Ovine Johne's disease: The risk associated with sheep imported into Western Australia

Johne's disease is not known to occur in Western Australia. For the sheep industry, the most significant risk of introducing the infection comes from the importation of sheep from interstate. The risk of introducing Johne's disease undetected in sheep imported from New South Wales was assessed quantitatively by using a stochastic simulation model. Results from the computer model were used to support decisions about controls on the importation of sheep. A significant increase in the prevalence of Johne's disease in New South Wales indicated a need to review the risk to the sheep industry in Western Australia. Inputs to the model were up-dated and a sensitivity analysis conducted to determine which parameters had the greatest effect on the risk calculations. Also, an assessment was made of the possible benefits to be realized if an improved serological test was developed. Consignments with infected sheep were predicted to be introduced every 77 to 333 years when the up-dated input parameters were used in the computer model. The sensitivity analysis indicated that no input parameter had a significant influence on the risk calculations. A doubling of the sensitivity of the agar gel immunodiffusion test, which was used to test all imports before transportation to Western Australia, increased the average interval between introductions of infected sheep by a factor of three. Although the prevalence of infected, but unidentified, flocks in New South Wales was estimated to have increased, the risk to the sheep industry in Western Australia was unchanged from a previous estimate because of the lower probability that flocks in the Australian Johne's Disease Market Assurance Program would be infected. On the basis of these calculations, alterations to current controls on the importation of sheep into Western Australia were not indicated.