Surveillance and risk assessment of Ovine Johne's disease in Australia

Abstract

Ovine Johne’s disease (OJD) was first diagnosed in Australia in 1980, and by January 1998, more than 200 infected flocks had been identified in New South Wales, Victoria and South Australia. In 1998, in response to the increasing number of infected flocks and industry concerns about the spread of the disease, a 6-year, AUD 40.1 million national program commenced, with the aim of controlling the disease while further research was undertaken to evaluate future control or eradication options. This research was undertaken as part of the national OJD program, to improve our understanding of the performance of existing and new diagnostic tests for the disease, to estimate disease prevalence and distribution and to develop a simple, objective system for assessing flock-risk of infection. In the first study, the sensitivities and specificities of an absorbed enzyme-linked immunosorbent assay (ELISA) and an agar-gel immuno-diffusion (AGID) test for the detection of Johne’s disease in sheep were estimated using data from six known infected and 12 assumed uninfected sheep flocks. Logistic regression analysis was used to test for significant effects of lesion score and condition score, with flock included in the model as a random effect. Estimated specificities were 95% (95% CI: 93.4, 95.6%) and 99% (98.4, 99.4%) for ELISA cut-point ratios of 2.4 and 3.6 respectively, and 100% (99.7, 100.0%) for the AGID. Estimated sensitivities were 41.5% (35.0, 48.3%), 21.9% (16.6, 27.9%) and 24.6% (19.1, 30.7%) for ELISA cut-point ratios of 2.4 and 3.6 and for AGID, respectively. ELISA and AGID sensitivities varied between flocks and between categories of lesion score and condition score. Sensitivity was highest in thin sheep and iv in sheep with multibacillary lesions. The effects of lesion type and condition score on test sensitivity were both significant in the logistic regression analysis for the AGID and ELISA at both cut-points and the flock effect was significant for the AGID but not for the ELISA at either cut-point. In the second study, the flock-level sensitivity of pooled faecal culture and serological testing using AGID were compared in 296 flocks in New South Wales during 1998. The overall flock-sensitivity of pooled faecal culture was 92% (82.4%, 97.4%) compared to 61% (50.5%, 70.9%) for serology (assuming that both tests were 100% specific). In low-prevalence flocks (estimated prevalence <2%), the flock-sensitivities of pooled faecal culture and serology were 82% (57%, 96%) and 33% (19%, 49%) respectively, compared to 96% (85%, 99.5%) and 85% (72%, 93%) respectively in higher-prevalence flocks (estimated prevalence ≥2%). In a third study, the results of abattoir surveillance for OJD carried out during 2000 were analysed to estimate the prevalence of infected flocks in three regions of New South Wales and in other States. A Bayesian approach was used to adjust apparent prevalence estimates for the assumed flock-sensitivity and flock-specificity of abattoir surveillance, and to allow for uncertainty about the true values of these measures. The 95% probability limits for flock-prevalence at 31 December 2000 were 0.04% – 1.5%, 8% – 15% and 29% – 39% for low, moderate and high prevalence regions of New South Wales respectively. The other States generally had an upper 97.5% probability limit of about 1% or less. Based on this analysis, there were probably between 2,000 and 3,700 infected flocks in Australia at 31 December 2000, with more than 80% of these in a relatively small geographic area of central and southern New South Wales. v The final part of the current research was to develop a simple quantitative model for flock-risk of OJD, based on estimated flock-prevalence and within-flock prevalence for different classes of flocks, depending on their location and status. A method is also described for modifying this risk-score for individual flocks, according to the presence and level of individual flock risk factors such as the use of vaccination, testing history and the presence and number of infected neighbours. This flock-based approach to risk assessment could be supported by varying degrees of regulatory control over sheep movements, or could be adapted to a deregulated environment, with sheep producers taking responsibility for their own risk management, rather than the State or Territory regulatory authorities.