Abstract

An observational case–control study was conducted to investigate risk factors for confirmed bovine tuberculosis (bTB) infection in cattle reacting positively to the single intradermal comparative cervical test (SICCT) in Northern Ireland in the years 1998, 2002 and 2006.Macroscopic lesions were detected at slaughter (positive visible lesion (VL) status) in 43.0% of reactor cattle, whilst 45.3% of those sampled were confirmed as bTB positive due to the presence of lesions or positive histopathology/mycobacterial culture (positive bTB status). In 97.5% of the reactors, the VL status and bTB status were either both negative or both positive. Generalized linear mixed model analyses were conducted on data of 24,923 reactor cattle with the variables herd identifier, local veterinary office (DVO) and abattoir being used as random effects within all the models generated at univariable and multivariable level. The other variables within the dataset were used as fixed effects. Significant risk factors associated with VL status and bTB status at multivariable level (p<0.05) included age at death, breed, sex, test year, net increase in skin thickness at bovine tuberculin injection site, epidemiological status of skin test, total number of reactors at the disclosure test, mean herd size and prior response to the skin test.These risk factors are likely related to the time since infection, the strength of the challenge of infection and the susceptibility of the animal. These findings are important as the detection of visible lesions and the confirmation of bTB are an integral part of the overall bTB control programme in Northern Ireland and the veterinary meat inspection and hygiene programme. The visible lesion status and bTB status of an animal can affect the way in which bTB breakdowns are managed, since failure to detect visible lesions and recovery of Mycobacterium bovis can lead to a less stringent follow-up after other risk factors have been taken into account.

Highlights

  • Bovine tuberculosis is a chronic, infectious and zoonotic disease of domestic and wild animals caused by Mycobacterium bovis

  • Most of the missing data were based on incomplete details on the abattoir the reactor animal was slaughtered in (n=4,895; all in 1998) and missing VL status and/or bTB status (n=1,632) of which the majority (n=1,089; 66.7%) were based on reactors in 1998

  • The visible lesion status and bTB status of an animal can affect the way in which bTB breakdowns are managed, since failure to detect visible lesions and recovery of M. bovis can lead to a less stringent follow-up

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Summary

Introduction

Bovine tuberculosis (bTB) is a chronic, infectious and zoonotic disease of domestic and wild animals caused by Mycobacterium bovis. Following aerosol exposure and phagocytosis, infected macrophages enter the lymphatic system and are carried to the lymph nodes This engulfment with macrophages in turn will activate other macrophages and draw helper T-cells to the area. Activated T-cells proceed to kill macrophages infected with mycobacteria leading to destruction of the surrounding tissue, which in combination with the dead or dying macrophages creates caseous necrosis forming a granuloma or lesion. Does this lesion or granuloma create a micro-environment in which infection can be controlled, it provides the mycobacterium with a niche in which it can survive (Miranda et al, 2012). The evolution of lesions is dynamic and different between individuals (Grosset, 2003)

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