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 ron1, D. Bergin1, F. Butler1, D. Prendergast2, S. Duggan2 & G. Duffy2

1UCD School of Agriculture, Food Sci. & Vet. Med. 2Ashtown 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%

• f 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

• f

results: Parallel study

• n

the incidence of Salmonella on pork oyster cuts (n=720) produced in the boning halls

• f

commercial pork abattoirs of Ireland.

Methodology

Scalding Stunning, killing and bleeding Evisceration Final washing Chilling Splitting and trimming Jointing Prevalence in caecal contents Stochastic regression Dehairing, singe, and polishing

R2 = 0.719 0.1 0.2 0.3 0.2 0.4 0.6 Proportion of slaughter pigs positive for Salmonella in caecal contents Proportion of pig carcasses positive for Salmonella (swabs) after evisceration

Meta-analysis

• Proportion of slaughter pigs carrying Salmonella entering the

slaughter lines (x)

• Proportion of resulting Salmonella-positive carcasses at the point
• f evisceration (y)

Source Proportion + caecal samples (x) Proportion + carcass (y) Duggan (2008) Sorensen (2004) Kranker (2003) Quirke (2001) Davies (1999) Morgan (1987) Oosterom (1985) 87/193 216/1658 22/122 61/419 256/2205 71/149 35/145 28/151 44/220 29/191 159/1665 6/117 42/419 155/2211 41/150 19/148 14/150 27/210

...Methodology

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.

xx

SS x Pc n s n t c Pc m y

2 ^

1 2

) 1 ( 1

2

n ChiSq s n

...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:

s y Normal Pev ,

^

Effect of splitting and trimming

 Alban and Stark (2005)  Prev. increase of 16%  Davies et al. (1999)  Incidence increase of 50%

Pev Csp Psp 1

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).

50 . , 16 . , Pert Csp

1 15 75 , 1 15 1 9 79 , 1 9 Beta Beta Rfr

Rfr Psp Pfr

Effect of chilling

 A reduction effect of chilling on

the recovery of Salmonella from pork carcasses has been

• bserved by many researchers.

 Thus,

data

• n

Salmonella prevalence

• n

pig carcasses before chilling and after chilling were taken from 9 published studies.

 Parametric

meta-analysis was conducted  Effect size measured was the “relative risk”

• f chilling.

Study 1 Study 2 Study 3 Study 4 Study 5 Study 6 Study 7 Study 8 Study 9 Fixed

• 5
• 4
• 3
• 2
• 1

1 2

Log relative risk (log pT/pC)

…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.

166 . , 868 . Normal

e Rch

X <= 0.303 2.5% X <= 0.581 97.5%

1 2 3 4 5 6 7

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)

Probability density

Pfr Rch Pch

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,

 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 (picd’) is

30 . , 20 . , Pert picd

Np icd

Pert p 30 . , 20 . , 1 1

'

…Increase in contamination in boning halls

 Contribution of inadequate cleaning and disinfection (Cicd) on a

particular day with respect to the cross-contamination that occurs during the first two production hours is

 With respect to all cross-contamination that occurs during a full

working day of 8 h., the contribution of inadequate cleaning and disinfection (Cicd’) is

 Thus, the prevalence of Salmonella in pork joints (Pj), which is

the final model output, was estimated as

' icd icd

p AP C

8 2

' icd icd

C C

'

1

icd

C Pch Pj

~(1-C’icd)% ~C’icd%

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)

 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

• f

prevalence

• f

Salmonella-carrier slaughter pigs (Pc).

 Pc for Ireland was estimated in the following way:

Model validation using Irish data

…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.

• Batches
• f

Salmonella- positive slaughter pigs entering the abattoir impart a force

• f

contamination during processing, that explains ~70% of the total contaminated carcasses at the beginning of the clean line.

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.

R2 = 0.719 0.1 0.2 0.3 0.2 0.4 0.6 Proportion of slaughter pigs positive for Salmonella in caecal contents Proportion of pig carcasses positive for Salmonella (swabs) after evisceration

...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
• f

Salmonella

• n

eviscerated pig carcasses in Ireland. Pev=11.4%, 95%CI: 10.4-12.5% Observed incidence

• Pearce et al. (2004) 

recovered 10% positive swabs after evisceration from an Irish abattoir.

