Most likely causes of infection and risk factors for tuberculosis in Spanish cattle herds

Introduction: This study aims to assess the most likely causes of Bovine tuberculosis (bTB) breakdowns in Spanish cattle herds and to identify the main risk factors at farm-level. Methods: Causes of bTB breakdowns were assessed through a qualitative risk-assessment based on decision-trees by analysing surveillance data from 3819 bTB breakdowns detected during 2014–2016. Results were compared to veterinary officers' (VO) opinions. Risk factors were identified through a case-control study with data from 196 bTB cases and 160 controls collected during 2014–2018. Results: The decision tree analysis identified residual infections and interactions with wildlife as the most frequent causes of breakdowns (36% each), followed by purchasing infected cattle (14%). These results were not supported by VOs' opinions. According to the regression models, the risk of bTB increased by sharing pastures (odds ratios [OR] = 2.7; 95% confidence interval [CI] = 1.6–4.4) and by increasing inwards cattle movements. The presence of wildlife reservoirs represented a significant risk for extensively-managed farms if other cattle farms are situated within a one-kilometre radius (OR = 2.3; 95% CI = 1.1–5.1). Discussion: To prevent bTB breakdowns, efforts should be devoted to decrease the likelihood of residual infections and improve farm biosecurity. The adoption of biosecurity measures might be influenced by farmers' perceptions, which should be carefully evaluated to ensure the effectiveness of such strategies.