FSIS Comparative

Risk Assessment for

Listeria monocytogenes

In Ready-to-eat Meat and 

Poultry Deli Meats

Prepared by

Risk Assessment Division

Office of Public Health Science

Food Safety and Inspection Service

United States Department of Agriculture

May 2010
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

Ready-to-eat Meat and Poultry Deli Meats Report

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Contributors

Heather Hicks Quesenberry1

Daniel Gallagher2

Sarah Endrikat2

David LaBarre1

Eric Ebel1

Carl Schroeder1

Janell Kause1

 

1 Risk Assessment Division, Office of Public Health Science, Food Safety and Inspection Service, U.S. 

Department of Agriculture, Washington DC.

2 Department of Civil & Environmental Engineering, Virginia Tech University, Blacksburg, VA.
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

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Acknowledgements

Many people within and outside the Food Safety and Inspection Service, U.S. 

Department of Agriculture contributed to the development of this risk assessment,

including Daniel Engeljohn, Carol Maczka, Regis Pouillot, James Lindsey, Dare 

Akingbade, Flora Tsui, and researchers of the National Alliance for Food Safety and 

Security.
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

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CONTENTS

Executive Summary.......................................................................................................... 1

Background..................................................................................................................... 1

Methods........................................................................................................................... 1

Results............................................................................................................................. 2

Conclusions..................................................................................................................... 2

1.0 Introduction................................................................................................................. 3

2.0 Methods........................................................................................................................ 6

2.1 Stage I: Prevalence and Level of L. monocytogenes in RTE Meat and Poultry deli 

meats at retail .................................................................................................................. 7

2.2 Stage II: Growth of L. monocytogenes from retail purchase to consumption ........ 14

2.3 Stage III: Deli Meat Consumption.......................................................................... 18

2.3.1 Consumer Storage Time and Temperature ...................................................... 18

2.3.2 Serving Sizes and Categories........................................................................... 21

2.4 Stage IV: L. monocytogenes Dose-response Relationship...................................... 22

3.0 Results........................................................................................................................ 23

3.1 Estimated Deaths and Illnesses by Slicing Location and Growth Inhibitor Use .... 23

3.2 Comparison with other food groups ....................................................................... 29

3.3 Sensitivity Analyses................................................................................................ 31

3.3.1 Consumer Storage Times / Temperatures........................................................ 31

3.3.2 Shelf Life .......................................................................................................... 34

3.3.3 Total Number of Deaths................................................................................... 37

3.4 Relative Impacts of Model Variables ..................................................................... 38

4.0 Conclusions................................................................................................................ 41

Appendix A: L. monocytogenes in Ready-to-eat Meat and Poultry Deli meat.......... 42

A-1 Data Collection Methods....................................................................................... 42

A-2 Statistical Analyses................................................................................................ 43

A-3 Study Results......................................................................................................... 44

A-3.1 Prevalence and Number of Samples ............................................................... 44

A-3.2 Logistic Regression ......................................................................................... 53

A-3.3 Comparison of Findings of the National Alliance for Food Safety and Security 

with those of the Food Processors’ Association....................................................... 56

A-4 Conclusions............................................................................................................ 57

References........................................................................................................................ 59
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

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FIGURES

Figure 1. a) Prevalence of L. monocytogenes in federally inspected facilities from the all 

RTE monitoring programs1 and b) Incidence of listeriosis per 100,000 from CDC 

FoodNet surveillance2

................................................................................................. 4

Figure 2. A conceptual model of the stages in this risk assessment and the critical inputs 

considered within each stage. ..................................................................................... 6

Figure 3. Cumulative density functions for the amount of L. monocytogenes in 

prepackaged compared to retail-sliced RTE deli meats.............................................. 8

Figure 4. Probability plots for fitted lognormal (base 10) distribution to observed levels 

for (a) retail-sliced and (b) prepackaged deli meat .................................................. 11

Figure 5. Fitted cumulative distribution plots and observed retail data for L. 

monocytogenes levels in retail-sliced versus prepackaged deli meats. Illustration (a) 

is over the range of observed L. monocytogenes levels. Illustration (b) is over the 

entire range of L. monocytogenes levels in deli meats used as an input into deli meat 

exposure pathway of the 2003 FDA-FSIS risk assessment model. .......................... 12

Figure 6. Quantile-quantile plot of fitted distributions for L. monocytogenes levels from 

retail-sliced and prepackaged deli meat. (A 1:1 reference line is included for visual 

comparison)............................................................................................................... 13

Figure 7. Relative frequency of storage time for retail-sliced versus prepackaged product.

