Salmonella detection and counting on pig carcasses and cutting lines in Italian slaughterhouses

During 2014-2015, 300 pig carcasses before chilling and 85 food contact surfaces (FCSs) at cutting lines were tested for Salmonella in three slaughterhouses (namely A, B and C) of Northern Italy. In slaughterhouses A and B, four carcass sites of 100 cm2 each (from both the exterior and interior side) were swabbed with a single sponge. In abattoir C, four 100-cm2 sites of the exterior and the interior side were swabbed with two independent sponges. The population average prevalence of Salmonella-positive carcasses (which takes into account the structure of the study design, with multiple samples collected in a single day) in slaughterhouses A and B was 12.3%, while in slaughterhouse C it was 11.2%. Presence of Salmonella on exterior and interior side of carcasses showed a low level of concordance (only 3/12 of the contaminated carcasses where positive on both sides). No significant difference was found for FCSs contamination in the three slaughterhouses, with a population average prevalence of Salmonella-positive FCSs of 19.9%. In addition, we found that the clustering due to the day of sampling account for more than 36% and 60% of the overall prevalence variation on carcasses and FCSs, respectively. Eight serovars were identified, with S. Derby as the most common type. The counting of Salmonella on carcasses showed large variability. It was low (< 0.0075 MPN/cm2) in 46.6% of the carcasses and as high as 2.7 MPN/cm2 in 4.7%. Specifically, we found that counts on carcasses fit with “heavy tailed” distributions (log-normal and Weibull with a small shape parameter), suggesting not negligible probability of episodes of high Salmonella contamination. The mean values of contamination obtained with the two distributions ranged from 0.235 to 0.435 MPN/cm2.