OVINE JOHNES-DISEASE - THE RISK ASSOCIATED WITH SHEEP IMPORTED INTO WESTERN-AUSTRALIA

Objective To assess the risk of Johne's disease not being detected in sheep imparted from New South Wales into Western Australia. Design A s tochastic simulation model. Procedure The process of importing sheep w as broken down into steps and numbers or probabilities assigned to eac h. Controls on the movement of sheep included surveillance tests in so urce flocks and serological tests on sheep in consignments before and after transportation to Western Australia. The model calculated the ri sk of occurrence of Johne's disease in Western Australia and the succe ss of the agar gel immunodiffusion test in identifying consignments wi th infected sheep. Results Negative surveillance tests in source flock s reduced the risk to about one twentieth of that when no surveillance tests were required. On average, Johne's disease was predicted to be introduced once in every 3 to 7 years when no testing of either the so urce flock or the sheep in consignments was required. When negative su rveillance tests only were required the interval increased to once in every 63 to III years and, with the additional requirement that all sh eep in each consignment must have a negative lest before and after tra nsport, the interval further increased to once in every 125 to 333 yea rs. When only sheep in consignments were tested, the interval was calc ulated to be 8 to 14 years. Conclusion A requirement that imports be d erived from flocks which had negative surveillance tests to Johne's di sease would provide significantly greater protection for the sheep ind ustry in Western Australia.