I have had mixed results with the WADDL Johne’s ELISA test. Drawing blood is no problem, but the ELISA test for sheep can throw false negatives. We had one animal that skinnied right out. She was isolated, and after euthanasia an analysis of tissue samples confirmed Johne’s.
A PCR test using a fecal sample seems to be a reasonable test, well at least for sheep.
That’s no good. I’ve never had any issues here but I keep a closed herd and any new animal gets quarantined for a month and all tests run.