Diagnosis and diagnostics.

There is always a catch it seems. So here in 2012, one would think that performing diagnostics to find a disease agent would be relatively easy, easy to replicate and low cost. Well – in the world of Johnes disease, one would be wrong on all counts. At the present we have blood tests, fecal DNA probes, fecal bacterial culture and lymph node culture as our in vivo (live animal) testing methods. Lets look at these:

Blood testing:

Bovine ELISA test:

-Tissue – Serum – test detects antibody to MAP

Upside – a strong positive test is pretty much diagnostic to identify an animal truely infected with MAP. It is cheap and easily done from sample collection to lab testing.

Down Side – most MAP infected animals test negative on the ELISA until they are well into clinical disease. By this time they have been shedding billions of MAP organisms into the environment, potentially infecting other younger animals. So major problem is poor sensativity (many negatives might in fact be positives)

So it is a decent screening test especially if there are alot of positive dairy animals. ELISA doesn’t work so well with beef cattle due to lower shed levels and more prolonged interval from infection to clinical manifestation.

MAP PCR (Polymerase Chain Reaction) Test

Sample – fecal material – test detects MAP DNA in the manure

Upside – a much more sensative test for detection of MAP DNA than the ELISA. And it is fast. Turn around at labs can be 1 to 3 days. The age at which this test can discover positives is much lower than the ELISA. This is a good screening test for a purebred operation (which has zero tolerance to Johnes disease) or beef herds due to difficulties with false negatives with ELISA. The test can be run on pooled samples.

Downside – Low cycle positives are pretty easy to interpret. High cycle positives (say > 35 cycles to positivity) are tricky. Is the bacteria from a clinically affected or truelly infected cow or is it a non-pathogenic pass through in the cows GI tract via exposure to contaminated feed or water? Also the test requires sophisticated (expensive) lab equipment and trained technical people to run it. Cost is a major issue. This test costs @ 8 – 10x more than the ELISA test. There is still the problem of false negatives – especially in cattle less than 1.5 – 2 years of age.

MAP Culture

Sample – fecal material – test detects MAP bacteria in liquid culture

MAP is a slow growing organism that requires special conditions to grow. So a qualified lab using specialized equipment is necessary to undertake the task of culturing (growing) the MAP. The Positives are verified via PCR.

Upside – the gold standard (Best test) available for MAP detection in live cattle.

Downside – This is a slow test – 42 days till the sample can be called negative. The problem of false negatives in young animals persists. Pass through is not as much of an issue as it is with PCR. But still must be considered. Cost is still an issue here as with PCR. Bottom section is from a cow with active Johnes disease. She was exhibiting granulomatous Ileitis). Notice how thickened the ridges are. (Thanks to http://johnes.org)

The picture below shows Ileum (distal small intestine). Top is normal