Observations on the introduction and dissemination of Salmonella in three previously low prevalence status pig farms in the United Kingdom.

In the United Kingdom a serological Salmonella surveillance scheme was run in pigs up to 2012. Farms that maintained a low seroprevalence (<10%) were recognised as "Platinum" pig farms. The aim of this study was to investigate the occurrence and distribution of Salmonella in three farms (17P, 18P and 46P) that had lost their "Platinum" status. Four visits to each farm were made over a period of 15 months. The sampling was carried out by collecting pooled pen floor faecal swab and environmental samples. All samples were tested for Salmonella by a modification of ISO6579 Annex D, and serovars were determined for all isolates. The Salmonella prevalence peaked in the Summer/Autumn months and all farms were still positive at the end of the study. The overall sample prevalence was higher in farm 17P (46%) and 18P (35%) than 46P (19%). Monophasic S. Typhimurium (mST) represented 77.8% of the Salmonella isolates, mainly from farms 17P and 46P. The mST isolated at the initial visit may have been introduced via other livestock present on farm or introduction into the herd of infected animals. The results of this study suggest that incursion of mST was likely to be the main cause of the loss of "Platinum" status and confirm that mST can persist in pigs and their environment.