The effect of feed withdrawal on the shedding of Salmonella typhimurium by swine

This project was designed to determine if feed withdrawal in conjunction with transportation-related stress caused increased shedding of Salmonella by carrier pigs. In this experiment, 48 pigs were challenged orally with S. typhimurium after weaning and allowed to grow under typical production practices. Antibiotics were not included in feeds. At monthly intervals, fecal and serum samples were collected from each pig. All pigs shed the challenge organism at least once during the experiment. By the sixth month, most pigs were negative for the challenge organism when cultured from feces. When pigs reached market weight (-240 pounds), they were split into 4 groups and subjected to one of the following feed withdrawal protocols: group I had no feed withdrawal, groups 2-4 had feed withdrawn at 6, 12 or 24 hours, respectively. Pigs in each group were transported -225 kilometers (4 hours), returned to the production facility and necropsied. Contents at the ileal-cecal junction were collected and cultured for the test organism. There was a direct correlation between the time of feed withdrawal and the number of pigs with S. typhimurium in intestinal contents. Pigs that had no feed withdrawal had the fewest number of positive pigs, whereas pigs that were off feed for 24 hours had the highest number of positive pigs. Serum samples were assessed using a mixed ELISA to detect antibodies against Salmonella. All pigs had detectable antibodies. Over the various bleedings, most pigs exhibited increases in anti-Salmonella antibodies over time. Some had modest drop-offs at the later times. In about I 0% of pigs, there was no increase or decrease in anti-Salmonella antibodies. In these non-responding pigs, the specific titers were lower than responding pigs, and above background levels. The results from this experiment complement our previous work and demonstrated that feed withdrawal when in conjunction with shipping contributes to shedding of Salmonella by pigs. Most pigs were not shedding the challenge organism just prior to slaughter, yet most (\u3e80%) had persistent infections. This indicates that the detection of infected, asymptomatic carriers using conventional culture methods is difficult and could complicate plans to develop programs to certify Salmonella-free pigs