Three-Phase Decline in Reported Farm-Level Fowl Typhoid Occurrence in the Republic of Korea, 2000–2024: Temporal Alignment with the 2016–2017 H5N6 HPAI Epidemic

This study characterized the long-term changes in reported farm-level fowl typhoid occurrence in the Republic of Korea from 2000 to 2024. Farm-level notification records (n = 1495) were obtained from the national livestock disease database. Annual counts were analyzed on a log(count + 1) scale using segmented linear regression. The number of breakpoints was selected by Bayesian information criterion (BIC) comparison of candidate segmented models, and BIC favored a two-breakpoint model. Breakpoints were estimated at 2009.0 (95% CI, 2004.7–2013.3) and 2016.5 (95% CI, 2014.1–2019.0). Phase 1 (2000–2009) declined significantly (annual percent change [APC], −16.3%; 95% CI, −25.9 to −5.5), Phase 2 (2009–2016) showed no significant change (APC, 3.7%; 95% CI, −10.3 to 20.0), and Phase 3 (2016–2024) showed an accelerated decline (APC, −28.0%; 95% CI, −34.9 to −19.8). Province-specific post-2016 slopes were not associated with cumulative provincial HPAI events during 2014–2024 (Spearman’s ρ = −0.143, p = 0.736). The post-2016 acceleration temporally overlapped with the 2016–2017 H5N6 HPAI epidemic, but the study did not measure the contemporaneous interventions or denominators needed for causal attribution.