Overview – read this first
Testing for Johne’s disease can seem complicated. We try here to give some general guidelines but every herd and every owner is different. We urge goat owners to work with their local veterinarian to find the right testing program for their herd.
First, let’s address: Why test?
There are a number of reasons to test your animals for Johne’s disease? In fact, I think every single goat herd should be tested. The best test for your goats depends on how the testing information will help you accomplish your goals. Diagnostic testing will help you to:
- Determine whether or not MAP-infected goats are present in your herd.
- Estimate the extent of MAP infection (prevalence) in your herd.
- Control MAP (decreased the prevalence) in an infected herd.
- Eradicate MAP from an infected herd – yes this is possible.
- Monitor your herd to insure it remains MAP-free (surveillance).
- Make a diagnosis for a sick animal, does it have Johne’s disease.
- Meet a pre-purchase or pre-shipping testing 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. Lots to consider! That’s why professional help is advised.
For all of these diagnostic assays, be sure you and your veterinarian use a laboratory that has voluntarily taken (and passed!) an annual “check test” to confirm that their test kits and methods are valid. The list of laboratories can be found here. Note: labs are listed by type of test – not all laboratories perform every type of test.
Having said all that, below we give some general recommendations.
Which Test Should I Use?
The two primary types of diagnostic tests look for either the organism that causes Johne’s disease (MAP, Mycobacterium avium subspecies. paratuberculosis) or the animal’s response to infection by MAP (antibody in the blood).
There are two types of these assays: (1) PCR, which looks for the MAP genetic material from living or dead MAP, and (2) Culture, which isolates the living MAP organism itself from manure or tissue. PCR has largely replaced culture as the most accurate and affordable way to detect MAP.
1. PCR, also called direct PCR: This test is used on manure or tissue samples. The assay looks for MAP’s genetic material instead of the living organism. Most labs provide a result in 7-10 days. The accuracy of culture and PCR are comparable and PCR is faster and less expensive.
Pooling of samples (5 per pool with pooling done by the testing lab) is the most accurate and affordable way to test your herd if you have a low or zero prevalence of infection. Request “Direct PCR (pooled)” on the submission form and list your animals on the submission form in the order by decreasing age (oldest first). This insures that the pools are created with the maximum likelihood of having the MAP-infected animals in the same pool.
Paraffin block PCR is a special form of PCR that only some laboratories perform. When tissues are collected at a necropsy, they are embedded in paraffin to be thin-sectioned, stained and examined microscopically. The pathologist is looking for characteristic inflammation in tissues and for MAP itself within cells. Your veterinarian can request a PCR test on a portion of the paraffin block.
Most labs also use a PCR to confirm that the organism isolated during culture is actually MAP and not one of its closely-related mycobacterial cousins that live in soil and water.
A sample submitted for culture is monitored for eight weeks or longer because MAP is a very slow growing organism. If the sample is heavily contaminated with MAP, a positive result may be had sooner, but it can take two months of incubation or more until the lab feels confident that no MAP organisms are present in the sample tested and can report a “culture negative” result. Culture is effective for testing any animal species and can be done on manure or tissue samples.
Pooling of manure samples reduces the cost of a whole 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. Older culture methods used solid culture media that were examined visually on a weekly basis for at least 3 months. Newer methods use liquid culture media and automated instruments to “read” the culture for up to 8 weeks.
Antibody (blood or milk) tests
These assays look for antibody produced by a MAP-infected animal using a technology called ELISA (enzyme-linked immunosorbent assay).
ELISAs detect antibodies in serum and the assay is performed in microtiter plates. To perform this test, 2-3 milliliters of blood is collected from an adult animal. The fluid part of blood samples (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 (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 as well as blood samples.
ELISAs are popular because they are fast and the least expensive of the available tests for Johne’s disease. However, they are designed for rapid, low-cost screening of large numbers of animals. ELISAs are less sensitive than MAP-detection assays (PCR and culture), typically being positive in roughly 30%-50% of the animals that MAP-detection assays will identify as MAP-infected. This is generally because antibody production occurs later in the course of a MAP infection, months or even years after an infected animal has been passing MAP bacteria in its feces.
ELISAs are >99% specific. This means that there is a <1% chance that a positive ELISA is a false-positive. Typically, this means that animals found ELISA-positive should have a confirmatory test done by PCR or culture.
There are many nuances to testing recommendations that cannot be explained here. The best advice is to discuss your specific needs for Johne’s disease testing with your veterinarian.
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 goat herd owners:
Question: Is MAP present in my herd?
Recommendation: Use targeted testing (ELISA or fecal PCR) of oldest or thinnest goats (10% or more of the herd).
Question: How many of my goats are infected?
Recommendation: A good estimate can be made by blood testing (ELISA) all goats after their second kidding or older.
Question: What test should I use to control MAP in my infected herd?
Recommendation: For commercial herds, blood testing (ELISA) on all goats after their second kidding or older is economical. See the Control section for further information.
Question: What test should I use to eradicate Johne’s disease in my herd?
Recommendation: Breeders must work to eradicate MAP. Pooled fecal culture is an economic way to eliminate the infection in the herd.
Question: Does this skinny goat have Johne’s disease?
Recommendation: After ruling our parasites, fecal PCR is best. Even better is necropsy (autopsy) where a pathologist examines the tissues and a microbiologist tried to detect MAP in tissues by PCR.
Question: What test do I need to sell and transport this goat?
Recommendation: This is determined by the agency managing the shipment or the receiving owner. If I were advising the buyer, I would recommend a test on source herd (all adults or at least 30 head) by fecal PCR (pooling acceptable). Buying young animals from such a herd fairly safe. If the herd owner could show you 3 years of whole-herd negative test results that would be even better.