Environmental determinants of Bacillus cereus food poisoning across the urban–rural gradient in the Republic of Korea, 2011–2024: a Bayesian zero-inflated analysis of 229 districts

Bacillus cereus is a spore-forming environmental bacterium and a common cause of foodborne illness, yet its population-level determinants are poorly characterised. We analysed 1,223 notified cases from the 229 districts of the Republic of Korea, 2011 to 2024; models used the 223 contiguous districts (3,122 district-years). Crude incidence was 3.7-fold higher in rural than urban districts, with a warm-season peak. With no spatial autocorrelation (Moran’s I = −0.005, p = 0.51), incidence was modelled with a Bayesian zero-inflated negative-binomial model with independent district and random-walk year effects, which outperformed the negative-binomial model and was well calibrated (ΔDIC ≈50, mid-PIT 0.500 against 0.494). Adjusting for demographic, environmental, healthcare and socioeconomic covariates, pasture land was associated with higher incidence (incidence–rate ratio 1.75, 95% credible interval 1.07 to 2.94), and in-area medical expenditure (0.55, 0.38 to 0.78), influenza-vaccination uptake (0.72, 0.53 to 0.96) and fiscal independence (0.66, 0.45 to 0.95) with lower incidence. In stratified models, household groundwater wells predicted higher rural incidence (1.89, 1.24 to 2.96) and the elderly-population rate higher urban incidence (1.47, 1.06 to 2.04). These exploratory findings are consistent with agricultural and private-water environments contributing to the rural excess of B. cereus.