Evaluation of eradication strategies for ovine Johne's Disease.

Abstract

Johne’s disease, caused by Mycobacterium avium subspecies paratuberculosis (Mptb), is spreading through domestic and non-domestic populations of ruminants worldwide. Ovine Johne’s Disease (OJD) is considered to be a relatively new disease in Australia (25 – 50 years) that at present has infected less than 10% of sheep flocks nationally. The great majority of infected flocks are concentrated around a number of foci of infection in the southern mainland and adjacent islands. OJD is highly infectious but difficult to detect early in the cycle of infection because of a lengthy incubation period (2 – 3 years) and insensitive tests for sub-clinically infected sheep. Currently there is international debate among gastroenterologists and microbiologists regarding Mptb’s involvement in the aetiology of Crohne’s disease in humans. In 1998 steering committees representing the sheep industries and state and federal governments agreed on a national program of regulation, education and research to contain and improve our understanding of the epidemiology and pathology of OJD under Australian conditions. Whole of flock destocking and restocking was considered to be one potential strategy to prevent further spread of the disease and underpin systematic regional eradication campaigns. This report presents the results of a field evaluation of the biological efficacy and economic viability of destocking and restocking strategies to eradicate OJD from farms in south eastern Australia. Of 41 flocks that were monitored for three years after restocking decontaminated farms, 28 (68%) presented with evidence of OJD. Among these (re)infected farms there was a substantial reduction in mean apparent prevalence between destocking and three years after restocking. Although equivocal, eradication failures appear to have been primarily of local origin (ie reinfection from neighbouring flocks and/or incomplete decontamination). Because of low efficacy, 20 year simulations of net farm income revealed that destocking and restocking was less profitable than vaccination as an OJD management option.