................................................................................................................................... 19

Figure 8. Storage times for retail-sliced and prepackaged product. ................................. 20

Figure 9. Relative frequencies for consumer storage temperature. .................................. 21

Figure 10. Estimated number of listeriosis deaths per annum for retail-sliced and 

prepackaged product based on the 4,000 dose-response simulations. ...................... 26

Figure 11. Cumulative density plots estimated number of deaths per annum for retailsliced and prepackaged product based on the 4,000 dose-response simulations...... 27

Figure 12. Comparison of estimated deaths per annum and deaths per serving across the 

various food groups. (a) deli meat categories combined into one category. (b) deli 

meat treated as 4 separate categories. ....................................................................... 30

Figure 13. Relative ratio of deaths with differing storage times between retail-sliced and 

prepackaged product. ................................................................................................ 34

Figure 14. Impact of varying shelf life for a) total estimated deaths, b) fraction of deaths 

from retail-sliced product, and c) risk ratio............................................................... 36

Figure 15. Recursive partitioning and regression tree for comparing importance of 

slicing location versus growth inhibitor usage. Consumer storage time temperature 

data from RTI............................................................................................................ 38

Figure 16. Box plots for each deli meat category by age group. Prepack = prepackaged, 

Retail = retail-sliced, GI = growth inhibitor. ............................................................ 39
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

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Figure 17. Interaction plots comparing the effect of growth inhibitor (GI) use and slicing 

location on the mean number of deaths from listeriosis. .......................................... 40

Figure A-1. Number of RTE samples by location of slicing (source).............................. 47

Figure A-2. Prevalence of L. monocytogenes in deli meat by location of slicing. ........... 48

Figure A-3. Number of deli meat samples collected per store. MN data are not included 

because stores could not be identified. ..................................................................... 50

Figure A-4. Number of RTE samples by deli meat type. Chub data are not included. One 

sample (not shown) did not include any listing for deli meat type........................... 51

Figure A-5. Prevalence of L. monocytogenes in RTE deli meats by deli meat type. Chubs 

were not included...................................................................................................... 52

Figure A-6. Prevalence of L. monocytogenes in RTE deli meats samples sliced at retail. 

The estimated store visit was based on similar sampling date and time. No sample 

times were provided for MN; thus, MN data not included. Thirty-seven total deli 

meat samples are shown............................................................................................ 53

Figure A-7. Graphical display of logistic regression results using deli meat sample 

prevalence at individuals stores as the dependent variable. MN data not included.. 55
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

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TABLES

Table 1. Survival analysis input for statistical distribution fitting for the level of L. 

monocytogenes in deli meats at retail. ................................................................................ 9

Table 2. Best fit maximum likelihood results and probability plot distribution parameters 

for retail-sliced deli meat. ................................................................................................... 9

Table 3. Best fit maximum likelihood results and probability plot distribution parameters 

for prepackaged deli meat................................................................................................. 10

Table 4. Quantiles from fitted lognormal distributions for retail-sliced and prepackaged L. 

monocytogenes levels........................................................................................................ 14

Table 5. Plant distribution and estimated fraction of production prior to the Interim Final 

Rule................................................................................................................................... 16

Table 6. Fitted Weibull distributions according to the deli meat category....................... 19

Table 7. Storage times for retail-sliced and prepackaged product.................................... 20

Table 8. Percent of deli meat production by slicing location and antimicrobial growth 

inhibitor use during July 2007. ......................................................................................... 22

Table 9. Estimated mean number of deaths per year from L. monocytogenes in deli meat 

among three populations stratified by age and four deli meat categories using the storage 

time that differed between prepackaged and retail-sliced product. .................................. 24

Table 10. Estimated mean number of illnesses per year from L. monocytogenes in deli 

meat among three populations stratified by age and four deli meat categories................ 25

