Why do you get animals positive for Johnes disease on the blood test but then negative on the faecal test?
There are 3 possible reasons for this: – False positive blood result: When an animal tests positive for the disease but in fact is not infected, it is termed a “false positive”. This may happen because antibody produced by the animal to other organisms may cross-react to the test. These cross-reactions can be temporary, in which case they may subside and the animal will be negative if tested several weeks or months later. This phenomenon affects all diagnostic tests to some degree and is the reason for the use of a second test. For Johne’s disease we use the faecal culture as the second test to confirm positives in some cases where there may be doubt about the herd status. In herds known to be infected there is little need for a second test. – Incorrect faecal culture result: The dung test is considered by some to be the best diagnostic test for Johne’s disease. It is almost 100% specific, meaning that if a positive result is obtained the bacteria is present in the faeces and the anima
