Enumeration of salmonella in feces of naturally infected pigs

Quantification of Salmonella in asymptomatic pigs can be used to institute control measures and to assess risk of carcass contamination during slaughter. The objective of this study was to quantify the fecal concentration of Salmonella in naturally infected pigs. Individual fecal samples (positive [n=443], negative [n=1225] determined by microbiological culture) were submitted for direct quantitative real-time polymerase chain reaction (q-PCR). Direct q-PCR categorized 99.6% (1220/1225) of culture negative samples as negative. For culture positive samples, 15.4% (68/443) were detected by q-PCR, but only 3.4% (15/443) were within the direct q-PCR quantifiable range (≥10³ colony-forming units [CFU]/g of feces). Of these latter samples, the concentration range was 1.06×10³ to 1.73×10⁶ CFU/g feces. Of the 15 samples with high Salmonella concentrations, seven were collected from one pig and three samples were collected from its penmates. Direct q-PCR may be an alternative to traditional culture-dependent methods for detection of pigs with high fecal concentrations of Salmonella, but not for detection of pigs shedding low concentrations of Salmonella, which represented the majority of pigs in this study. When high shedding was detected it was clustered within a single pig and its penmates. These data contribute to quantitative risk assessments of the association between concentrations of Salmonella shed by pigs during the finishing phase and risk of carcass contamination at slaughter.