In this article
- What is Johne’s disease?
- Species affected
- Transmission
- Clinical signs
- Diagnostic recommendations
- Sample/specimen collection guidelines
- Interpreting results
- Key takeaways
What is Johne’s disease?
Johne’s disease is a chronic, progressive intestinal disease of ruminants caused by the bacterium Mycobacterium avium subspecies paratuberculosis, also known as MAP. The organism infects the small intestine, leading to thickening of the intestinal wall, reduced nutrient absorption, and eventually diarrhea and severe weight loss. The disease typically progresses slowly over time; however, once infected, animals remain infected for life.
Species affected
Johne’s disease primarily affects cattle, sheep, goats, and other ruminants such as deer, elk, and bison. While disease clinical signs vary by species, all infected animals can shed MAP and contribute to herd or flock transmission.
Transmission
Infection usually occurs early in life, most commonly through ingestion of MAP from contaminated manure, milk, colostrum, water, feed, or the environment. Calves and young stock are particularly susceptible. MAP can also be transmitted in utero from infected dams. Adult-to-adult transmission is less common but can occur in heavily contaminated environments.
Clinical signs
Animals can carry the infection for years without showing any visible signs of disease. When clinical disease develops signs typically include:
- Progressive weight loss despite a normal appetite
- Diarrhea
- Especially in cattle
- Less common in small ruminants
- Reduced milk production
- Bottle jaw
- Poor body condition
- May be difficult to assess in sheep due to their thick fleece.
- Decreased fertility
There is no effective treatment to eliminate the bacteria. Once clinical signs are apparent, the infected animal’s condition will continuously worsen until death.
Diagnostic recommendations
Johne’s disease testing strategies vary depending on the stage of infection and whether the purpose is to diagnose a single animal or assess herd infection levels. TVMDL offers several diagnostic tests, each with specific strengths and considerations based on disease progression and testing goals.
Serological tests
Agar Gel Immunodiffusion (AGID): Detection of antibodies to M. avium ssp. paratuberculosis in serum.
Complete Fixation (CF) [Referral Test]: Detection of antibodies to M. avium ssp. paratuberculosis in serum. Please note, referral tests are not performed at TVMDL; a processing and shipping fee will apply.
Milk (ELISA): Detection of antibodies to M. avium ssp. paratuberculosis in milk. This test is particularly useful for herd screening and identifying animals more likely to be shedding MAP.
Serum or Plasma (ELISA): Detection of antibodies to M. avium ssp. paratuberculosis in serum or plasma. This is another helpful diagnostic tool used for herd screening, monitoring infection trends, and identifying animals with a higher likelihood of shedding.
Molecular test
Real-time polymerase chain reaction (rtPCR): Detection of M. avium ssp. paratuberculosis DNA. Acceptable sample types include one or more of the following: feces, intestinal content or fresh intestine.
This test is highly sensitive and specific, making it excellent for confirming infection and identifying animals that are actively shedding MAP.
Bacteriological test
Solid culture: Detection of M. avium ssp. paratuberculosis by culture in a fecal sample. This test option is highly specific; however, it requires a longer turnaround time (16 weeks).
Before requesting a culture, it’s very important to consider that MAP sheds inconsistently. An infected animal is not shedding the bacteria constantly; it may shed large amounts on one day and little to none on another. It is important to note that an infected animal can have a negative fecal test result even though it carries the infection, potentially allowing Johne’s disease to spread undetected.
Pathologic evaluation
Necropsy: Post-mortem examination and dissection of a whole animal to harvest tissues and determine a cause of death. During necropsy, the intestines and associated lymphoid tissues are examined for gross changes consistent with Johne’s disease.
Histopathology: Histologic evaluation of postmortem tissues to identify characteristic microscopic lesions associated with Johne’s disease and support diagnostic confirmation.
Sample/specimen collection guidelines
Fecal or intestinal samples
- Place samples in tubes and clearly label with animal ID and sample number.
- Change gloves between samples to avoid cross-contamination.
- Use clean plastic sleeves or bags for collection.
- Ship samples as soon as possible, preferably overnight on cold packs.
- Avoid freezing samples if possible.
Blood/serum samples
- Allow blood to clot before transferring clear serum to a separate tube.
- Clearly label the tube with animal ID.
- Handle gently to avoid hemolysis, which can reduce test accuracy.
- Ship promptly with cold packs; avoid freezing unless instructed by the lab.
Milk samples
- Collect in clean, labeled containers with animal ID.
- Avoid contamination with dirt or debris.
- Ship with cold packs and as soon as possible to maintain sample quality.
Fresh carcass
- The fresher the carcass, the greater the diagnostic yield.
- Keep carcass chilled until delivery.
Avoid freezing to maintain integrity of gross and microscopic lesions.
Interpreting results
Johne’s testing must be interpreted in the context of age, species, disease stage, and herd history. Herd-level patterns are often more informative than results from an individual animal, particularly for surveillance and control programs. Herd-level testing is especially helpful for clients seeking to:
- Monitor disease status over time
- Identify trends rather than one-time results
- Guide management decisions to reduce transmission
Additionally, a negative test does not guarantee an animal is uninfected; ongoing testing is essential for monitoring and preventing Johne’s disease.
Positive ELISA: Indicates exposure and a higher likelihood of MAP shedding.
Negative ELISA: Does not rule out infection, especially in young or early-stage animals.
Positive fecal PCR: Indicates the animal is shedding MAP and poses a transmission risk.
Negative fecal PCR: Does not exclude infection, as shedding may be intermittent or below detection limits.
Positive culture: Strong evidence for infection.
Negative culture: Does not rule out infection, as MAP is shed intermittently.
Necropsy: Gross changes observed during necropsy, such as thickened intestines or enlarged lymph nodes, provide strong visual evidence of Johne’s disease and help guide further tissue sampling.
Histopathology: The microscopic study of collected tissues can confirm infection by revealing characteristic lesions and the presence of acid-fast positive MAP within intestinal macrophages.
Key takeaways
Veterinary diagnostic testing plays a central role in identifying infected and high-risk animals, allowing producers and veterinarians to make informed decisions to protect herd health and productivity.
Some key recommended testing practices for Johne’s disease include:
- Use multiple test types when possible
- Perform repeated testing over time
- Test different age groups strategically
- Consider testing before introducing new animals to the herd or before breeding
- Maintain proper sample collection and handling
For more information on Johne’s disease and TVMDL’s diagnostic services, visit tvmdl.tamu.edu or call the College Station laboratory at 888.646.5623 or the Canyon laboratory at 888.646.5624.