Johne’s Testing Guidelines
When testing cattle for Johne’s disease it is important to select the appropriate test animals, consider the purpose for testing, and then select the diagnostic test that matches the testing purpose.
For screening purposes, the serum ELISA is the best test. The PCR is best used as a confirmatory test.
The KSVDL reports out both a test interpretation and the ELISA S/P ratio. The S/P ratio provides much more information than the test interpretation; therefore, the ratio-values should be used to help manage individual animals.
In the table below are guidelines that can be used with the S/P to help producers manage Johne’s disease.
|Recommendations for using the Johne’s ELISA S/P test results (used only with IDEXX ELISA)*|
|S/P ratio||Test interpretation||Test follow-up||Comment|
|0.0 – 0.09||Negative||Blood test in one year to confirm|
|0.10 – 0.24||Suspect||Blood test or PCR in 6 months to one year||15 times more likely to be positive compared to negative animal|
|0.25 – 0.39||Low positive||Blood test or PCR in 6 months to one year||Can be either – or + by fecal PCR|
|0.40 – 0.99||Positive||PCR if confirmation is necessary||Likely shedding in feces, colostrum and milk|
|1.0 – 10.00||Strong positive||PCR if confirmation is necessary||Shedding is likely; is in later stages of disease; clinical disease may follow|
|*Adapted from Dr. M.T. Collins and IDEXX Corporation.|
For most herds, the most practical testing plan would include using ELISA for initial screening and PCR to confirm shedding is occurring. Using the PCR as a screening test may miss positive animals because animals in the earlier courses of the disease may be antibody positive but not shedding, and some animals will either be shedding at low levels or perhaps be intermittent shedders, both increasing the possibility of false negatives.1
1Rebecca Mitchel, et al. Differences in intermittent and continuous fecal shedding patterns between natural and experimental Mycobacterium avium subspecies paratuberculosis infections in cattle. Veterinary Research (2015) 46:66.
Available Johne’s disease tests:
- Johne's Antibody ELISA (SES-1059)
Sample: 0.5 ml serum (red top) or 0.5 ml whole blood (purple top)
- Johne's Fecal PCR (BMB-1200)
Sample: Fresh feces (5 gms)
Laboratory can pool up to 5 samples for $32.00.