The purpose of this Pork Checkoff-sponsored study was to determine what Salmonella serotypes are present in pigs and their environment on commercial swine farms, and to further characterise and compare Salmonella from different sources, according to Dr Siddhartha Thakur of North Carolina State University.

Salmonella isolates were obtained during a longitudinal study sampling pigs and their environment from 30 commercial swine farms in North Carolina. This study followed 10 groups of pigs from farrowing to slaughter, sampling the pigs, farm environment, pig carcasses and the slaughter environment at various stages of production.

Sampling was carried out from October 2008 to December 2010 at various stages of production, including once at farrowing (seven to 10 days old), twice at each of nursery (four and seven weeks of age) and finishing stages (16 and 26 weeks of age) and finally, once at slaughter.

During the farrowing stage, a cohort of 35 healthy piglets per farm (four piglets per sow) were selected and ear-tagged for identification; subsequently, sampling followed the same cohort of pigs at different sampling stages during farm and slaughter stages.

Salmonella isolates collected during this study were sent to the National Veterinary Services Laboratories (NVSL) to determine the serotype. A representative subset (n=272) of Salmonella was genotyped using Pulsed-Field Gel Electrophoresis (PFGE) to determine the specific strain, or PFGE fingerprint profile, at different stages of production.

Comparisons were then made between PFGE strains and serotypes found in pigs and the farm environment.

Even though the focus of this grant was at the farm level, for providing more in depth information to the scientific community, the researchers compared fingerprint profiles of Salmonella isolated from carcasses of the same pigs at slaughter with the farm isolates.

Serotyping revealed 22 different serotypes found in pig and environmental samples on farm and at slaughter, with Salmonella Typhimurium being the most common.

Genotyping analysis revealed 47 clusters containing 100 per cent similar Salmonella isolates among pig and environmental isolates, including feed, water, soil, lagoon, floor swabs and slaughter lairage both within and between cohorts.

In addition, 41 unique strains were also detected. This indicates that certain serotypes and strains are present in pigs and their environment throughout the pork chain, irrespective of the farm or stage of production.

The NCSU group also found evidence highlighting the clear role played by the environment in the persistence and dissemination of Salmonella to conventionally reared pigs at farm and slaughter.

March 2014