Risk factors for reporting bovine tuberculosis-like lesions and confirmation of Mycobacterium bovis from bovine tuberculosis-like lesions from routine carcass inspection in cattle in New Zealand 2019-2021 : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Studies (Epidemiology) at Massey University, School of Veterinary Science, Massey University, Palmerston North, New Zealand

Abstract

Aim: In New Zealand, surveillance for bovine tuberculosis (bTB) is changing as the eradication programme matures. The transition from on-farm testing of livestock to predominantly slaughter surveillance for the detection of infection and providing evidence of disease absence has begun. The aim of this research is to provide data to support a critical evaluation of the effectiveness of slaughter surveillance for bTB in New Zealand. Specifically, the thesis explores risk-factors associated with i) bTB-like lesion submissions from cattle at routine carcass inspection and ii) confirmation of Mycobacterium bovis from bTB-like lesions submitted from cattle at routine carcass inspection. Methods: Lifetime and bTB data were extracted from the National Animal Identification and Tracing database and Disease Management System database for cattle slaughtered for human consumption through registered meat processing plants between 1 January 2019 and 30 September 2021. The data sets were combined using the lifetime animal identification number and collated using R Statistical software (v4.1.2; R Core Team 2021). Animal-level data included sex, age, number of lifetime movements between farms, the bTB herd status of the herd where the animal was sent to slaughter from, the disease control area the animal was sent to slaughter from, the season and year of slaughter, and the meat processor identifier. Multivariable logistic modelling was used to identify risk-factors for bTB-like lesion submission from routine carcass inspection, and risk-factors for confirmation of bTB from bTB-like lesions submitted from routine carcass inspection. Results: During the study period, there was one bTB-like lesion submitted for every 6378 cattle carcasses inspected. bTB was confirmed in one bTB-like lesion for every 60 bTB-like lesions submitted. During routine slaughter surveillance a bTB-like lesion was more likely to be submitted from an animal that came from an infected (OR 3.9, 95%CI 2.66 - 5.73) or suspended (OR 2.08, 95%CI 1.4 - 3.1) status herd, moved less than three times prior to slaughter (OR 1.17, 95%CI 1 - 1.27), and came from an area outside of movement control areas (OR 1.37, 95%CI 1.05 - 1.79). bTB was more likely to be confirmed from an animal that came from and infected status herd (OR 8.26, 95%CI 2.09 - 32.59), was female (OR 4.39, 95%CI 1.23 - 15.72), and came from a movement control area (OR 81.47, 95%CI 5.5 - 1192.37). The baseline risk of bTB-like lesion submission and confirmation of bTB differed between meat processing plants. Conclusion: These findings indicate that meat processing plant-specific factors, not just animal-specific factors alone, influence whether a bTB-like lesion is submitted for testing or confirmed as bTB. Better understanding of factors influencing the probability for a meat processing plant to detect and submit bTB-like lesions would enable more tailored policy-making to improve the overall submission rate. Identifying risk-factors for bTB-like lesion submissions and risk-factors for bTB-like lesions confirmed as bTB may identify enhancements to both on-farm surveillance, and a more efficient, risk-based approach to slaughter surveillance. The content of this research represents a first step for the overall evaluation of bTB slaughter surveillance as fit-for-purpose to support bTB eradication. A complete evaluation, based on this research, is warranted to determine required outputs and metrics for effective slaughter surveillance of bTB, and identify key areas for enhancement.