Using meta-analysis to underpin a risk assessment model for the estimation of prevalence of Salmonella spp. on pork joints produced in Irish slaughterhouses Ursula Gonzales-Barron 1 , Ilias Soumpasis 1 , Francis Butler 1 & Geraldine Duffy 2 1 UCD School of Agriculture, Food Sci. & Vet. Med. 2 Ashtown Food Research Centre, Teagasc
Introduction Foodborne salmonellosis is a major public health issue and requires concerted efforts to control the pathogen in the food supply. Pork is one of the main sources for human salmonellosis (5-30% of human cases). The primary source of Salmonella in the whole pork production chain is the Salmonella - infected animal.
...Introduction In the slaughter process, contamination of 30% of Salmonella -positive carcasses arises from cross-contamination of other infected pigs in the slaughterhouse. Numerous researchers have observed that there is a strong association between the proportion of sub-clinically infected pigs entering the slaughter lines (carrying or excreting Salmonella ) and the proportion of contaminated carcasses at the point of evisceration.
Objective To estimate the prevalence of Salmonella on pig carcasses and pork joints produced in Ireland using quantitative risk assessment techniques. To this effect, a stochastic relationship between Salmonella prevalence in pigs’ caeca and Salmonella prevalence on eviscerated carcasses was built taking into account sensitivities. Validation of results: Parallel study on the incidence of Salmonella on pork oyster cuts (n=720) produced in the boning halls of commercial pork abattoirs of Ireland.
Methodology Prevalence in caecal contents 0.35 Proportion of resulting Salmonella- positive eviscerated carcasses 0.30 Stunning, killing r 2 = 0.77 0.25 and bleeding 0.20 Weighted 0.15 Scalding regression 0.10 0.05 0.00 Dehairing, singe, 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Proportion of slaughter pigs carrying Salmonella and polishing in caeca Evisceration Splitting and trimming Final washing Meta-analysis Chilling Jointing
Proportion of slaughter pigs carrying Salmonella entering the slaughter lines (x’) x = s C / n C = positive caecal samples / total number of - samples Correcting x for culture sensitivities (Se) of Salmonella - x’ Source Proportion + Culture Culture Estimated Corrected caecal samples Se protocol Se Prop+ caec samples ( x ’) ( x ) Duggan (2008) 87/193 PCR-MSRV 0.950 0.880 99/193 Sorensen (2004) 216/1658 NMKL 0.450 0.450 480/1658 Kranker (2003) 22/122 NMKL 0.450 0.450 31/122 Quirke (2001) 61/419 RV+BGA 0.950 0.460 133/419 Davies (1999) 256/2205 DIASSALM 0.892 0.892 287/2205 Morgan (1987) 71/149 RV+XLD 0.710 0.710 100/149 35/145 0.710 0.710 49/145 28/151 0.710 0.710 39/151 Oosterom (1985) 44/220 TB+BGPRA 0.860 0.797 55/210
… Proportion of slaughter pigs carrying Salmonella - Se values of culture protocols were relative to faecal contents and NOT to caecal contents in pigs. - x’ cannot be regarded as true prevalence. - Corrections provided a reliable metric to equalise the studies minimising the level of between-study heterogeneity.
Proportion of resulting Salmonella positive carcasses at the point of evisceration (x’) y = s S / n S = positive carcass swabs / total number of carcass swabs. - Se of carcass swabs for every protocol could not be found in the literature. - Malorny et al. (2003): PCR method in relation to traditional culture methods for - pig carcass swabs (Se=0.985). Reasonable to correct s S in terms of PCR. - Source Proportion + Relative Swab extent Estimated Corrected carcass Se (cm 2 ) protocol Se Prop+ carcass ( y ’) ( y ) Duggan (2008) 29/191 1 1000 1 29/191 Sorensen (2004) 159/1665 0.985 1400 0.985 161/1665 Kranker (2003) 6/117 0.985 300 0.550 11/117 Quirke (2001) 42/419 0.985 Entire 0.985 43/419 Davies (1999) 155/2211 0.985 1000 0.985 157/2211 Morgan (1987) 41/150 0.985 1000 0.985 42/150 19/148 0.985 0.985 19/148 14/150 0.985 0.985 14/150 Oosterom (1985) 27/210 0.985 1000 0.985 27/210
Adding weights to individual studies (x’, y’) data pairs have been extracted from different studies - presenting different degrees of precision. In meta-analysis, a common method of weighting individual - estimates is by their inverse variances. Source Log-relative risk se(RR) Weight Effect size - (log RR) ( ω i ) parameter: RR Duggan (2008) -1.217 0.184 29.27 Sorensen (2004) -1.097 0.084 141.03 Kranker (2003) -0.994 0.326 9.39 ' ' n s n s Quirke (2001) -1.129 0.161 38.45 2 S S C C se ( RR ) i ' ' Davies (1999) -0.606 0.094 111.75 s n s n S S C C Morgan (1987) -0.874 0.143 48.94 -0.968 0.244 16.83 -1.018 0.289 11.94 1 Oosterom (1985) -0.712 0.214 21.88 i 2 se RR i Fixed effects meta-analysis for RR, U test =368 P<0.001
Stochastic weighted linear regression - Non-parametric Bootstrap technique added uncertainty to m , c , and s . - B Bootstraps samples {( x’ 1, y’ 1 ) * , ( x’ 2, y’ 2 ) * … , ( x’ 9, y’ 9 ) * } are created, where each (x’i, y’i)* is a random sample with replacement from {( x’ 1, y’ 1 ), ( x’ 2, y’ 2 ) … , ( x’ 9, y’ 9 )}. - For every Bootstrap sample, a weighted linear regression was fitted to the data using ω i m B , c B and s B were calculated. - 50 000 Bootstrap samples were taken. - Distributions were fitted for m , c and s .
