Determining the sensitivity of abattoir surveillance for ovine Johne's disease

The objective of this study was to determine the sensitivity of abattoir surveillance of intestinal tract lesions for detecting ovine Johne's disease (OJD) under normal meatwork conditions.

The design of this study was a diagnostic test validation. The three OJD inspectors were the diagnostic test and follow-up histopathological examination was used for test validation.

Approximately 1200 sheep were procured from known high prevalence OJD infected farms. The sheep viscera were tagged (numbered) and then examined as they were processed on the abattoir line by three experienced meat inspectors. Their observations were independently recorded on a cassette tape. Specified sections of viscera were prepared and subjected to histopathological examination and these results were compared with the inspector diagnoses.

The sensitivity of abattoir inspection for OJD varied between inspectors from 53 percent to 87 percent. The specificity varied from 97 to 100 percent. It appeared that the level of sensitivity for detecting disease was higher in lines of sheep where the disease was more prevalent. It also appeared that formal training was an important aspect in ensuring a high level

Abattoir surveillance is a very economical and rapid method of assessing the OJD status of sheep. On the basis of these results it is reasonable to suggest that abattoir surveillance has a sensitivity of approximately 70 percent. This technique is useful as an ancillary to other testing regimes for negative assurance programs where a sheep identification system is used.