Effect of transportation and mixing with unfamiliar pig on Salmonella susceptibility in market weight pigs

There is increasing evidence that stress can have a significant deleterious effect on food safety through a variety of potential mechanisms. However, there is very little research conducted to determine the potential effects of specific pre-slaughter stressors on Salmonella infection and carriage in pigs. Understanding when pathogen loads are the highest or when animals are most susceptible to infection is critical to determine when intervention strategies for pathogen control may be most effective, and consequently, increase pork safety. Therefore, this study was conducted to determine the effect of two common pre-slaughter stressors, transportation and mixing with unfamiliar pigs, on the susceptibility of marketweight pigs to a low-dose Salmonella challenge. A total of 40 market-weight pigs were randomly assigned to one of the following four treatments: 1) control, 2) mixing with another pig for 6 hours, 3) transportation for 1 hour, and 4) transportation for 1 hour followed by mixing with another pig for 6 hours. Immediately after the transportation treatment, all pigs were individually inoculated with 104 cfu of Salmonella Typhimurium. After 6 hours, the pigs were euthanized and subjected to necropsy for sample collection, including ileal and cecal contents, ileal tissue, and mesenteric lymph node. All samples were processed for the isolation and enumeration of the challenge strain. Even though a low challenge dose was used, infection and shedding were established in all market-weight pigs used in this study. Pigs subjected to any of the stress treatments had higher (P<0.05) levels of Salmonella in the ileum, whereas only pigs subjected to both stressors combined (i.e., transportation and mixing) had higher (P<0.05) Salmonella levels in their cecum, compared to control pigs. Therefore, it is concluded that pre-slaughter transportation and mixing with unfamiliar pigs increases the susceptibility of market-weight pigs to a low-dose Salmonella challenge. Introduction Colonization of swine by Salmonella, and its subsequent dissemination along the pork production and processing chain is a major public health and economic issue for the pork industry worldwide. There is some evidence that stress in farm animals can have a significant deleterious effect on food safety through a variety of potential mechanisms (Rostagno, 2009). However, there is very little research conducted to determine the potential effects of specific pre-slaughter stressors on Salmonella-infected pigs. Moreover, there is almost no research to determine how stress affects the susceptibility of pigs to Salmonella infection/colonization. The gastrointestinal microbiota may be disturbed by many factors, causing levels of pathogens and shedding from unapparent carriers (i.e., subclinically infected animals) to be affected. For instance, during the process of being transported from production farms to abattoirs, pigs are exposed to a variety of potential stressors before slaughter (Warriss, 2003; Averos et al., 2008). Consequently, many believe that the number of animals carrying and shedding Salmonella, as well as its levels in the gastrointestinal tract will be increased in response to stressors. It is also believed that the pig’s susceptibility to new infections will increase. However, although these assumptions are widely accepted, definitive proof still lacks, as most of the current knowledge is based on limited scientific evidence. Therefore, this study was conducted to determine the effect of two common pre-slaughter stressors, transportation and mixing with unfamiliar pigs, on the susceptibility of market-weight pigs to a low-dose Salmonella challenge. Material and Methods A total of 40 market-weight pigs were randomly assigned to one of the following four treatments: 1) control, 2) mixing with another pig for 6 hours, 3) transportation for 1 hour, and 4) transportation for 1 hour followed by mixing with another pig for 6 hours. Immediately after the transportation treatment, all pigs were individually inoculated with 104 cfu