effect of chilling on prevalence of Salmonella spp. on pig - PowerPoint PPT Presentation

A meta-analysis study of the effect of chilling on prevalence of Salmonella spp. on pig carcasses Ursula Gonzales Barron, Donal Bergin and Francis Butler UCD School of Agriculture, Food Science and Veterinary Medicine

INTRODUCTION  Meta- analysis refers to ‘the statistical analysis of a large collection of results from individual studies, such as experimental studies, opinion surveys and causal models, for the purpose of integrating the findings’.  The primary aim of meta-analysis is to produce a more precise estimate of the effect of a particular intervention or treatment, with an increased statistical power, than is possible using only a single study.

…INTRODUCTION  There is a need for conducting meta- analysis in the field of food safety, to start identifying, appraising and summarizing the results of otherwise unmanageable quantities of research, so that policy- makers and decision-makers can access trustworthy and concise information on effectiveness of interventions to control and prevent food-borne illnesses in humans.

OBJECTIVE  The overall objective: introduce a traditional parametric approach of meta- analysis with the purpose of synthesizing findings of prevalence studies of pathogens within the food processing chain.  Specific objective: investigate whether there is support in the sampled population of studies for the causal inference that the chilling stage within pork production had a statistically-significant decreasing effect on Salmonella prevalence of pig carcasses

METHODOLOGY  A meta-analysis begins with the formulation of a focused study question: population, intervention or treatment and outcome. Problem statement : Estimation of the overall effect of chilling on the Salmonella prevalence of pig carcasses during pork production. Population: Eviscerated pig carcasses post-meat inspection in slaughterhouses. Intervention or treatment : Chilling stage during pork processing, which includes cooling and posterior cold storage (18-24 hours) at ~5 C. Measured outcome : Presence of Salmonella spp. on the pig carcass surface.

…METHODOLOGY  The effect size ( θ ) refers to the degree to which the hypothetical phenomenon (i.e, decrease in Salmonella prevalence due to chilling) is present in the population (i.e, pig carcasses during processing at slaughterhouses).  For studies to be compatible, meta- analysis converts the effect size into a ‘ parameter ’ or common metric that permits direct comparison and summation of the independent studies.

…METHODOLOGY  Effect size parameterisation (θ) chosen  Relative risk is defined as the probability of the outcome in the treatment group relative to the probability in the control group.

…METHODOLOGY Table 1. Occurrence of Salmonella -contaminated pig  carcasses before and after chilling as detected in individual studies Study Study reference Pre-chilling (control) Post-chilling (treatment) number 1 s C f C n C s T f T n T 1 Oosterom et al. (1985) 27 183 210 12 198 210 2 Saide et al. (1995) 3 267 270 1 269 270 3 Davies et al. (1999) 7 18 25 3 22 25 UCD study (2000) 2 4 3 160 163 1 162 163 5 Quirke et al. (2001) 6 413 419 1 418 419 6 Booteldoorn et al. (2003) 138 232 370 12 63 75 7 Bouvet et al. (2003) 7 113 120 3 117 120 8 Lima et al. (2004) 5 25 30 4 26 30 9 Prendergast et al. (2008) 18 153 171 5 156 161

…METHODOLOGY  Effect size parameterisation (θ) chosen  Relative risk s n T T ln RR ln i s n C C  The se(θi) gives an indication of the degree of precision to which each study estimates the effect size: a small se(θi) indicates a precise estimate, usually from a large study. 0 . 5 f f T C se se (ln RR ) i s n s n T T C C

…METHODOLOGY  Since different studies estimate the true effect size with varying degrees of precision, a weighted average is used to combine individual study estimates.  A common method of weighting individual estimates is by means of their inverse variances 1 i 2 se i

…METHODOLOGY  A fixed-effect meta-analysis makes the fundamental assumption that each study is estimating the same underlying effect size, with a random error that stems only from a chance factor associated with subject-level sampling error. i i

…METHODOLOGY  The global null hypothesis that the effect size in all studies is equal to zero is tested by comparing the statistic 2 i i U i with the chi-squared distribution with one degree of freedom.

