Bovine tuberculosis (TB) in herds with long-duration of official freedom during a period of national resurgence of infections

Herds which experience long periods of bTB freedom enjoy lower prospective breakdown risk. However, during periods of rising infection levels, as seen in Ireland in recent years, the number of breakdowns in low-risk herds can also increase. In this context, an analysis was undertaken to investigate the breakdown risk of long-term officially TB-free herds (≥ 10 years without a breakdown, “C10” herds), during 2016–2024 in Ireland. Furthermore, we performed a cohort study using a mixed effects logistic regression model to explore the correlates of breakdown probability for C10 herds over an 18-month risk period. The length of time a herd had been officially free was associated with a decline in the risk of breakdown during the at-risk period from January 2023 to June 2024; for example, 17.1% of herds officially free for less than 1-year (C0) had a breakdown during the at-risk period, relative to 4.6% of C10 herds. C10 herds had greater risk of breakdown during the risk period if they were a dairy (OR: 1.806; 95%CI:1.599–2.041; P < 0.001) or drystock herd (OR: 1.242;; 95%CI:1.092–1.413; P < 0.001), relative to breeding non-dairy herds; had larger herd size (OR: 1.547 for each additional unit of loge-herdsize; 95%CI: 1.475–1.622; P < 0.001); if they had any evidence of a breakdown in the past (beyond 10 years) (inverse OR: 1.348; 95%CI: 1.245–1.458; P < 0.001), and with increasing inward movements of animals (e.g. ≥ 90 vs. < 30 moves, OR: 2.395 (95%CI:1.881–3.049;P < 0.001). However, the effect of inward movement varied by herd-type, with movements being more important in non-dairy herds. There was no evidence to suggest that C10 herds in areas (district electoral divisions) wholly or partially designated for badger vaccination were at increased, or decreased, risk of breakdown relative to areas not designated for badger vaccination (P > 0.96). These data provide additional evidence of the importance of bTB history as a risk factor for bTB breakdown.