A Study on Risk Factors for Bovine Tuberculosis in the Disease-Free Regions of Italy.

Animal tuberculosis in cattle (TB) has been controlled in many European countries through long-standing eradication programs, yet sporadic breakdowns continue to occur in officially tuberculosis-free (OTF) areas, challenging the sustainability of disease freedom. This study aimed to identify and quantify herd-level and area-level risk factors associated with TB occurrence in Italian OTF regions in order to support risk-based surveillance strategies. A national longitudinal open-cohort study was conducted using data from the Italian Veterinary Information System, including approximately 300,000 herd-year observations from 2022 to 2025. The outcome was the occurrence of at least one TB breakdown per herd-year, analyzed using a discrete-time hazard modeling approach based on a binomial generalized linear mixed model with province-level random effects. The incidence of TB remained very low but increased over time, and significant spatial clustering was observed. Higher TB risk was associated with larger herd size, a previous history of TB, non-OTF herd status, proximity to recent breakdowns, number of animals purchased, transhumance practices, and a shorter time since acquisition of OTF status at provincial level. These findings highlight that, even in disease-free contexts, TB risk is heterogeneous and driven by identifiable factors, supporting the refinement of targeted, risk-based surveillance to maintain OTF status over time.