Longitudinal study to evaluate the association between thermal environment and Salmonella shedding in a midwestern US swine farm

The objective of this study was to document the association between the thermal environment in the barn and Salmonella shedding in finishing pigs. For this purpose, individual fecal samples from 900 finishing pigs (8 collections per pig) were repeatedly collected from 18 cohorts (50 pigs per cohort) on 3 sites of a multi-site farrow-to-finish production system in a longitudinal study. Pen temperature and humidity were measured every 2. min during the study period. The thermal parameters of interest were: hourly average temperature, minimum and maximum temperature, hourly temperature variation, temperature humidity index (THI) and cumulative number of hours/degree above and below the thermal neutral zone at the pen level prior to fecal sampling for 6 time periods (12. h, 24. h, 48. h, 72. h, 1 week and 1 month). Additional potential risk factors at the individual (e.g., sex, health events), cohort (e.g., mortality, morbidity, Salmonella status of the nursery) and pen level (e.g., type of pen) were also evaluated. Multilevel logistic models using generalized linear models, with random intercepts at pig, pen and cohort levels to account for clustering (individual samples nested within pigs, pigs nested within pens, pens within cohorts) were constructed. Site (A, B, C) was considered as a fixed effect in order to control for clustering within site. The outcome variable was Salmonella fecal status of the individual sample. Cold exposure (temperatures below the thermal neutral zone) and exposure to a THI > 72 were both positively associated with risk Salmonella shedding. Nursery Salmonella status was positively associated with Salmonella shedding and pig age was negatively associated with Salmonella shedding. In the multilevel intercept-only model the largest proportion of model variance was associated with the individual fecal sample (44.8%) followed by cohort (24.5%), pen (20.5%) and pig (10.2%). The present study allowed the investigation of the association of time-variant thermal factors and Salmonella shedding. Interventions that target the thermal environment may have an effect on reducing Salmonella shedding in swine and also improve pig well-being and production efficiency. Alternatively, thermal parameters may be used to identify groups of pigs at high risk for Salmonella shedding. Future studies should be performed to investigate the cost-efficacy of interventions to improve the thermal environment of swine.