Table 11. Summary of estimated listeriosis death (%) by deli meat product category. ... 27

Table 12. Statistical comparison of mean number of estimated listeriosis deaths by deli 

meat type........................................................................................................................... 28

Table 13. Estimated mean number of deaths per serving among the three age groups and 

four deli meat categories................................................................................................... 28

Table 14. Comparison of NFPA versus NAFSS L.monocytogenes Prevalence Data...... 29

Table 15. Estimated mean number of listeriosis deaths per year from deli meat among 

three populations stratified by age and four deli meat categories..................................... 32

Table 16. Summary of estimated percent deaths by product category. ............................ 33

Table 17. Estimated mean number of deaths and illnesses per annum by fraction of 

consumer storage time between retail-sliced and prepackaged product........................... 33

Table 18. EGR for product with and without growth inhibitor by shelf life. ................... 35
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

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Table 19. Estimated percent listeriosis deaths by deli meat category for 80 day shelf life.

........................................................................................................................................... 37

Table 20. Estimated percent listeriosis deaths by deli meat category when total deaths 

across all food groups are reduced by 50%. ..................................................................... 37

Table A-1. Prevalence of product samples1 and stores visited based on sampling 

locations. ........................................................................................................................... 44

Table A-2. Prevalence of product samples (retail-sliced, prepackaged) and stores visited 

based on quarter of year.................................................................................................... 45

Table A-3. Prevalence of product samples (retail-sliced, prepackaged) and stores visited 

based on time of day (AM versus PM). ............................................................................ 45

Table A-4. Prevalence of only retail-sliced product and stores visited based on time of 

day (AM versus PM)......................................................................................................... 46

Table A-5. Prevalence of product samples (retail-sliced, prepackaged, and chubs) and 

stores visited based on store type (major grocery chain versus other grocers)................. 46

Table A-6. Prevalence of L. monocytogenes in retail-sliced and prepackaged deli meat by 

site..................................................................................................................................... 48

Table A-7. Results of logistic regression for store prevalence as function of processing 

type, store type, and time of day indicator variables. Data for MN not included. N=613.54

Table A-8. Prevalence of L. monocytogenes in sliced deli meat by site and processing 

type from the Food Products Association (Gombas et al. 2003)...................................... 56

Table A-9. Overall results of statistical tests for prevalence of L. monocytogenes on RTE 

meat and poultry deli meats by location, season, time of day for slicing at retail, and by 

deli meat type.................................................................................................................... 57
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-toeat Meat and Poultry Deli Meats Report

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Executive Summary

FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-to-eat Meat and 

Poultry Deli Meats

May 2010

BACKGROUND

Listeria monocytogenes (L. monocytogenes) is an important foodborne pathogen, estimated to 

cause approximately 2,500 illnesses, 2,300 hospitalizations, and 500 deaths each year in the 

United States. In an effort to understand better the sources of foodborne L. monocytogenes 

infection, the Food and Drug Administration (FDA) and the Food Safety and Inspection Service 

(FSIS), working collaboratively, developed a quantitative microbial risk assessment for L. 

monocytogenes that compared the risk of listeriosis among twenty-three categories of ready-toeat (RTE) foods. The results of the risk assessment, completed in 2003, indicated that deli meats 

pose the greatest risk for listeriosis, accounting for approximately 1,600 illnesses per year.

Based on these findings, FDA and FSIS conducted a preliminary analysis using the 2003 FDAFSIS L. monocytogenes risk assessment model to evaluate the relative risk of illness from L. 

monocytogenes on deli meat sliced and packaged at federally-inspected processing 

establishments (prepackaged deli meat) compared to deli meat sliced at retail facilities. This risk 

assessment contained industry data for L. monocytogenes on retail deli meat from delicatessens 

in California and Maryland (Gombas et al., 2003). The results of this risk assessment indicated a 

high percentage of listeriosis cases related to deli meats were associated with those sliced at 

retail. Because these results, however, were based on limited retail L. monocytogenes 

contamination data for deli meats, FSIS sought to gather additional data specifically to examine 

the relative risk of illness from prepackaged deli meat compared to deli meat sliced at retail 

facilities more closely. Therefore, the U.S. Department of Agriculture, Agricultural Research 

Service funded the National Alliance for Food Safety and Security (NAFSS) – a consortium of 

twenty-five research universities – to conduct a four-state study in which prepackaged deli meat 

and deli meat sliced and packaged at retail were analyzed for the prevalence and level of L. 

monocytogenes (Draughon, 2006). 

