Submitted to: Current Issues in Intestinal Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 24, 2005
Publication Date: February 20, 2006
Citation: Callaway, T.R., Morrow, J.A., Edrington, T.S., Genovese, K.J., Dowd, S.E., Carroll, J.A., Dailey, J.W., Harvey, R.B., Poole, T.L., Anderson, R.C. 2006. Social stress increases fecal shedding of Salmonella typhimurium by early weaned piglets. Current Issues in Intestinal Microbiology. 7:65-72. Interpretive Summary: Pigs are weaned at an early age and are transported in mixed groups for long distances. Mixing of groups of pigs together causes social stress, through fighting and establishment of a dominance hierarchy. Stress causes changes in the immune system, making animals more susceptible to colonization by bacteria that can cause disease in animals or in humans. The effects of social stress on intestinal populations of food borne pathogenic bacteria have never before been demonstrated. In this study, pigs were exchanged daily between "mixed" pens, creating a constant social stress as the dominance hierarchy was re-established, control pigs were not exchanged. One "seeder" pig was inoculated with Salmonella Typhimurium in each pen containing "contact" pigs that were all quickly colonized by Salmonella. Fecal swabs were collected each day, and by day 5, more mixed pigs were shedding Salmonella than were controls. Upon necropsy, mixed pigs were more susceptible to tissue invasion by Salmonella, and rectal Salmonella populations were higher in this group. Collectively, the results indicate that social stress of weaned pigs may increase the susceptibility to and/or fecal shedding of Salmonella.
Technical Abstract: "Segregated early weaning" (SEW) of pigs reduces exposure to pathogenic bacteria but on arrival at grower facilities pigs may be co-mingled without regard to farm of origin. The present study was designed to examine the effect of mixing (social) stress on populations of Salmonella enterica Typhimurium in SEW pigs. Piglets (7 d old; n = 28 in each of 2 replicates) were separated into 2 treatments (control and "mixed" [e.g., one piglet would be swapped each day between "mixed" pens] groups) of 2 pens per treatment (7 piglets/pen). One (n = 1) "seeder" pig/pen was inoculated with 10**9 CFU of S. Typhimurium via oral gavage. Each seeder was placed with non-inoculated "contact" piglets (n = 6). A "contact" piglet was swapped each day between the "mixed" pens for 5 d; pigs in control pens were not exchanged. On d 5, the incidence of fecal Salmonella shedding was higher in the mixed contact pigs (P< 0.05). Rectal Salmonella and cecal coliform populations in mixed pigs were significantly (P< 0.05) greater than in control pigs but cecal Salmonella populations were not different. Mixed pigs were more susceptible to tissue invasiveness (i.e., Salmonella-positive tonsils and lymph nodes) than control pigs. These results indicate that social stress of weaned pigs may increase susceptibility to and/or fecal shedding of Salmonella.