Testing to control Johne's disease in dairy herds in Victoria

There are over 500 dairy herds enrolled in the Victorian Johne's disease (JD) test-and-control program. The objectives and components of the program and the progress in 36 herds that had completed 4 or more annual herd tests were reported at the previous colloquium. There are now nearly 400 herds that have completed 5 or more annual herd tests. There has been a slow and interrupted decline in ELISA reactor prevalence but a more marked decline in clinical case rates. A crude estimate of the ELISA sensitivity indicates it to be low. The number of ELISA positive animals detected at the first test round (T1) was about one-third of the number of animals from that round that developed clinical disease or became ELISA positive at later test rounds. Because the age distribution of clinical cases and reactors is similar, it appears that the test is identifying imminent clinical cases. The test is read at a cut-point that yields a very high diagnostic specificity.