Seasonal Presence of Salmonella spp., Salmonella Typhimurium and Its Monophasic Variant Serotype I 4,[5],12:i:-, in Selected United States Swine Feed Mills

This study evaluated the seasonal prevalence and distribution of Salmonella spp., Salmonella enterica serovar Typhimurium (ST) and its monophasic variant 4,[5],12:i:- (STM), in selected swine feed mills across the United States. Eleven facilities were selected for this study and 12 sites were sampled within each mill during fall 2016, early spring 2017, and summer 2017. Samples were evaluated following the USDA-FSIS guidelines for Salmonella isolation and culture positive samples were analyzed by polymerase chain reaction (PCR). A multiplex real-time PCR was used to differentiate ST and STM from other serotypes. Associations between season, mill, and sample site with Salmonella presence were investigated using generalized linear mixed models. Both season (p < 0.007) and mill (p < 0.005) were significantly associated with Salmonella spp. presence. Fall months were associated with a higher Salmonella prevalence (13.2%) compared with early spring and summer. A total of five isolates, among the 383 samples were serotyped as ST and STM. These two serotypes showed a similar seasonal presence throughout the study, being found during fall and summer seasons. These findings demonstrated the seasonal presence of Salmonella spp. in feed mills and the role of these environments as potential pathogen entry route into the human food chain.