A Preliminary Simulation

Model for Prevalence of

Salmonella spp. during Pork

Processing in Ireland

U. Gonzales-Barron1, 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%of Salmonella-positive carcasses arises fromcross-contamination of other infected pigs in theslaughterhouse.

Numerous researchers have observed that thereis a strong association between the proportion ofsub-clinically infected pigs entering the slaughterlines (carrying or excreting Salmonella) and theproportion of contaminated carcasses at thepoint of evisceration.

Objective

To estimate the prevalence of Salmonella on pigcarcasses and pork joints produced in Irelandusing quantitative risk assessment techniques.

To this effect, a stochastic relationship betweenSalmonella prevalence in pigs’ caeca andSalmonella prevalence on eviscerated carcasses.

Validation of results: Parallel study on theincidence of Salmonella on pork oyster cuts(n=720) produced in the boning halls ofcommercial 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

0.1

0.2

0.3

0 0.2 0.4 0.6

Proportion of slaughter pigs positive for Salmonella

in caecal contents

Pro

po

rtio

n o

f p

ig c

arc

as

se

s p

os

itiv

e f

or

Sa

lmo

ne

lla

(s

wa

bs

) a

fte

r e

vis

ce

rati

on

Meta-analysis

- 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 +

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.

xxSS

xPc

nsntcPcmy

2

^ 12 )1(

1 2

nChiSq

sn

...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:

syNormalPev ,^

Effect of splitting and trimming

Alban and Stark (2005) Prev. increase of 16%

Davies et al. (1999) Incidence increase of 50%

PevCspPsp 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).

50.0,16.0,0PertCsp

11575,115

1979,19

Beta

BetaRfr RfrPspPfr

Effect of chilling

A reduction effect of chilling on

the recovery of Salmonella from

pork carcasses has been

observed by many researchers.

Thus, data on Salmonellaprevalence on 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”

of 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 0 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.

166.0,868.0NormaleRch

X <= 0.303

2.5%

X <= 0.581

97.5%

0

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)

Pro

babili

ty d

ensity

PfrRchPch

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

30.0,20.0,0Pertpicd

Npicd Pertp 30.0,20.0,011'

…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

'icdicd pAPC

82'icdicd CC

'1 icdCPchPj

~(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 DATA 242 1098

Pc=Beta(242+1,1098-242+1)

The model’s ability to produce accurate estimates, andintrinsically the effectiveness of the modeling capabilities ofmeta-analysis, were appraised using Irish data for theinput parameter of prevalence of Salmonella-carrierslaughter 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 of Salmonella-

positive slaughter pigs

entering the abattoir impart

a force of 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

0.1

0.2

0.3

0 0.2 0.4 0.6

Proportion of slaughter pigs positive for Salmonella

in caecal contents

Pro

po

rtio

n o

f p

ig c

arc

as

se

s p

os

itiv

e f

or

Sa

lmo

ne

lla

(s

wa

bs

) a

fte

r e

vis

ce

rati

on

...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

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)

Cum

ula

tive p

robabili

ty

...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%

0

5

10

15

20

25

30

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Output distribution

Validation results

Pro

ba

bil

ity

de

ns

ity

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

output 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 jointing0

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 modelPro

babili

ty d

ensity

Mean = 0.034

Mean = 0.039

...Results and Discussion

Key parameters having

more influence on

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

Variable

s

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

of slaughter pigs in Ireland (Pc)

Sa

lmo

ne

lla

in

cid

en

ce

on

po

rk c

uts

(P

j)

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 Salmonellaspp. on Pig Carcasses And Pork Joints using aQuantitative 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 ofAgriculture, Fisheries and Food.

- QPORKCHAINS, an EU 6th Frameworkproject.

Thanks for your attention