Testing Accuracy

How accurate are current JD tests?

This is a common question.

To be accurate, a test must not only be good at detecting infected animals (sensitivity) but also at correctly giving non-infected animals a negative result (specificity).

The sensitivity of JD tests overall is low, that is the rate of false negatives is high, especially in young or recently infected animals. Sensitivity does increase as the infection progresses in the animal, meaning the probability of missing the infection (false negative results) decreases in older animals or in animals in the advanced stage of the disaese. Therefore groups of adult animals are tested to increase the probability of detecting the infection in the herd or flock to an acceptable level. So, the current tests have adequate sensitivity when used as a herd/flock test, but a negative test on an individual animal gives very little assurance that the animal is not infected.

The specificity of JD tests is very good, meaning false positive results are very rare. On average in Australia, the AGID test produces about one false positive in one thousand tests and the ELISA produces about two false positives in one thousand.

As there is still the very small chance of a result being false positive, all reactors to screening tests are followed-up.

Why aren’t animals tested when they are young?

Even though most animals become infected when they are young, the Johne’s bacterium, M paratuberculosis, grows very slowly. In the early stages the immune system is not stimulated strongly enough for it to produce detectable antibodies, so blood tests are usually negative. Similarly, little or no shedding of bacteria in the faeces occurs early in infection, so faecal culture tests will also be negative. So, although most animals get infected when they are young, with the current tests there is usually no point testing an animal until it is at least two-years-old, when the tests are more likely to pick up the disease. Research is being undertaken at present to see if the minimum age for testing can be lowered.

The exceptions to this are deer and alpaca, where disease has progressed more quickly in cases in Australia. Consequently animals in the Alpaca Market Assurance Program (AlpacaMAP) are tested by faecal culture as one-year-olds. (A MAP has not been developed for deer yet.) Goats are tested for the Market Assurance Program at a minimum age of 12 months, but the Check Test applies to goats two-years-old and over.

Why are groups of animals tested, rather than individual animals?

Sampling rates for the Market Assurance Programs and for surveillance programs are designed to give a high probability of detecting infection, even if a small proportion of the tested animals are infected. Finding one infected animal indicates that there is JD infection in the herd or flock, as there will almost certainly be other infected animals in the early stages of the disease that have not reacted to the test, or were not tested.

Page Updated: 15 January 2010