Determination of individual animal-level sensitivity of abattoir surveillance for ovine Johne's Disease.

Abstract

This project aimed to estimate the sensitivity of abattoir surveillance for ovine Johne’s disease (OJD) at an individual animal level. Currently abattoir surveillance for OJD occurs in 21 meatworks around Australia and since late 1999 over 16 million sheep have been examined by this method. Abattoir inspection is an extremely efficient and economical method of regional surveillance for OJD but could also be used for negative assurance purposes if the sensitivity were known. The trial involved three inspectors working in different OJD prevalence areas and compared their diagnoses with the gold standard of histopathology for approximately 1200 sheep from known infected properties. The sensitivity level for the best inspector was 87.3%. A “dummy run” of presumed negative sheep yielded a very low false positive rate. This trial has established that for the lines of sheep examined, an average sensitivity of 70% is applicable. However for the purposes of a flock negative assurance system in a low prevalence flock, a sensitivity level of 50% would be more appropriate. This potential use of this methodology for flock assurance purposes relies on an accurate identification system for sheep.