Residency time and other risk factors for large bovine tuberculosis breakdowns in Irish cattle herds

Bovine tuberculosis (bTB) poses challenges to sustainable livestock production, and to animal and human health globally. Its prevalence increased in Irish cattle herds between 2016 and 2023. Over the same period, bTB breakdowns (periods of trade restriction and enhanced surveillance within cattle herds due to bTB detection) in dairy herds saw an increase in the average and variability of bTB case numbers. To help inform Irish national control policy, we investigated the risk factors for large bTB breakdowns. Specifically, we set out to characterise the relationship between cattle residency times and case counts in Irish herds. For breakdowns ending between 2012 and 2023 (N = 37,176 breakdowns in 24,730 herds) we describe the association between herd size, cattle residency times, breakdown history, initiating test type, year, inward movements, neighbourhood burden and bTB breakdown case counts of greater than one, four and ten cases, using logistic regressions. For a subpopulation of 15,834 skin test initiated breakdowns, we investigated the same risk factors for increasing standard skin reactor counts using zero-truncated negative-binomial regression. We show that increasing herd size and cattle residency times are the strongest predictors of increased case counts. Herd management and initiating test type are also important but play smaller roles. Dairy and fattener herd types, and risk-based skin tests are associated with the largest breakdowns. Slaughterhouse-initiated breakdowns were associated with single cases. Interferon-γ surveillance policy evolved over the study period. When cases detected by this test type were excluded, year did not account for much variation within the estimated models. Herd- or animal-level measures of neighbourhood burden added the least explanatory ability to our models. Our results demonstrate that, while herd size and residency times are important factors driving large case counts, metrics of neighbourhood risk are not. This suggests that, following initial introduction of infection, cattle-to-cattle transmission within herds plays a more important role in amplifying infections rather than further introduction of infection from the neighbourhood. Targeting larger breeding herds for prevention of introduction of infection may assist with the control of bTB. The causes and consequences of larger breakdowns in fattener herds warrant further research.