Rapid Infection of Pigs Following Exposure to Environments Contaminated with Different Levels of Salmonella Typhimurium

Pigs reared in an environment free of Salmonella species or on farms with low levels of infection may acquire infection during transport to the abattoir or while held in lairage. We designed a study to determine if pigs could become infected with S. Typhimurium when placed in a contaminated environment similar to that observed in commercial lairage. In addition, quantitative examination of salmonellae in all environmental and animal samples was undertaken. In order to simulate a naturally contaminated environment, animals experimentally infected with a challenge strain of S. Typhimurium (PT12) were used to seed the trial pen environment with salmonellae. In trial 1, pigs were exposed to a highly contaminated environment (5.4 log₁₀ CFU/100 cm²) for 2, 3, or 24 hours. Following these exposure periods, pigs were euthanized and samples including gastrointestinal and associated lymphoid tissue were analyzed for the challenge strain. S. Typhimuirum PT12 was detected in at least one sample type analyzed from each pig after exposure for ≥2 hours. The most frequently contaminated samples were tonsils (100% positive), followed by segments of the ileocecal junction (94.4% positive) and cecal contents (89% positive). Quantitative analysis conducted on cecal contents and ilocaecal junction segments revealed that similar numbers of organisms (1.1–2 log ₁₀ /g) were isolated at all timepoints. In trial 2, pigs were exposed to a less contaminated environment (2.65 log ₁₀ CFU/100 cm²) for periods of 1, 3, 6, or 24 hours. S. Typhimuirum PT12 was not detected in any sample from pigs euthanized after exposure of 1 hour. The challenge strain was recovered from the cecal contents of pigs after exposures of 3, 6, and 24 hours, and from the tonsil of one pig after exposure for 6 hours. These results highlight the need to reduce the environmental load of Salmonella spp. in lairage holding pens in order to reduce the numbers of infected pigs entering the slaughter process.