Johne’s disease, Article Four, part four

Testing - Choosing the Right Test for the Right Purpose

Prepared and edited by Christine Rossiter and Don Hansen

Members of the AABP Food Safety Committee and the National Johne’s Working Group

Have a strategy

Despite some limitations, testing will be indicated and useful in most Johne’s disease farm plans. However, if you and your client are lacking a plan for interpreting and using the tests, the results can prompt confusion, waste time, and erode confidence in efforts to prevent or control Johne’s disease. To get the most from an investment in testing, veterinarians and cattle producers should discuss testing strategy issues as outlined in Section C in the Johne’s Disease Plan Manuals for Veterinarians printed in the May 1999 issue of The Bovine Practitioner.

Using tests for different purposes

The choice of Johne’s test and strategy will be client-specific. Some examples of testing choices are provided.

1. Diagnosis for a cow with clinical signs of Johne’s disease.

Remember that for most diseases, establishing a diagnosis is an imperfect process usually accomplished through a series of steps that rule-out the different disease possibilities. For an example, you have a herd with no confirmed history of Johne’s disease and a 6-year-old cow with diarrhea, gradual weight loss, no fever and a good appetite. There could be ten reasons why this cow is showing these signs including Johne’s disease. By thorough examination and preliminary testing, you could rule out most diseases and estimate that there is a 20% chance this cow has Johne’s disease, i.e., your pretest probability for Johne’s disease is 20%. Now, which test to use?

The most rapid and least costly tests to confirm this "suspected case of Johne’s disease" are the ELISA and AGID. Since the AGID performs best in animals with clinical signs, in this case, the performance of the AGID and ELISA are similar. The probability of a positive test being true (PVP) is 82% and the probability of a false-positive is at least 18%. Table 1.

On the other hand, the best positive predictive values come with the fecal culture test. In this case a positive culture test is virtually 100% i.e., identification of M. avium subs. paratuberculosis. Disadvantages to fecal culture here are the delay in results (8 to 16 weeks) and higher cost.

If the serology test result was negative, we could be about 85% confident that the cow is not infected, leaving a 15% chance of a false-negative result. Regardless of a negative test result, it would be a wise disease management decision to isolate this cow and submit a fecal sample for culture as a second test for this test-negative clinical suspect, particularly since Johne’s disease has not been confirmed in the herd before. Occasional cows with clinical Johne’s disease don’t have detectable antibody. An alternative would be to collect appropriate tissue samples at slaughter for histological examination or culture.

A second scenario may be a 3 year-old cow with the same signs in a herd with an established Johne’s infection and culling 3% of cows per year with Johne’s. Your impression is that an estimated 50% of cows are infected. The probability that this animal has Johne’s is +50%. The PVP of a positive ELISA test now is +95% and may offer enough added confidence to cull the heifer immediately.

makes johne’s ma nagement a routine and active issue on the farm.

Where short interval or small group testing may not be appropriate in beef herds, timing is. To accommodate management in cow calf herds, one thorough strategy is to test all cows 1-2X per year, timed to have results available before calving or breeding.

6. Testing immature replacements.

Unfortunately, because of the slow progression of M. avium subs. paratuberculosis infections from Stage I to Stage III, no current test has demonstrated sufficient sensitivity in young cattle, <20 months old and in Stages I or II, to warrant recommendation.

7. Testing herd additions.

Herds become infected with Johne’s disease by introducing infected, asymptomatic animals. The risk increases with each addition. The best way for a herd to stay free of Johne's is to remain closed. However, that is not always realistic. Obtaining cattle from a low risk source is the only reliable way to know that any individual is low risk. Thus, securing animals from herds that are one time or repeatedly test-negative reduces the risk of introducing Johne’s to the minimum. Testing individuals and introducing only those that are negative can reduce risk to limited degrees only. The strategies listed below are in order of decreasing risk of Johne’s in new animals. Low risk history means no history of Johne’s and critical management practices are in place:

No risk assessment

Test additions before buy

Test before and after buy

Test repeatedly after buy

Low risk herd history

Low risk herd history plus testing, even if some are positive

Low risk history and 30 random animals tested negative

Low risk history, < 5% AP herd test

Low risk history, negative herd test

Low risk history, repeated negative tests

If you buy, you are likely to buy Johne’s. Put a plan in place to prevent its spread.

8. The Voluntary Johne’s Disease Herd Status Program for Cattle

A task force of the Johne’s Committee of the U.S. Animal Health Association developed a voluntary Johne’s disease herd-status program in 1998. It is a scientifically sound, affordable program that encourages producers to identify their herds as low risk.

NOTE: The National Johne’s Working Group of the U.S. Animal Health Association has reviewed the information in this article. Some material has been adapted and edited with the kind permission from M.T. Collins, Johne’s Information Center, web site at http://www.vetmed.wisc.edu/pbs/johnes/

---