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 sheep depends on what the information will help you accomplish. Diagnostic testing helps in:
- Determining whether or not MAP is present in your flock
- Estimating the extent of MAP infection in your flock
- Controlling MAP in an infected flock
- Eradicating MAP in an infected flock
- 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 diagnostic plan that best meets your needs. This plan should outline the type of test, when to test, which animals to focus on, the cost of testing, how to interpret the results and what actions to take based on test 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: http://www.aphis.usda.gov/animal_health/lab_info_services/approved_labs.shtml (labs are listed by type of test – not all laboratories perform every test)
The two primary types of diagnostic tests look for either (A) the organism that causes Johne’s disease (i.e., MAP, Mycobacterium avium ss. paratuberculosis) or (B) 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 primarily in manure samples.
- 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 adults of 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 flock 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 (positive for) MAP.
- There is a strain of MAP found in some flocks that takes particularly long to incubate (3-6 months). Be sure your lab is aware they are testing sheep (vs. another type of ruminant that usually harbor a faster growing strain)
- Direct PCR. This test is used for manure samples from adult sheep. 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 depending on the length of culture incubation.
- Paraffin block PCR. When tissues are collected at a necropsy, they are fixed with formalin and 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.
- 2-3 milliliters of blood is collected from an adult sheep. 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 sheep is infected and shedding MAP on your premises.
- The ELISA is designed for testing large numbers of samples quickly (results in a few days) and this makes it a low-cost test.
- The ELISA kit to use for sheep (individual milk or blood) is the Prionics ELISA. It appears to have less of problem with false positives due to antibody from another disease (CLA, or caseous lymphadenitis) than other kits. Tell your lab this is the antibody ELISA kit you want.
- 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 sheep 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 sheep 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 but can be attempted by experienced labs with sheep samples. Blood tests are available for cattle, sheep, goats (and deer in some countries).
Recommended testing approaches
There are a variety of ways to test your flock that will give you the information that you need. The best testing program can be developed by you and your veterinarian since you know your operation best: its goals, resources, other animal health issues, etc.
Here are some approaches that have worked well for other sheep flocks:
- Is MAP present in my flock?
- Culture of 5-10 environmental fecal samples collected at high sheep traffic areas on the farm.
- Targeted testing (ELISA or fecal culture) of oldest or thinnest sheep (10% or more of the flock).
- How many of my sheep are infected?
- A good estimate can be made by blood testing (ELISA) all adult sheep.
- In a dairy flock, another option would be milk ELISA of the same population at dry off.
- What test should I use to control MAP in my infected flock?
- Blood testing (ELISA) sheep 3 years or older. See the Control section for further information.
- What test should I use to eradicate Johne’s disease in my flock?
- Pooled fecal culture is an economic way to eliminate the infection in the flock.
- Does this skinny sheep have Johne’s disease? (Making a diagnosis for a sick sheep: weight loss, diarrhea)
- ELISA or fecal culture/PCR if previous cases have been seen in the flock.
- If the infection has never been confirmed in the flock, fecal culture/PCR is recommended, followed by necropsy if positive and culture of tissues (ileum and mesenteric lymph nodes).
- What test do I need to sell and transport this sheep?
- This is determined by the agency managing the shipment or the receiving owner.
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