Johne's is rarely seen in goats over seven years old. There are three commonly available tests for diagnosing Johne's Disease . Culturing fecal matter to detect the organism is the most accurate, but the bacteria grows slowly and the test takes six weeks to four months to complete. If the animal being tested is not shedding the organism in its feces, it can test negative even though it may really be infected. Repeat testing on suspect goats is essential.
Tests detect antibodies and are done on blood samples.
Each test has its own shortcomings. Because antibodies appear relatively late in the disease, antibody tests in general have poor sensitivity. The advantage of using the fecal culture positive test is that no other organism looks like Mycobacterium paratuberculosis , so false positives do not occur. It also tells other important information, such as how much of the organism is being shed. Interestingly, the organism cannot be cultured in the feces of sheep.