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 cattle depends on what the information will help you accomplish. Diagnostic testing helps in:
- Determining whether or not MAP is present in your herd
- Estimating the extent of MAP infection in your herd
- Controlling MAP in an infected herd
- Eradicating MAP in an infected herd
- 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 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 (positive for) MAP.
- Direct PCR. This test is used for manure samples from adult cattle. 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. Both have been validated for domestic agriculture species, primarily cattle. NOTE: If you have vaccinated your cows for Johne’s disease, you cannot use blood tests to screen for the infection.
- 2-3 milliliters of blood is collected from an adult cow. 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 cow 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.
- 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).
Recommended testing approaches
There are a variety of ways to test your herd 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 dairies:
1. Is MAP present in my herd?
- Culture of 5-10 environmental fecal samples collected at high cattle traffic areas on the farm.
- Targeted testing (ELISA or fecal culture) of oldest or thinnest cows (10% or more of the herd).
2. How many of my cows are infected?
- A good estimate can be made by blood testing (ELISA) all cows in their second lactation or older at dry off.
- Another option would be milk ELISA of the same population at dry off.
3. What test should I use to control MAP in my infected herd?
- Blood testing (ELISA) cows in their second lactation or older at dry off. See the Control section for further information.
4. What test should I use to eradicate Johne’s disease in my herd?
- Pooled fecal culture is an economic way to eliminate the infection in the herd.
5. Does this skinny cow have Johne’s disease? (Making a diagnosis for a sick cow: weight loss, diarrhea)
- ELISA or fecal culture/PCR if previous cases have been seen in the herd.
- If the infection has never been confirmed in the herd, fecal culture/PCR is recommended, followed by necropsy if positive and culture of tissues (ileum and mesenteric lymph nodes).
6. What test do I need to sell and transport this cow?
- This is determined by the agency managing the shipment or the receiving owner.
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