Johne’s Disease Testing and Diagnosis FAQs
Animal Health Australia (AHA) coordinates the National Johne’s Disease Program for Australia’s livestock industries.
How accurate are current Johne’s disease 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 Johne’s diease (JD) tests overall is low, (i.e. the rate of false negatives is high), especially in young or recently infected animals. Sensitivity increases as the infection progresses in the animal, so the probability of missing the infection (false negative results) decreases in older animals or in animals in the advanced stage of the disease.
Testing groups of adult animals increases the probability of detecting the infection in the herd or flock to an acceptable level.
Current JD tests have adequate sensitivity when used as a herd or 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, so false positive results are very rare. On average in Australia:
- AGID test (used mainly in sheep) produces about 1 false positive in 1000 tests
- ELISA (used mainly in cattle) produces about 2 false positives in 1000 tests.
One ELISA test (Parachek®) is approved for use in sheep and goats, but has a higher false positive rate (about 1 false positive per 100 tests).
There is a very small chance of a result being false positive; so 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 (Mycobacterium paratuberculosis), grows very slowly.
In the early stages of the disease, 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, it’s not useful to test an animal until it’s at least 2 years old when the current tests are more likely to detect the disease. (Research is being undertaken 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. Animals in the Alpaca Market Assurance Program (AlpacaMAP) are tested by faecal culture as 2-year-olds. (A MAP has not been developed for deer yet.)
Goats are tested for the GoatMAP at a minimum age of 12 months, but the check test applies to goats 2-years-old and over.
Why are groups of animals tested, rather than individual animals?
Sampling rates for the MAPs 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.
- BJD testing and diagnosis
- National Johne’s Disease Project
- Johne’s disease market assurance programs (MAPs)