METHODS

Data from the NAFSS study, described in Appendix A of this risk assessment report, were used 

as inputs to the deli meat exposure pathway developed by modifying the above-mentioned 2003 

FDA-FSIS L. monocytogenes risk assessment model for RTE foods. The pathway consists of 

four distinct stages. The Retail Stage determines the level of L. monocytogenes in prepackaged 

deli meats and in deli meats sliced at retail. The Growth Stage uses an exponential growth rate 

function to model growth of L. monocytogenes in deli meat between purchase at retail and 

consumption. The Consumption Stage uses information about deli meat serving sizes and the 

number of servings consumed to estimate consumer exposure to L. monocytogenes in deli meat. 

Lastly, by integrating the predicted exposure with a dose-response relationship, the Dose-
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-toeat Meat and Poultry Deli Meats Report

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Response Stage predicts the probability of death from consuming L. monocytogenes on deli 

meat. 

The modified model considered four categories of deli meats: retail-sliced versus prepackaged 

and with or without growth inhibitor. Consumer storage times were based on a consumer survey 

conducted by RTI International, Tennessee State University, and Kansas State University (Cates 

et al. 2006). The results of the survey indicated prepackaged deli meat was stored for statistically 

significant longer periods than deli meat sliced at retail. The survey did not find any difference 

for storage temperature. 

RESULTS

This risk assessment, using current retail contamination data for deli meat (Draughon, 2006) and 

current consumer behavior data for deli meats (Cates et al., 2006) indicates that of those 

listeriosis cases and deaths attributed to deli meats, approximately 83% are associated with deli 

meats sliced at retail. The estimated mean number of deaths per year from L. monocytogenes in 

retail-sliced deli meats was 166.9 (95% CI: 164.5 – 169.3). In contrast, the estimated mean 

number of deaths from prepackaged product was 34.1 (95% CI: 33.4 – 34.9). Similarly, 919.6 

(906.8-932.4) illnesses were attributed to retail-sliced product while 188.6 (184.7-192.4) 

illnesses were attributed to prepackaged product.

Of the four categories of RTE deli meat, most of the predicted deaths were attributed to retailsliced product (which had a higher starting concentration) without growth inhibitor (which 

allowed for greater growth rates). Almost 70% of all predicted deaths fell into this category. 

The results illustrate the significant interaction between slicing location and use of growth 

inhibitor.

Sensitivity analyses indicated that the percentage of deaths attributed to retail-sliced deli meats 

was not appreciably affected by consumer storage time, product shelf life, or total number of 

deaths.

CONCLUSIONS

Of those illnesses and deaths from L. monocytogenes from deli meat consumption, 

approximately 83% are attributed to deli meat sliced and packaged at retail facilities (Endrikat et 

al., 2010). The remainder is from prepackaged deli meat. Similar results were obtained by 

Pradhan et al. (2010) in a study that compared the risk of listeriosis in both retail-sliced and 

prepackaged ham and turkey. Studies are needed to determine how contamination of deli meat at 

retail occurs and to design effective interventions for reducing listeriosis associated with the 

consumption of deli meat sliced at retail.
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1.0 Introduction

In 2000, the Food and Drug Administration (FDA) and the U.S. Department of Agriculture’s 

Food Safety and Inspection Service (USDA/FSIS) began a risk assessment to identify which

ready-to-eat (RTE) foods pose the greatest risk for listeriosis in the U.S. (FDA-FSIS 2003). Deli 

meat was found to pose the greatest risk of listeriosis among all RTE food categories. Based on 

these results and in response to public comments on the FSIS proposed rule Performance 

Standards for the Production of Processed Meat and Poultry Products (66 FR 12589), FSIS 

developed a risk assessment for L. monocytogenes in RTE meat and poultry products (FSIS 

