Effect of a test and control program for bovine Johne's disease in Victorian dairy herds 1992 - 2002

To report on progress in Johne's disease (JD) control in infected dairy herds participating in the Victorian Johne's disease Test and Control Program (TCP).

Clinical histories and JD testing data recorded by the Department of Natural Resources and Environment (now called Department of Primary Industries) were analysed for 542 dairy herds participating in the TCP. The herds were required to conduct annual herd tests of cattle 2 years old and older with an enzyme linked immunosorbent assay (ELISA), cull the reactors and manage the younger cattle to minimise infection.

Testing of over 680,000 animals identified over 10,000 reactors giving an average prevalence of reactors at the first whole-herd test (T1) of 1.78%. There was a relatively rapid increase in the incidence of clinical disease before the TCP started and then it markedly declined. There was a slow and interrupted decline in reactor prevalence, with a marked peak occurring at the fourth herd test (T4). The average age of reactors and clinical cases was 5.7 and 5.9 years, respectively. Of the reactors and clinical cases detected during the TCP, 87% and 95% respectively, were born before the TCP started. Thirty herds completed the program by achieving three successive negative whole herd tests and 91 herds dropped out because of inability to comply with the agreed requirements of the program. There were no home-bred reactors born after the start of the program in 253 (47%) herds and of the 522 herds that were tested more than once, there were 319 (61%) herds in which no home-bred reactors were detected after the first year of testing. The number of ELISA positive animals detected at T1 appeared to be only about 26% of the animals from that round that subsequently became positive or developed clinical disease at later test rounds.

The TCP caused a marked decline in the number of clinical cases, probably because animals in which clinical disease was imminent were detected by testing and removed. A reduction in prevalence of reactors occurred only when most herd members were born after the TCP started. The sensitivity of the ELISA appears to be low based on the large number of reactors that were negative at T1 but were positive at later tests. Low sensitivity of diagnostic tests and the long incubation period of the disease limits meaningful analysis of the program until it has continued for some years. Measures adopted in the TCP have not broken the cycle of infection in many participating herds. It is unsure if this was because of poor compliance with control recommendations or a poor understanding of methods of transmission by scientists. Eradication is not feasible in the short-term.