Efficiency of slaughterhouse surveillance for the detection of bovine tuberculosis in cattle in Northern Ireland

Abstract

Post-mortem examination continues to play an important surveillance role in the bovine tuberculosis (bTB) eradication programme in Northern Ireland. It is estimated that 18-28% of new bTB herd breakdowns are disclosed by the detection of bTB lesions in animals routinely slaughtered. The purpose of this study was to compare the performance of different slaughterhouses in Northern Ireland in detecting bTB-lesioned animals at routine slaughter (LRS) and to apply the findings to maximize the sensitivity of bTB slaughterhouse surveillance. Univariate statistical analysis on cattle slaughtered in Northern Ireland during 2011-2013 revealed that the risk of LRS disclosure varied between slaughterhouses, ranging from 0·08% to 0·54%. Furthermore, the risk of confirmation of these LRS as bTB varied between slaughterhouses, ranging from 57·9% to 72·4%. Logistic regression modelling of selected risk factors found that the risk of LRS disclosure increased with age, and was higher in purchased animals, during winter months, in animals coming from high bTB incidence areas and in animals slaughtered from herds with a bTB restriction in the last 2-3 years. Adjusting for these selected factors, the risk of LRS disclosure and bTB confirmation changed very little from the univariable analysis, suggesting that differences in disclosure risks between slaughterhouses were likely to be due to factors related to the slaughterhouses, rather than to the risk status of the animals presented. Examination of procedures within these slaughterhouses is recommended to identify ways that could increase the sensitivity of their bTB surveillance.

Risk factors and variations in detection of new bovine tuberculosis breakdowns via slaughterhouse surveillance in Great Britain

“…Compared to Scottish animals, the odds of being a SLH case are on average 21 times higher for animals living mostly in the HFT region of England, and 12 times higher for animals living mostly in the HFT region of Wales. Qualitatively similar effects were observed for analogous risk region (patch) variables in earlier studies [ 10 , 12 , 13 ].…”

“…As expected, there is a strong increasing effect of age, such that older animals are more likely to be picked up as slaughterhouse cases than younger animals (the odds of animals >60 months old being SLH cases are on average 5.3 times higher than the odds for than animals (0, 18] months old). Again, this general increase of risk with age was also observed in earlier studies in ROI and NI [ 10 , 12 ], and is likely a proxy for an increased probability of infection, and also an increased duration of infection (thus increasing the time for the development of visible lesions).…”

“…This is qualitatively similar to the earlier study in the ROI [ 10 ], where the risk of submission and confirmation (in general) was smaller in animals that were sourced from herds that had been clear of bTB infection for longer. In NI [ 12 ] the risk of disclosure seems to be smaller in herds that have recently cleared infection, but then rises again for herds that have been clear for longer. This may reflect differences in the underlying epidemiology, or effectiveness of testing, in NI compared to GB.…”

“…Similar research was conducted in Northern Ireland (NI) [ 12 ], again focusing on risk factors for disclosure of bTB in slaughterhouses, and for the probability of confirmation. This study looked at the effects of age, sex, season, patch incidence (e.g.…”

The impact of the number of tuberculin skin test reactors and infection confirmation on the risk of future bovine tuberculosis incidents; a Northern Ireland perspective

“…This complements the above logic relating to residual bTB infection in herds. Furthermore, the poor sensitivity of abattoir inspection in finding gross bTB lesions alongside reported variation between abattoirs [34][35][36][37] supports the findings from the current study and questions a policy where the follow-up SCITT regime after disclosure of a bTB herd incident is determined by bTB confirmation status.…”