beliefs, sexual orientation,
and marital or familial status.
Superscripts refer to references. The reference list and image credits are posted at www.uvm.edu/extension/johnesdisease