Determination of Individual Animal - Level Sensitivity of Abattoir Surveillance for Ovine Johne's Disease

Summary

Ovine Johne’s disease (OJD) is a fatal enteric infection that is difficult to diagnose and control. In recent years this disease has become increasingly important as surveillance strategies have found infected flocks in different areas in Australia. The disease is highly topical and receives wide coverage in both the rural and mainstream media. Recently funds have been allocated to research many aspects of this disease such as the efficacy of vaccination and organism survival. However an economic, rapid and highly accurate diagnostic test still eludes researchers around the world.

Visual and manual examination of sheep viscera by inspectors as they pass through an abattoir is an extremely rapid and economic method of diagnosing OJD. Unfortunately until this time there has been no rigorously conducted study to estimate the ability of inspectors to detect truly infected sheep. Nationally abattoir surveillance has been increasingly adopted as a methodology for detecting infected viscera and this surveillance activity is now a requirement for maintaining Protected and Control zone status. Since late 1999, approximately 16 million sheep1 have been examined for OJD at abattoirs with a total of 2398 infected lines detected as positive. In total, 4% of the lines examined were positive with most of these lines emanating from the heavily infected areas of NSW. It is apparent that the vast majority of sheep viscera examined are in fact negative.

In the absence of a perfectly sensitive diagnostic test for OJD, sheep producers wishing to buy uninfected stock have the option of purchasing from a “lower risk” source such as a Market Assurance Program (MAP) accredited flock. There are significant costs and risks associated with being involved in such programs and uptake has not been high. Abattoir surveillance is currently conducted in all states in Australia and much of the data generated is in fact negative. It would seem sensible to utilise this information for the purpose of providing some assurance of freedom from OJD. It is necessary to have a good estimate of the sensitivity of abattoir surveillance to do this.

This project aimed to establish the sensitivity of abattoir surveillance for OJD under conditions encountered in a meat works. Three OJD inspectors were involved in the trial from different OJD prevalence areas in Australia. Approximately 1200 sheep from OJD infected farms were examined by the inspectors and subsequently with follow – up histopathology. The ability of inspectors to detect gross lesions varied from 53% to 87%. The best performing two inspectors (74% and 87%) had a high level of agreement between their diagnoses despite coming from very different prevalence areas (WA and NSW). The third inspector had not undergone the formal OJD inspector training which may have contributed considerably to the lower level of sensitivity. The results suggest that abattoir inspection can detect about 70% - 75% of histologically positive animals However, for the purposes of setting a sensitivity level for a negative assurance scheme, it is reasonable to use a sensitivity of 50% to account for the lower level of heavily infected animals in low prevalence flocks..

A small control line of presumed negative sheep were inspected at the abattoir without confirmatory histopathology. These two hundred sheep yielded an extremely low false positive rate with only one inspector misclassifying one sheep. The false positive rate was higher for the infected lines. Since confirmatory histopathology is performed on the most diagnostic lesions in such circumstances, it is extremely unlikely that a flock of sheep would be falsely labelled as infected because of gross pathology. However the use of gross pathology — or indeed any test based on abattoir surveillance — is contingent upon a widely adopted, accurate method of sheep identification.

The use of gross pathology also relies upon properly trained and accredited OJD inspectors. The best inspector in this trial, with such a high sensitivity, would be well placed to assess other inspectors under line conditions to ensure minimum inspection standards are met.

When a sheep identification system and inspector accreditation system are in place negative abattoir data for OJD could be used in a flock assurance scheme, or as part of providing an assessment of the risk of the flock having OJD (risk-based trading). Use of abattoir surveillance for such purposes would probably occur at the request of the individual producer initially and could supplement other diagnostic testing regimes such as pooled faecal culture and serological check tests. This would aid producers immensely in trading in “OJD assured negative” sheep or in a risk-based trading system with very little, if any cost to the producer.