Johne’s, CAE, and CL Testing for Goats: Serology 101
How goat testing labs determine blood test results.
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We use goat blood testing as a biosecurity measure to detect and prevent the spread of incurable goat diseases in our herd. The “why” of blood testing is easy. CAE and CL testing for goats allows us to control diseases.
The “how” — drawing blood samples — can be learned online or with a mentor.
The “what” is a question unanswered for many:
- What do serology tests do?
- What don’t they do?
- What should we do with the results?
Cultures and PCR, or “polymerase chain reaction,” are tests used to detect pathogens — virus or bacteria — that attack the body. However, for diseases such as CAE (caprine arthritis-encephalitis), CL (caseous lymphadenitis), and Johne’s disease, pathogens may not be reliably present for tests to detect. Rather than relying on detecting the pathogen, serology uses blood to measure antibodies as an indication of infection. Antibodies are proteins produced by the body designed to combat specific infections. If serology is negative, there are no detectable antibodies. If it is positive, the goat has antibodies, which means it encountered the pathogen at some point. ELISA, “enzyme-linked immunosorbent assay,” is a serology test.
CAE, CL, and Johne’s are lifelong, contagious conditions — once infected, always infected. With these diseases, the presence of antibodies indicates infection. The exception is if the animal has been vaccinated, which is currently only a possibility with CL.
For other pathogens that a goat is able to clear from its system, positive serology will only indicate exposure and not necessarily active infection. The question is — where did exposure occur? In their own body? An infected herdmate? A previous herd? Because of this, and the relatively inexpensive nature of serological testing, it often works best as a herd screening and monitoring tool, rather than to determine individual disease status.
Testing individual goats or testing a herd at only one point in time are not reliable indicators of a clean animal or clean herd. When adding an animal to an existing herd, CAE and CL testing for goats from the herd of origin are the best indicator of an animal’s likelihood of exposure. A herd with multiple positive serology results is highly likely to have infection present.
Michele Rupert, DVM, CVA, owner of Rupert Ranch, LLC in North Carolina, cautions, “I know someone who bought a goat that tested negative for Johne’s, CAE, and CL in 2017. There was no recent testing at the time of sale in 2020. When tested at the new farm, she was seropositive for Johne’s and CL. It turned out that the test from the previous farm had a couple of goats positive for Johne’s. There was a three-year time-lapse of no testing data. Had that farm tested annually, they could have caught it sooner, had less environmental contamination in their pastures/barns, and not sold someone an infected goat, putting another herd at risk.”
What causes a false positive or false negative?
There are many reasons for unexpected results. How the sample is collected and stored is one of the primary reasons. Labs will specify how to collect and ship specimens. The timing of sample collection can also influence test results. For most tests, goats must be at least six months old to eliminate the possibility of detecting antibodies the kid received from the dam. Results can also be affected by stress, just prior to or after kidding, recent vaccination, or in animals with inflammation or high parasite loads. False negatives can be a result of an infection with antibodies too low to be detected, as there is a delay between infection and antibody production.
There are those that say testing is a waste of time and money because the number of false positives and negatives make the tests invalid. However, being on alert for a possible infection is much better than being unaware. You can’t manage a problem and minimize the impact if you don’t know it exists. While a false positive is alarming, it is better than a false negative. False positives can be ruled out by subsequent testing.
Understanding the meaning of sensitivity and specificity of a test is valuable for interpreting your results. “For screening a herd, it’s important to have a test with high sensitivity, because you don’t want potential false-negative results to cause you to miss positive animals,” states VMRD veterinarian Siddra Hines, DVM, Ph.D., DACVIM. “However, it’s always important to balance this with specificity. If the specificity is too low, you end up with too many false positives.” To put this in perspective, the only USDA-licensed ELISA test for CAE (manufactured by VMRD) has a published specificity of 99.6%. This means 4/1000 truly negative animals could falsely test positive — but the remaining 996 that test positive should truly be positive. Amanda Grimm, MS, a scientist with VMRD, explains, “Like FDA approval for humans, the USDA requires rigorous evaluation to ensure tests meet USDA standards for quality and performance. Only USDA-licensed tests are legal for diagnostic purposes in the US; unlicensed tests can only be used for research.”
What is a test cutoff?
