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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