Ovine Johne’s Disease – Our Situation

Earlier this year we had the very unpleasant surprise of having an OJD positive “Pooled Faecal Culture” from a routine sampling that we conduct to maintain a disease free flock.

This positive came with NO CLINICAL SIGNS of disease in our flock, so I decided to continue testing to help confirm the diagnosis from the state run laboratory. As it was pointed out to me, “I am in denial” and “mistakes do not happen”.

1) We blood tested all animals in our stud that were over 2 years of age. The blood test is not very sensitive but it is very specific, as described in the Animal Health Australia webiste quoted below:

“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”.

Of the 388 animals tested, we had 18 positives. 3 of the oldest animals were post mortemed by a DAFFWA veterinarian and showed no visual changes. I am still waiting on the culture results.

Interestingly 2 of the 6 sheep imported from protected areas of Australia tested positive and another 2 sheep from that region had died unexpectantly from weight loss previously and resulted in Kaya undertaking worm tolerance testing as that was the suspected problem.
Given that most infection occurs before an animal is 12 months of age, this raises a few interesting questions that the authorities do not want to answer.

2) We retested the original sheep involved in the “Pooled Faecal Culture” (PFC) and this time sent duplicate samples to 2 different government run laboratories in Australia. One lab returned a negative result from the 5 pools and the other returned 1 potential positive from the 5 pools.

3) Many of our clients have tested their flocks around Australia and to this point I know of more than 10 who have reported negative results and ZERO who have reported positive results.

If you are unfortunate enough to live in SA or QLD, then a negative result does not hold any significance, even though the same testing is required to commence a “Market Assurance Program”.

CONCLUSION
Regardless of any further information, we are classified as infected and our clients are by association classified as suspect, regardless of testing results.

The entire OJD system in Australia is a flawed system which has very little influence on the transfer of the disease, BUT does provide trade protection for certain regions of Australia.

When purchasing sheep you will rely on an “Animal Health Statement”. This AHS will state the “status” of the particular consignment, based on abattoir monitoring, PFC and vaccination.

Abattoir monitoring is detecting the disease after it is already well established and possibly 5-20 years after a flock has been exposed.

PFC is only detecting the disease consistently once it is at medium to high levels and is only relevant when a vendor has performed the test. If a vendor lives in certain regions of Aust, there is no requirement to test, so detection of OJD is minimal.

“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 disease”

So what this really means is, if you import sheep onto your property, you can rely on the AHS, but then you should “flip a coin” to confirm that you do not bring in unwanted issues.

My recommendation to any “seed stock producer” is to vaccinate all lambs using Gudair and never test because, as I have discovered, “If you do not know you have a problem then you do not have a problem”.