Estimation of prevalence of Salmonella on pig carcasses and pork joints, using a quantitative risk assessment model aided by meta-analysis.

This risk assessment study aimed to estimate the prevalence of Salmonella on pig carcasses and pork joints produced in slaughterhouses, on the basis that within groups of slaughter there is a strong association between the proportion of Salmonella-positive animals entering the slaughter lines (x) and the resulting proportion of contaminated eviscerated pig carcasses (y). To this effect, the results of a number of published studies reporting estimates of x and y were assembled in order to model a stochastic weighted regression considering the sensitivities of the diverse Salmonella culture methods. Meta-analysis was used to assign weights to the regression and to estimate the overall effect of chilling on Salmonella incidence on pig carcasses. The model's ability to produce accurate estimates and the intrinsic effectiveness of the modeling capabilities of meta-analysis were appraised using Irish data for the input parameter of prevalence of Salmonella carrier slaughter pigs. The model approximated a Salmonella prevalence in pork joints from Irish boning halls of 4.0% (95% confidence interval, 0.3 to 12.0%) and was validated by the results of a large survey (n = 720) of Salmonella in pork joints (mean, 3.3%; 95% confidence interval, 2.0 to 4.6%) carried out in four commercial pork abattoirs as part of this research project. Sensitivity analysis reinforced the importance of final rinsing (r = -0.382) and chilling (r = -0.221) as stages that contribute to reducing considerably the occurrence of Salmonella on the final product, while hygiene practices during jointing seemed to moderate only marginally the amount of contaminated pork joints. Finally, the adequacy of meta-analysis for integrating different findings and producing distributions for use in stochastic modeling was demonstrated.

An appraisal of the use of meat-juice serology monitoring data for estimating prevalence of cecal Salmonella carriage of pigs at slaughter by means of herd-level and animal-level simulation.

Some attempts have been made to elucidate the association between positive serology and Salmonella detection by bacterial culture in individual pigs and pig herds. This study aimed to appraise whether the existing knowledge on such association provides grounds for the utilization of serology monitoring data for predicting Salmonella subclinical infection of pigs entering the abattoir. Serology test results of pig carcasses (taken at abattoirs) originating from 436 representative active herds in Ireland were utilized to estimate the overall cecal Salmonella carriage of Irish slaughter pigs. To this effect, two separate simulations were conducted using (i) herd-level regression data and (ii) animal-level sensitivity (0.2890) and specificity (0.8895) data, which were extracted from published articles. The herd-level approach estimated a moderate prevalence of cecal Salmonella carriage of 0.222 (sigma = 0.094; 95% confidence interval [CI]: 0.069 to 0.431), which matched closely the mean prevalence value from the surveys' validation data of Salmonella-positive cecal samples (n = 1,098) obtained at Irish abattoirs (0.215; 95% CI: 0.192 to 0.240). The animal-level simulation generated an output distribution with slightly more uncertainty (sigma = 0.102 and 95% CI: 0.146 to 0.537) and a higher estimate of cecal carriage (0.312), which was an effect of the low relative sensitivity of serology, common under field conditions. While the herd-level simulation appeared to be technically more appropriate, since its correlation is only moderate, further elucidation of other factors related to subclinical infection should be attained for their incorporation in prospective dynamic on-farm models, which would be useful in the ultimate goal of estimating the risk of carcass contamination during slaughter.