Johne’s Disease in Pygmy Goats (Part 1)
By Elaine Krieg, DVM and Nic Everett, Ph.D.
This article is a summary of a more detailed technical review and update on Johne’s Disease (JD) in goats that is available at Johnes Disease in Pygmy Goats NPGA final.pdf. This update was prompted by an increase in requests for guidance on JD testing and the fact that there have been recent improvements in JD testing in cattle and sheep. Part 1 of this article reviews current knowledge of JD and JD testing based on recent published literature while Part 2 will review new data from applying these tests to detecting JD in pygmy goat herds.
Johne’s is a worldwide, contagious, debilitating bacterial disease of ruminants including cattle, sheep, goats, llamas and alpacas. It is caused by Mycobacterium avium subsp. pseudoparatuberculosis (MAP) that can survive, but not grow, in soil for many months. There is usually a long delay (more than 6 months) between infection, becoming infectious to others, and death from rapid or sustained weight loss. It is this delay that makes the disease insidious - it enables undetected spread of the disease and makes reliable testing, herd security and sanitation important.
Young kids are the most susceptible to infection by ingestion of feces, colostrum or milk from infected adults. Unfortunately, infected does shed increased numbers of MAP bacteria (as well as coccidia) around kidding time, so the kidding barn is an environment that requires particular attention. In utero transmission of MAP is believed to be low (less than 10%) unless the doe is a high shedder of MAP or is showing clinical symptoms of JD. In cattle, MAP bacteria have been shown to be present in the semen of infected bulls. It is not known if this route of infection is important in pygmy goats. However, infected bucks are likely to be shedding MAP in their feces and so breeding to an infected buck represents an unnecessary risk. The greatest risk for introducing MAP infection into a previously JD-free herd is by the purchase of an infected animal. Thus it is an advantage to maintain a closed herd or to scrutinize new additions carefully. Remember that it is possible for an infected animal to test negative at the time of purchase and then develop JD months later. It is a good idea to isolate new animals from the main herd until they have been retested negative at least 6 months later.
It is the opinion of JD experts that the risk of cross-infection in the show ring is very low. But a wise precaution would be to not put young kids into the confines of a show pen with does of unknown status, especially with other does that have kidded recently. There is always some risk of infectious diseases when attending shows, particularly at fairgrounds where the barn may have been recently used for cattle, sheep or other goats.
There are a number of different types of tests that can be diagnostic of MAP infection and JD disease. But none of them are perfect and they have different pros and cons in different herd situations that are best evaluated with your veterinarian. If a previously untested goat dies after rapid or sustained weight loss that did not respond to diet, deworming or antibiotics, the possibility of JD should be suspected and the animal submitted for necropsy, histology and PCR or culture of mesenteric lymph nodes. If positive, the testing needs to progress to animals that are not currently showing clinical symptoms.
Historically, the “gold standard” of diagnostic tests for JD has been the isolation and culture of MAP bacteria from feces followed by definitive identification by microbiology. A positive test allowed a positive diagnosis of JD with high confidence. But it was recognized that the culture test identified less than 50% of animals that eventually developed JD. Also, a negative culture test result could take 3 months or more to obtain, during which time the “false negative” animals could infect others in the herd.
PCR (polymerase chain reaction) now allows detection of MAP DNA in feces without the need to culture the bacteria. Most test labs can report these results in 1-2 weeks of sample submission. PCR tests are reported to have equivalent sensitivity and specificity to culture tests. So a positive PCR result can equally support a positive diagnosis but a single negative result does not justify a conclusion of a non-infected animal. The test has an almost 100% success rate with animals that are shedding significant numbers of MAP bacteria, but produces a false negative result if an infected animal is shedding low numbers of bacteria or has not yet reached the shedding stage of the disease.
Blood tests may have the potential to detect early stage infections because they detect antibodies that the animal has produced in response to the bacteria, not the bacteria themselves. The test is an immunoassay using a technique known as ELISA. The literature suggests that goats may show an ELISA-positive result before becoming PCR-positive, but this has not been established in pygmy goats. As with other tests, an infected young animal may produce a negative ELISA test result and still represent a future disease risk. Remember the incubation period can be 6-12 months or more, especially in a low stress environment. This emphasizes the importance of repeated testing.
Explanation and Recommendation
Antibodies to MAP were not detected. The animal is either not infected or at a very early, undetectable stage of infection. Retest in 6-12 months to increase confidence of result.
