Pieces of the Puzzle: Johne's Diagnostic Testing

The diagnosis of Johne's Disease is made difficult primarily by the cause of the disease itself, Mycobacterium paratuberculosis.  An animal can become infected at a very young age, but does not become clinically ill until 2-10 years later, when weight loss and diarrhea occur.  According to recent data, it is estimated that 22% of U.S. dairy herds are infected with M. paratuberculosis.  The expansion of dairy herds across the U.S. has facilitated the spread to uninfected herds and, with a cost ranging from $100-$200+ per cow in herds with high prevalence, producers and veterinarians are looking for detection methods to decrease or eliminate prevalence in their herds.

  To understand the testing methods, it is important to understand the general stages of the disease since testing depends on the stage of the animal's disease (in general, the later the stage of disease, the higher the likelihood the test will be positive).  The stages include Stage 1, silent infection of calves and young stock; Stage 2, inapparent carrier state of adults; Stage 3, clinical disease state of adult cattle; Stage 4, advanced clinical disease of cattle.  For every animal that is in the advanced stage of the disease, there are 10-15 silently infected calves and 6-8 carriers in a herd.

  The major types of tests for Johne's disease can be divided into two categories: fecal culture or antibody (ELISA or AGID) tests on serum.  When choosing a testing method for one cow or a group of cows it is important to consider the level of the disease status, the sensitivity and specificity of the tests chosen, and the percentage of cows in the herd that are known to have clinical disease.  The following table is a summary of the comparison between fecal culture and ELISA tests.

Diagnostic Test	Sample and Detection	Specificity	Sensitivity	Turnaround Time
Culture	Feces- Detects organism	High specificity	Ranges are 40-50%	Weeks to months
ELISA	Serum- Detects antibodies	Lower specificity	Depends on the level of diseases - ranges from 15-75%	Days

Fecal culture is the gold standard for diagnosing true positives.  ELISA has been a good screening test for many testing protocols.

  A former disadvantage of the fecal culture is the length of time it takes to complete culture growth.  However, new breakthroughs in technology, such as new molecular techniques in combination with new automated technology at ADDL, can identify highly infected animals within 2-3 weeks and animals with low levels of infection in 4-8 weeks compared with months that were usually needed.  Other types of tests to evaluate cell mediated immunity are also being evaluated. The Johne's skin test and gamma interferon assay may be used more frequently in the United States in the near future.

  In summary, there is no definitive answer for the "best test" for Johne's disease.  A great deal of thought and selection should go into the testing process when a producer and veterinarian choose the diagnostics for an animal or herd.

-by Sarah Stewart, Class of 2005

-edited by Dr. Leon Thacker

References:

1. Carpenter TE, Gardner IA, Collins MT, Whitlock RH: 2004.  Effects of Prevalence by ELISA and fecal culture on the risk of introduction of M. avium ssp. paratuberculosis infected cows into dairy herds. J Vet Diag Inv 16:31-38.

2. Kalis CH, Collins MT: 2003.  Specificity of two tests for the early diagnosis of bovine paratuberculosis based on cell mediated immunity, the Johnnin skin test and gamma interferon assay.  Vet Microb (1-2): 73-76.

3. Steeves S, Wu CC: Nov 4, 2004.  New technologies to battle little known widespread cattle disease. Purdue Agricultural Communication Service. Purdue University.

4. Smith BP: 2004.  Large Animal Internal Medicine. Mosby.

5. Stabel JR, Wells SJ, Wagner: 2002.  Relationships between fecal culture, ELISA, and bulk milk test results for Johne's Disease in US Dairy Herds. J Dairy Science. 85: 525-531.

6. Van Schaik G, Stehman SM, Schukken Y, Rossiter, Shin S:. Pooled fecal culture samples for Mycobacterium avium ssp paratuberculosis at different herd sizes and prevalence.

7. Wells SJ, Witlock RH, Wagner BA, Collins J, Garry F: 2002.  Sensitivity of test strategies used in the Voluntary Johne's Disease Herd Status Program for detection of Mycobacterium paratuberculosis infection. JAVMA 220: 1053-1057.