Evaluation and Comparison of Two Methods of Abattoir Surveillance for Detection of Ovine Johne’s Disease

Summary

This project was designed to determine whether properties infected with ovine Johne’s disease (OJD) could be identified by routine monitoring of cull sheep at abattoirs for lesions of OJD or the presence of Mycobacterium avium subsp. paratuberculosis.

Thirty-five lines of sheep that met predetermined criteria for inclusion in the trial were obtained from 17 known infected properties (average line 330, range 50 to 683). The criteria were designed to ensure that all the trial lines were infected with OJD. In most cases the trial lines were probably heavily infected. Over a period of six months, these trial lines were delivered to abattoirs throughout New South Wales for slaughter, most to the two major export works. One line was killed in Victoria.

Trained inspectors were stationed in abattoirs to examine not less than 50% and up to 95% of the abdominal viscera from all lines of adult sheep slaughtered during each kill shift (about 10 to 15 lines per shift at the two major export abattoirs). Where visible lesions suggestive of OJD were observed, fixed tissue samples were taken for confirmatory histopathology from up to three suspect sheep in each suspect line. Inspectors were not told the identity of the trial lines, but were aware that there was a trial line to be killed during the particular shift.

The inspectors detected gross lesions suggestive of OJD in 34 (97%) of the 35 eligible trial lines. The only trial line in which lesions were not detected by inspectors was a line of cross-bred ewes introduced as adults to a property on which OJD had only been reported at low prevalence in merinos. Microscopic lesions diagnostic (31 lines or 91%) or suggestive (3 lines or 9%) of OJD were identified in all 34 lines detected by inspectors. The average proportion of sheep in these 34 lines with gross lesions suggestive of OJD as reported by inspectors was 21% (range <1% to>90%).

Tied-off loops of terminal ileum were also taken from 10 randomly selected sheep in each line of sheep killed during the shift for pooled intestinal culture (PIC). Apart from 9 lines selected at the abattoirs as negative controls on the basis that these lines originated from areas not known to be OJD infected, all intestinal samples from non-trial lines were discarded on receipt at the laboratory.

For trial and control lines to be tested, the mucosa was scraped from approx 25 sq cm of each of the 10 tied-off loops of terminal ileum after opening by sterile technique. The pooled scrapings from each line were submitted for intestinal culture by the BACTEC method, with positive cultures subjected to PCR and REA for the IS900 sequence of M. ptb.

Of the 33 culture results available from the 35 trial lines, 19 (58%) were positive for M. ptb and 14 (42%) were negative. The crossbred line described above in which no visible lesions of OJD were detected by inspectors was positive on PIC. PIC detected as positive for OJD only 18 (53%) of the 34 lines detected by visual examination and histopathology.

The apparent lower sensitivity of PIC in this trial may simply reflect the small random sample (10) of sheep in the PIC method used in comparison to the larger sample examined visually (> 200 sheep in most trial lines), rather than lower sensitivity of the culture method per se.

Five lines of sheep obtained from infected properties were retrospectively excluded from the trial on the basis of later epidemiological information that indicated they did not meet the selection criteria. All of these lines were negative on both histopathology and culture.

For 7 of the 9 negative control lines (from properties not known to be infected with OJD), no visible lesions of OJD were detected. In 2 of the 9 control lines, however, inspectors reported 2% and 4% of sheep with suspect lesions, but all fixed tissue samples submitted from these lines were negative on histopathology.

Positive PIC results however were obtained from 2 of the 7 control lines in which no visible lesions were detected, one line from an area of Western NSW where OJD has not been reported to date, the other from an area where OJD had not been reported at the time. Whether these positive PIC results result from true flock infection or transient environmental contamination of cull sheep during transportation or holding in abattoir lairage is uncertain.

The results of this trial indicate that visual and tactile monitoring of the viscera of cull sheep at slaughter for lesions suggestive of ovine Johne’s disease is a highly sensitive and reliable strategy for the detection of OJD infected flocks in which deaths from OJD are occurring.

The estimated 95% confidence intervals of 91.5% and 100% for the sensitivity estimate of 97% obtained from these data suggest a practical working estimate of the sensitivity of visual/tactile examination of viscera for lesions suggestive of OJD in lines of >300 adult sheep as a screening test for the selection of samples for a definitive histopathology test would be 90% for any sheep population in which OJD has been recognised for many years.

Whether this abattoir screening technique will be sufficiently sensitive for the routine detection of flocks that have been recently infected with OJD or flocks with persistent low prevalence infection is however less certain.