Evidence of serum antibodies above background levels. May be in early stages of infection and are 5-15 times more likely to be MAP infected than the ELISA-negative animals above. Isolate from young animals and retest, do not use colostrum.
Low level of serum antibodies to MAP, but above manufacturer’s suggested cut-off. Odds are 16:1 that animal is infected but may be currently low risk of transmitting infection by shedding in feces. Isolate from young animals and retest by fecal PCR.
Moderate level of serum antibodies to MAP. Odds are at least 30:1 that this animal is infected and is likely to be shedding MAP bacteria in feces and milk. If confirmed by PCR/culture, animal should be culled from herd.
High level of serum antibodies. Odds are over 200:1 that animal is infected and shedding large numbers of bacteria in feces and milk. May soon develop clinical JD disease symptoms. Cull from herd unless retests do not confirm.
Table 1. Interpretations for dairy cattle from a herd known to be MAP-infected and individuals tested with an ELISA kit with cut-off S/P = 0.25 (adapted from Collins, 2002). S/P = Sample/Positive Control.
ELISA tests are significantly less expensive than PCR or culture tests. So they can be a useful tool in initial testing of a herd to help select any candidate suspect animals for subsequent PCR testing. The manufacturers of ELISA test kits for JD suggest cut-off values to distinguish between a positive and negative result that are appropriate for cattle. Experts in JD testing suggest that a single cut-off value is misleading and that any value significantly above background should be considered potentially positive (see Table above). This is supported by a recent result with a MAP-infected pygmy goat buck that was PCR-positive, had enlarged mesenteric lymph nodes that were positive for acid-fast bacteria, and showed thickening of the small intestine wall but gave ELISA S/P results of only 0.166 and 0.189 on two separate occasions. This is why it is important to obtain quantitative results from the test labs and not rely on their standard positive/negative interpretations that are based on cattle data. For example, the ELISA test report for the buck described above was “negative”, which was misleading to the owner.
One contributing reason for modest symptoms and low test scores in pygmy goats may be the common use of medicated feed that contains monensin (Rumensin) to control coccidiosis. In dairy cows, monensin usage was associated with reduced occurrence of ELISA-positive milk tests, and calves fed monensin had fewer culture-positive fecal samples. Even without monensin, it is known that infected goats shed fewer MAP bacteria than cattle or sheep.
Studies are in progress to evaluate multiple pygmy goat herds using multiple tests at multiple test centers. The resulting data should provide useful guidance for future control of JD risk and will be reported in Part 2 of this article.
If you still have questions after reading the more detailed article on the NPGA web site, contact Dr. Elaine Krieg at firstname.lastname@example.org or (530) 305-3144.
Collins, M.T., 2002. Interpretation of a Commercial Bovine Paratuberculosis Enzyme-Linked Immunoabsorbent Assay by Using Likelihood Ratios. Clinical and Diagnostic Laboratory Immunology 9(6): 1367-1371. This article is available free of charge at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC130105/?tool=pubmed
Johne’s Information Center and JTC ( Wisconsin ) at http://johnes.org/
Johne’s Information Central (National Johne’s Education Initiative) at http://johnesdisease.org/
New York State Cattle Health Assurance Program, Johne’s Module at http://nyschap.vet.cornell.edu/module/johnes/section4/johnes4.asp
USDA-APHIS Johne’s Disease Information at http://www.aphis.usda.gov/animal_health/animal_diseases/johnes/index.shtml
USDA-approved Testing Laboratories at http://www.aphis.usda.gov/animal_health/lab_info_services/approved_labs.shtml
Updated NPGA Bite Check
As agreed upon by the HER, BSC, and JTC, a visual examination of the mouth will be performed by viewing the mouth from both the side (for under and over bites) and the front (for deviation to either side). If there is any doubt as to the correctness of the bite, a more invasive manual examination can then be done. This would be done with either exam gloves or sanitizing before and after the exam. By modifying the procedure now used by NPGA judges to examine the mouth there is less chance of contagion being spread during judging but correct bites will remain an important part of the judging procedure. The local show chairs should provide latex free exam gloves and hand sanitizer with the active ingredient of 4% chlorhexidine.
This document is for informational purposes only and is in no way intended to be a substitute for medical consultation with a qualified veterinary professional. The information provided through this document is not meant to be used in the diagnosis or treatment of a health problem or disease, nor should it be construed as such.