A Preliminary Simulation Model for Prevalence of Salmonella spp. - PowerPoint PPT Presentation

A Preliminary Simulation Model for Prevalence of Salmonella spp. during Pork Processing in Ireland U. Gonza zales-Barron ron 1 , D. Bergin 1 , F. Butler 1 , D. Prendergast 2 , S. Duggan 2 & G. 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.  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 0.3 Proportion of pig carcasses positive for Stunning, killing Salmonella (swabs) after evisceration and bleeding 0.2 Stochastic Scalding 0.1 regression R 2 = 0.719 0 0 0.2 0.4 0.6 Dehairing, singe, Proportion of slaughter pigs positive for Salmonella in caecal contents and polishing Evisceration Splitting and trimming Final washing Meta-analysis Chilling Jointing

...Methodology Proportion of slaughter pigs carrying Salmonella entering the - slaughter lines (x) Proportion of resulting Salmonella -positive carcasses at the point - of evisceration (y) Source Proportion + Proportion + caecal samples carcass (x) (y) Duggan (2008) 87/193 29/191 Sorensen (2004) 216/1658 159/1665 Kranker (2003) 22/122 6/117 Quirke (2001) 61/419 42/419 Davies (1999) 256/2205 155/2211 Morgan (1987) 71/149 41/150 35/145 19/148 28/151 14/150 Oosterom (1985) 44/220 27/210

Stochastic linear regression Using classical statistics, uncertainty was added to - ‘ m ’, ‘ c ’ of the relationship between the proportion of Salmonella -positive caecal samples ( x ) and the proportion of Salmonella -positive pig carcasses after evisceration ( y ), and the standard deviation ‘ σ ’ of the additional unexplained variation. 2 2 n 1 s Pc x ^ 1 ChiSq ( n 1 ) y m Pc c t n 2 s n SS xx

...Stochastic 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 y , s

Effect of splitting and trimming  Alban and Stark (2005)  Prev. increase of 16%  Davies et al. (1999)  Incidence increase of 50% Csp Pert 0 , 0 . 16 , 0 . 50 1 Psp 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 Psp Rfr Rfr Beta 15 1 , 75 15 1

Effect of chilling  A reduction effect of chilling on the recovery of Salmonella from pork carcasses has been Study 1 observed by many researchers. Study 2 Study 3  Thus, data on Salmonella Study 4 Study 5 prevalence on pig carcasses Study 6 Study 7 before chilling and after chilling Study 8 Study 9 were taken from 9 published Fixed studies.  Parametric meta-analysis was -5 -4 -3 -2 -1 0 1 2 Log relative risk (log p T /p C ) 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 Pch Rch Pfr 2 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 ~ C’icd % icd 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 Positive samples Total samples Duggan et al (Ireland) 87 193 UCD study (Ireland) 85 471 Quirke et al. (Ireland) 61 419 Casey et al. (Ireland) 9 15 POOLED ED DATA 242 242 1098  Pc=Beta(242+1,1098-242+1)

… Model validation using Irish data The model’s output (Pj) was compared to the results of an - extensive survey of Salmonella incidence in pork joints produced in the boning halls of four representative Irish abattoirs. This separate survey study of Salmonella prevalence and - counts on pork joints covered an aspect within the same research project, and it is explained in detail in Prendergast et al. (2008). The risk assessment model was developed in Microsoft - Excel using the @Risk add-in and run for 20000 iterations using Latin Hypercube sampling.

Results and Discussion The principal sources of microbial contamination during the processing of - pigs (Hald et al., 2003): Contamination from carrier animals (sub-clinically infected) - Cross-contamination from equipment, machinery inadequately - disinfected from previous batches, staff practices. Batches of Salmonella - - 0.3 positive slaughter pigs Proportion of pig carcasses positive for Salmonella (swabs) after evisceration entering the abattoir impart a force of contamination 0.2 during processing, that explains ~70% of the total 0.1 contaminated carcasses at R 2 = 0.719 the beginning of the clean 0 line. 0 0.2 0.4 0.6 Proportion of slaughter pigs positive for Salmonella in caecal contents

...Results and Discussion The pool of Irish surveys on Salmonella prevalence in caecal contents of - slaughter pigs ( Pc ) led to an estimate of 22.1% (95% CI: 19.7 – 24.6%) Observed incidence - Cross-sectional study at four pig abattoirs in Northern Ireland, 31.4% (161/513) of slaughter pigs tested positive for Salmonella in caecal culture. UK national survey  23% (578/2509) of the caecal samples were - Salmonella positive in 34 pig abattoirs. France national survey  24.8% (256/1030) of caecal samples tested - positive for Salmonella in 18 pig abattoirs. Denmark and The Netherlands have reported significantly lower - incidence of Salmonella -carrier slaughter pigs (8.0% and 8.5%)  severity of the application of national programmes for Salmonella control at farm level !!

...Results and Discussion Prevalence of - Salmonella on eviscerated pig 1.0 carcasses in Ireland. 0.8 Cumulative probability Pev =11.4%, 95%CI: 0.6 10.4-12.5% 0.4 Observed incidence 0.2 Pearce et al. (2004)  0.0 - 0.09 0.10 0.11 0.12 0.13 0.14 recovered 10% positive Prevalence of Salmonella on eviscerated pig carcasses (Pev) swabs after evisceration from an Irish abattoir.