… Stochastic weighted linear regression m average 0.0193, standard deviation 0.0435 - c average 0.2909, standard deviation 0.1344 - s average 0.0502, standard deviation 0.0193 - Having defined an estimate of prevalence of - Salmonella -carrier slaughter pigs ( Pc ), the Salmonella prevalence on eviscerated pig carcasses ( Pev ) is approximated as: Pev Normal m Pc c , s
Effect of splitting and trimming Alban and Stark (2005) Prev. increase of 16% Davies et al. (1999) Incidence increase in 50% Csp Pert 0 , 0 . 16 , 0 . 50 Psp 1 Csp Pev Effect of final rinsing Davies et al. (1999): Salmonella -positive results from two abattoirs before final rinsing (15/75) and after final rinsing (9/79). Beta 9 1 , 79 9 1 Pfr Pev Rfr Rfr Beta 15 1 , 75 15 1
Effect of chilling A reduction effect of chilling on the recovery of Salmonella from pork Study 1 carcasses has been Study 2 Study 3 observed by many Study 4 Study 5 researchers. Study 6 Study 7 Thus, data on Salmonella Study 8 Study 9 prevalence on pig carcasses Fixed before chilling and after chilling were taken from 9 -5 -4 -3 -2 -1 0 1 2 published studies. Log relative risk (log p T /p C ) Parametric meta-analysis was conducted Effect size measured was the “relative risk” of chilling.
...Effect of chilling RR for chilling = Probability of encountering Salmonella - positive carcasses after chilling relative to the probability of encountering Salmonella -positive carcasses before chilling. The distribution of the reduction factor for the overall effect of the chilling operation ( Rch ) was therefore approximated by meta-analysis conducted on RR. X <= 0.303 X <= 0.581 7 2.5% 97.5% 6 Normal 0 . 868 , 0 . 166 Rch e Probability density 5 4 3 2 Pch Rch Pfr 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Reduction in detected Salmonella prevalence due to chilling (Rch)
Increase in contamination in boning halls Berends et al. (1998): AP=0.67 of inadequate disinfection in cross contamination of pork with Salmonella spp. during the first 2 hours of production when the risk factor prevails, it provokes about two thirds of the total cross- contamination during the first production hours. Based on survey of Salmonella presence in Irish boning halls, the probability of inadequate cleaning and disinfection was defined as, p icd Pert 0 , 0 . 20 , 0 . 30 According to Berends et al. (1998), disinfection takes place 1-4 times a day ( Np=Discrete (1,2,3,4) ). The probability that disinfection is poorly performed at least once a day ( p icd ’) is ' Np p 1 1 Pert 0 , 0 . 20 , 0 . 30 icd
… Increase in contamination in boning halls Contribution of inadequate cleaning and disinfection ( C icd ) on a particular day with respect to the cross-contamination that occurs during the first two production hours is ' C AP p icd icd With respect to all cross-contamination that occurs during a full working day of 8 h., the contribution of inadequate cleaning and disinfection ( C icd ’ ) is ~( 1- C’icd )% ' C C 2 8 icd icd ~ C’icd % Thus, the prevalence of Salmonella in pork joints ( Pj ), which is the final model output, was estimated as ' Pj Pch 1 C icd
Model validation using Irish data The model’s ability to produce accurate estimates, and intrinsically the effectiveness of the modeling capabilities of meta-analysis, were appraised using Irish data for the input parameter of prevalence of Salmonella -carrier slaughter pigs ( Pc ). Pc for Ireland was estimated in the following way: Source x=s C /n C Culture Culture x’=s C ’/n C protocol Se Duggan et al. 87/193 PCR-MSRV 0.880 107/193 Quirke et al. 61/419 RV+BGA 0.460 133/419 UCD study 85/471 TB+BGPRA 0.797 99/471 POOLED DATA 339/1098 Pc=Beta(339+1,1098-339+1)
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