The presence of antibodies to M. paratuberculosis makes blood testing possible, but it doesn't protect the host against the disease. Without the cell-mediated response to keep the infection in check, it can spread, and the animal will begin to shed organisms in its feces. When testing an animal or a herd, it's important to understand a couple of general concepts about tests and test results. For example, if you test an animal in the preclinical stages of M. paratuberculosis infection, before diarrhea develops, the animal will likely test negative - but it's a false negative. The measures of a test's accuracy are its sensitivity and its specificity.
Sensitivity refers to the ability of a test to detect infected animals.
The lower the sensitivity, the more false-negatives will occur. Tests with low sensitivity underestimate disease in an infected herd. Specificity refers to the test's ability to detect noninfected animals. It's the probability or likelihood of a negative test result in animals known to be free of infection. A test with low specificity will produce more false positives than a highly specific test.