In ELISA tests, chemical reactions cause a color change that correlates to antibodies in the sample. The color development is measured, producing a number to compare to the “cutoff.” Test manufacturers establish cutoff numbers by testing many different animals of known positive or negative status. Individual labs may interpret tests using slightly different parameters. If a result falls close to the cutoff — it may be marked “suspect.” This can occur in early infection, or in animals that are immunosuppressed. Some individuals may have unique properties in their blood that interfere with the test, causing a higher-than-normal reaction even without antibodies present. Whenever results are unclear or unexpected, the lab should rerun the sample to confirm the result. If still unclear, a new sampling should be done four to six weeks later. If the animal is positive, antibody levels should increase in that time.
If a disease is suspected through testing or symptoms, it is vital to work with a veterinarian to determine the next steps. Multiple diagnostic methods may be needed to determine the animal’s status, and herd management strategies.
Even animals with no clinical signs can be seropositive. In the case of CAE, as much as 90% of infected goats remain asymptomatic for years or life (https://waddl.vetmed.wsu.edu/animal-disease-faq/cae.) It is a lentivirus — like the human HIV virus — that can have a variable incubation period before detection. For this reason, a single test does not indicate that the animal is disease-free. The disease is spread by bodily fluids, making any encounter with an infected animal a risk for those who are uninfected.
CL status can also not be determined by a single negative test. It is a disease indicated by the presence of abscesses and is spread by direct contact with bacteria. There are two tests used to detect CL: serology and bacterial culture.
If the animal has visible abscesses, culturing the contents is the most definitive way to diagnose CL. Animals can have internal abscesses that shed bacteria before external abscesses appear. Early detection through serology, allows the producer to have carcasses screened for internal abscesses, and monitor for external abscesses. Because antibodies can result from either infection or vaccination, the Merck Veterinary Manual recommends CL vaccination only in herds where CL is already present.
Johne’s is spread by contact with contaminated feces. It has a very long incubation period, and detection is rare prior to 18 months of age, even in infected animals. It can be diagnosed by serology or fecal testing.
There are different types of fecal tests for Johne’s — culture and PCR — and they can be expensive. Culture shows that the bacteria are active and can replicate, but can take five to 16 weeks to get results. PCR results can be available in days, as it only detects the presence of bacteria without determining if it is active. According to APHIS, only 40% of infected cattle are identified by even the most sensitive culture technique. Some animals are not actively shedding the bacteria at the time of testing.
In Dr. Rupert’s experience, “Because of the intermittent shedding that occurs throughout the disease, the fecal test can miss a true positive. For example, I’ve seen a seropositive doe that was emaciated and presented clinically for Johne’s. She tested negative on fecal PCR, but I still recommended culling her regardless of PCR results. I’ve also seen a case that looked clinically normal, tested seropositive, and was also positive on fecal PCR. It’s a very frustrating, tricky disease to manage.”
If not diagnosed while the animal is living, necropsy of the lower small intestine and lymph nodes can also determine Johne’s status.
Goat owners are often cautioned to use only an accredited lab for accurate results. Accreditation is a quality control measure that audits a lab’s records and procedures. The most notable accreditation is AAVLD (American Association of Veterinary Laboratory Diagnosticians.) It is important to be informed as a consumer. If a lab is private — not attached to a university or government agency — it is not eligible for AAVLD accreditation. Amardeep Khushoo, Ph.D. explains, “Private labs can meet and even exceed accreditation standards. At UBRL, we do assay validations, run internal controls, review L-J plots and perform continuous quality assessments. We also participate in annual proficiency for Johne’s disease serology tests submitting our results to USDA/APHIS for validation of quality and standards. We have rigorous Quality Assurance and Quality Improvement programs. When we report, we consider the impact on the people (and their animals) that will receive the result and offer support and education. Our mission is to eradicate these diseases. We make testing convenient, cost-effective, and easy to understand for everyone including 4-H groups, backyard, and hobby farmers. We need participation from a vast majority of people to test for and eliminate these pathogens from our environment.”
Regardless of where you perform testing, it is important to work with your veterinarian to interpret results and develop a disease management plan that is right for your herd. “There is no “one-size-fits-all” strategy for disease management,” says Dr. Hines, “So much is dependent on your herd, the living environment, potential sources of risk, and herd history. What is feasible or economical for someone else may not work in your situation. However, virtually everyone benefits from some type of disease monitoring program, tailored to their specific needs.”
Karen Kopf and her husband Dale own Kopf Canyon Ranch in Troy, Idaho. They enjoy “goating” together and helping others goat. You can learn more about them at Kopf Canyon Ranch on Facebook or kikogoats.org
Originally published in the September/October 2021 issue of Goat Journal and regularly vetted for accuracy.