Did you know that there are a number of reasons to test your animals for Johne’s disease? And that there are times when testing for MAP (the organism that causes Johne’s disease) may not be needed? The best test for your animals depends on what the information will help you accomplish. Diagnostic testing helps in:
- Determining whether or not MAP is present in a herd
- Estimating the extent of MAP infection in a herd
- Controlling MAP in a herd known to be infected
- Making a diagnosis for a sick animal
- Seeing if MAP is present in the environment
- Meeting a purchase or shipping requirement
Once your veterinarian knows the reason(s) you want to test for Johne’s disease, s/he can tailor a surveillance program that best meets your needs. This program should outline the type of test, when to test, which animals to focus on, the cost of testing and how to interpret the results.
For any of these assays, be sure you and your veterinarian use a laboratory that has voluntarily taken (and passed!) an annual “check test” to confirm that their methods are valid. The list of laboratories can be found here. (labs are listed by type of test – not all laboratories perform every test)
The two primary types of diagnostic tests look for either the organism that causes Johne’s disease (MAP, Mycobacterium avium ss. paratuberculosis) or the animal’s response to infection by MAP (antibody in the blood or milk).
A. Organism-based tests. There are two types of these assays: (1) Culture, which isolates the living organism itself from manure, tissue or environmental samples and (2) PCR, which looks for the MAP genetic material from living or dead MAP.
- Culture: A sample submitted for culture is monitored for seven weeks or longer because MAP is a very slow growing organism. If the sample is heavily contaminated with MAP, a positive result may be detected in a week or two, but it can take two months of incubation or more until the lab feels confident that no MAP organisms are present and can report a “culture negative” result.
- Culture is effective for testing any species (manure or tissue samples).
- Environmental samples (soil, water, grass, etc.) may also be tested by culture
- Pooling of manure samples
- This approach reduces the cost of a herd test
- Individual samples are collected, then the laboratory mixes the samples (usually 5 samples per pool, 1 pool per culture).
- If a pool is test positive, the 5 animals contributing to the pool are then tested individually to find which one(s) are shedding MAP.
- Direct PCR. This test is used for manure samples, and to date has been validated in cattle only. The assay looks for MAP’s genetic material only instead of the living organism. Most labs provide a result in less than a week. The sensitivity of culture and PCR are believed to be generally comparable, although culture may perform a bit better since more and more organisms appear for detection during incubation.
- Paraffin block PCR. When tissues are collected at a necropsy, they are embedded in paraffin to be examined microscopically. The pathologist is looking for damage to the tissues and for MAP itself within cells. Your veterinarian can request a PCR test on a portion of the paraffin block.
- Culture isolate PCR identification – many labs use a type of PCR to confirm that the organism isolated during culture is actually MAP vs. one of its closely-related cousins.
B. Antibody (blood or milk) tests. These assays look for antibody produced by an infected animal. There are two common types of blood types: the ELISA and the AGID. Both have been validated for domestic agriculture species, primarily cattle.
- 2-3 milliliters of blood is collected from an adult animal. The fluid part of the sample (serum) is tested for anti-MAP antibody.
- The amount of antibody found (if any) is compared with positive and negative controls, and an interpretation is then assigned to the ELISA result. These numeric results (the actual amount of antibody) are useful: the higher the test result, the greater the certainty that the animal is infected and shedding MAP.
- The ELISA is designed for testing large numbers of samples quickly (results in a few days) and this makes it a low-cost test.
- A number of ELISA kits have been approved for use in milk from individual cows (not bulk tank) as well as blood samples.
- The only commercially produced ELISA validated by the USDA for sheep and goats is the Prionics ELISA.
- AGID. This assay also targets antibody in the blood (it is not used for milk). It is useful especially if the purpose is to investigate a diagnosis for a sick animal. It can be used for cattle, sheep and goats but does not produce a numerical result (results are reported simply as positive, negative, suspect).
When and what animals to test?
Test adult animals. Due to the biology of MAP infection, only adult animals produce the targets needed by diagnostic tests. That is, calves, kids, lambs etc. are infected while very young but they do not shed the organism with any frequency (so the organism detection assays will be negative) nor do they produce antibody (so the blood/milk tests will be negative). That is why it is recommended that diagnostic tests be used only for animals at least 18 months old.
Again, any species’ manure or tissue, ruminant or otherwise, can be tested by culture. Direct PCR has been validated for cattle manure only so far. Blood tests are available for cattle, sheep, goats (and deer in some countries).
SELECT THE SPECIES OF PARTICULAR INTEREST TO YOU AT THE LEFT FOR MORE INFORMATION ON DIAGNOSTIC TESTING (for instance, dairy cattle).
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