0.0 0.2 0.4 0.6 0.8 1.0 0.09 0.10 0.11 0.12 0.13 0.14

Prevalence of Salmonella on eviscerated pig carcasses (Pev) Cumulative probability

...Results and Discussion

• After splitting and trimming, reduction in Salmonella levels may be related

to the removal of residual contamination during the spraying of the carcasses, performed in order to remove bone and blood clots.

• Thus, Salmonella prevalence on finished carcasses (after final rinsing

and before chilling) was estimated to be Pfr=8.6%, 95%CI: 3.7-17.1% Observed incidence

• While figures cannot be directly comparable since they have not been

corrected for culture protocol sensitivities nor are methods fully harmonised among countries, according to the last EFSA report, the average Salmonella incidence on carcass swabs detected among European slaughterhouses in 2006-2007 was 8.3% (95% CI: 6.3-11.0%).

...Results and Discussion

Output distribution of Salmonella prevalence in pork cuts produced in Ireland

Irish survey: Sampling pork cuts in boning halls Mean: 3.33% (24/720) 95% CI: 2.02 - 4.64%

X <= 0.0179 5.0% X <= 0.0725 95.0% 5 10 15 20 25 30 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Output distribution Validation results

Probability density Salmonella prevalence in pork joints

Mean = 0.033 Mean = 0.039

...Results and Discussion

Distribution of Salmonella prevalence in pork joints in Ireland as an

• utput of the regressional model in contrast to a previously-developed

stage-by-stage model

Stage-by-stage model

• No meta-analysis
• No

stochastic regression

• Contamination and

decontamination factors were used for each stage from bleeding to jointing

5 10 15 20 25 30 35 0.00 0.05 0.10 0.15 0.20

Salmonella prevalence in pork joints

Regressional model Stage-by-stage model

Probability density

Mean = 0.034 Mean = 0.039

...Results and Discussion

 Key parameters having

more influence

• n

model’s output.

 Reaffirmation

that final rinsing and chilling are efficient at controlling carcass contamination  CCP

 Expected

that final rinsing with hot water would further decrease Salmonella prevalence

• 0.382
• 0.221

0.106

• 0.030

0.019

• 0.6
• 0.4
• 0.2

0.0 0.2 0.4 0.6

Rfr Rch Csp Np

Correlation Coefficients Variables

picd

...Results and Discussion

 Effect of Pc on Pj  If Pc=5%  Pj~=1.7%

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Incidence of Salmonella in caecal contents

• f slaughter pigs in Ireland (Pc)

Salmonella incidence on pork cuts (Pj)

Conclusions

 A risk assessment model was built on the assumption

that the occurrence of Salmonella-infected carcasses post-evisceration is proportional to the total contamination introduced by the carriers animals themselves entering the slaughter lines.

 The model output for the Salmonella prevalence on

pork joints produced in boning halls was successfully validated with the results of an extensive survey carried out in four large Irish abattoirs.

...Conclusions

 Final rinsing and chilling had strong impact on the

prevalence of Salmonella on pork joints.

 A parametric meta-analysis showed to be effective in

risk assessment.

 According

to this model, interim cleaning and disinfection in the boning halls has only a marginal effect on diminishing the amount of contaminated pork joints produced.  as long as contaminated carcasses are being processed, about 90% of the cross-contamination is unavoidable.

 Data gaps:

 Lack of research on presence of Salmonella on the

skin of pigs entering the abattoir;

 Efficiency/inefficiency of scalding process;  Sensitivity values cannot be found for all culture

protocols.

...Conclusions

Ongoing work

 Incorporation of sensitivity values for the diverse culture

protocols  Corrections for true prevalence.

 The manuscript:

Ursula Gonzales-Barron, Ilias Soumpasis, Francis Butler and Geraldine Duffy. “Estimation of Prevalence of Salmonella

• spp. on Pig Carcasses And Pork Joints using a

Quantitative Risk Assessment Model aided by Meta- analysis”

Future Work

 This model only considers the contamination

force inflicted by Salmonella carrier pigs. Future models should incorporate parameters for the cross-contamination from equipment, machinery, staff practices, etc.

Acknowledgments

• SafeFood

and the Irish Department

• f

Agriculture, Fisheries and Food.

• QPORKCHAINS, an EU 6th Framework

project.

Thanks for your attention