Effect of group size and chlortetracycline on Salmonella in swine

Abstract

The objective of this study was to evaluate the effect of stocking density and subtherapeutic chlortetracycline on Salmonella prevalence in swine. From one farm, four finisher barns with eight pens per barn were selected for entry into the study. Two barns received subtherapeutic chlortetracycline in the diet, and 2 rooms received the same diet without antimicrobials. Within each barn, alternate pens were assigned to high or low stocking density. Overall 6% of all fecal samples were positive. Salmonella was cultured from all barns. Of the 30 pens included in the study, 21 (70%) were positive at least once. Preliminary data suggests that there was no difference in the proportion of pen samples positive or the odds of a pen to be positive associated with stocking density. There were increased overall proportion of positive samples and increased odds of a pen to be Salmonella positive if it was in a barn that received chlortetracycline.Introduction Animal stocking density has been proposed as a potential risk factor for Salmonella shedding in swine. Stocking density has known impacts on growth performance in swine (Hyun and Ellis, 2001;Hyun et al., 1998), but data regarding animal density and marketing group as risk factors for shedding of salmonellae are sparse. In a study of US swine, groups of finisher pigs categorized as having high Salmonella prevalence were more likely to be stocked at higher pig densities (ie, less space allowance per pig) at the time of sampling, compared to low prevalence groups. (Funk et al., 2001) The association with stocking density itself was unclear, as the variation at the time of finisher sampling was accounted for by the number of pigs that had been marketed prior to sampling. Linton et al. (1970) identified higher prevalence of infection in pens with higher pig density, but this result was not confirmed on subsequent sampling in the same herd.Subtherapeutic antimicrobial use, in particular for the purpose of growth promotion, is under increasing scrutiny regarding its contribution to antimicrobial resistance in human pathogens. It is currently unclear what the effect of subtherapeutic antimicrobial use has on the prevalence of Salmonella. Few on-farm clinical trials evaluating the effect of subtherapeutic antimicrobials on Salmonella prevalence have been conducted.The objective of this study was to evaluate the effect of group size/stocking density and subtherapeutic chlortetracycline on Salmonella prevalence in swine.