…METHODOLOGY  Assuming that there is a common effect size in all studies, the overall fixed effect θ and its standard error se(θ) are estimated by 1 i i se i i

…METHODOLOGY  Because meta-analyses are often performed retrospectively, in many situations it might be expected that differences in the study protocols will produce heterogeneity. A large-sample test for heterogeneity in effect size parameter across studies exists, and it is based on the Q statistic: 2 2 Q U i i i i

…METHODOLOGY  If the hypothesis of homogeneity across studies is rejected, then there must be additional sources of variability other than carcass-level sampling error. Under such a condition, a random-effects model can be assumed. i i i

RESULTS AND DISCUSSION  The effect size parameterization of ln-RR led to a highly-significant U statistic (p<0.001) providing strong evidence of the reduction due to chilling on Salmonella prevalence on pig carcasses.  All individual studies presented a negative estimate of ln-RR, which shows their agreement on the beneficial effect of chilling.  The Q statistic for the ln-RR parameterization was not statistically significant (p=0.96), indicating that there was no strong evidence of heterogeneity among studies  Fixed-effects approach more suitable.

…RESULTS AND DISCUSSION Table 2. Fixed-effects meta-analysis for the effect size  parameterization of ‘ln - relative risk’ of Salmonella presence on pig carcass after chilling in relation to control (before chilling) Study Effect size Standard error Relative weight ( θ i ) ( se(θ i ) ) ( ω i ) number 1 -0.811 0.333 9.021 2 -1.098 1.151 0.754 3 -0.847 0.629 2.524 4 -1.098 1.149 0.757 5 -1.792 1.078 0.861 6 -0.846 0.273 13.42 7 -0.847 0.678 2.176 8 -0.223 0.619 2.609 9 -1.211 0.493 4.107 U = 27.304; (1 df) p<0.001 Q = 2.448; (8 df) p=0.964 Overall effect θ : -0.868 Standard error of overall effect se(θ) : 0.166

…RESULTS AND DISCUSSION  Forest plots use point estimates of the Study 1 individual studies along Study 2 with their confidence Study 3 Study 4 intervals and may help Study 5 to reveal discernable Study 6 Study 7 patterns in the data Study 8 among studies. Study 9  The marker size Fixed illustrates the -5 -4 -3 -2 -1 0 1 2 contribution of each Log relative risk (log p T /p C ) study to the overall effect estimate

…RESULTS AND DISCUSSION  Proportion of Salmonella -positive carcass after chilling relative to before chilling was: 0.4197 with a 95% CI of 0.303 – 0.581.  The meta-analysis of the studies identified, indicated that chilling, on average, would be expected to reduce the number of carcasses with detectable Salmonella by a factor of 2.38 (1/0.4197) (95% CI: 1.720- 3.299).

CONCLUSIONS  Because of the systematic approach of meta- analysis and its reliance on actual data, the effect size outcome distribution can be used instead of, or in addition to, expert judgment in quantitative risk assessment models.  Hence, it is expected that the normal distribution of the effect size of chilling on Salmonella prevalence from the RR meta-analysis (Prevalence after chilling/Prevalence before chilling) ~ e N(-0.868, 0.166) will provide a more precise and realistic input distribution of the chilling stage for risk assessment models of this pathogen along the pork production process.

…CONCLUSIONS  This meta-analysis has confirmed that chilling can be an effective control for pathogenic Salmonella spp. when the operation is properly undertaken, as it may reduce by ~2.4 the detected incidence of this pathogen on pig carcass surfaces.  Finally, this meta-analysis has helped identify: (i) data gaps in the existent literature with regards to sensitivities for the whole range of Salmonella detection protocols, (ii) a common methodological flaw in the available research, which is the lack of standardization for Salmonella detection in swab samples of pig carcasses.

ACKNOWLEDGMENTS SafeFood and the Irish Department of Agriculture, Fisheries and Food.