2003) that focused on federally inspected processing plants. The risk assessment model predicted 

that the use of post-lethality interventions and antimicrobial growth inhibitors significantly 

lowered the public health risk of listeriosis compared to either control if used independently or 

compared to sampling alone. Post-processing lethality treatments that reduced L. monocytogenes 

in products formulated or processed to inhibit the growth of any remaining L. monocytogenes 

were predicted to be the most effective in protecting public health. Both the 2003 FDA-FSIS and 

2003 FSIS Listeria risk assessments served as the scientific basis for FSIS’ interim final rule for 

the control of L. monocytogenes during processing (“Control of Listeria monocytogenes in

Ready-to-Eat Meat and Poultry Products,” 68 FR 34208; June 6, 2003 (revised January 1, 2006); 

9 CFR 430).

These controls appear to be successful at federally inspected establishments. As shown in Figure 

1a, the prevalence of L. monocytogenes has declined since 1990. However, the incidence of 

foodborne listeriosis has been relatively constant and has not changed significantly since 2001 

(Figure 1b). Thus, strides to reduce L. monocytogenes contamination in RTE meat and poultry 

product samples collected in federal establishments are not being translated into improvements in 

public health.
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-toeat Meat and Poultry Deli Meats Report

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Year

Figure 1. a) Prevalence of L. monocytogenes in federally inspected facilities from the all RTE

monitoring programs1 and b) Incidence of listeriosis per 100,000 from CDC FoodNet 

surveillance2. 1 http://www.fsis.usda.gov/Science/Micro_Testing_RTE/

2 http://www.cdc.gov/foodnet/factsandfigures/2009/Table1b_all_incidence_96-09.pdf

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Percent Positive

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

19901991199219931994199519961997199819992000200120022003200420052006200720082009

Incidence per 100,000 Population

a) FSIS Regulatory Testing for 

 L. monocytogenes in RTE Products

b) Incidence of Foodborne Illness:

 Listeria

National Health Objective:

0.24 cases per 100,000
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Subsequently, in 2004, FDA and FSIS did a preliminary analysis using L. monocytogenes 

contamination data for retail deli meat from California and Maryland (Gombas et al. 2003) to 

estimate the relative risk of listeriosis from deli meat sliced and packaged in FSIS-inspected 

processing establishments (hereafter termed prepackaged) versus those sliced and packaged at 

retail facilities. Results suggested that deli meat sliced and packaged at retail posed the greater 

risk, accounting for approximately 80% of all listeriosis cases from deli meat.

In 2006, researchers with the National Alliance for Food Safety and Security (NAFSS) – a 

consortium of 25 research universities – completed a study of L. monocytogenes contamination 

in prepackaged RTE meat and poultry deli meats and those sliced and packaged at retail from 

California, Minnesota, Georgia, and Tennessee (Draughon 2006). FSIS adapted the FDA-FSIS

(2003) risk assessment model to examine data from the NAFSS study and reanalyze the 

comparative risk of listeriosis from prepackaged RTE deli meat versus RTE deli meat sliced and 

packaged at retail. This report describes the analysis and its findings. 
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2.0 Methods

This analysis uses the deli meat exposure pathway from the risk assessment model developed 

and utilized in a previous L. monocytogenes risk assessment (FDA-FSIS, 2003) that estimated 

risk of death attributable to 23 ready-to-eat (RTE) food categories. This analysis separates the 

deli meat category into prepackaged deli meats

3

and those sliced at retail establishments. 

Because of increased use of antimicrobial growth inhibitors in deli meat, each deli meat type is 

divided into those with or without antimicrobial growth inhibitors. Consistent with the FDAFSIS (2003) risk assessment model, this analysis considers four conceptual stages (Figure 2).

Retail occurrence

• Product type 

(prepackaged or 

retail-sliced deli 

meat)

Growth

• Exponential 

growth rate (EGR)

• Growth inhibitor 

usage

• Storage time

• Storage 

temperature

Consumption

• Serving size

• Number of 

servings

Dose-response

• Age of consumer 

(neonatal, 

intermediate, 

elderly)

Outputs

• Annual illnesses and deaths

• Annual mortality by Age

• Risk of death per serving

Figure 2. A conceptual model of the stages in this risk assessment and the critical inputs 

considered within each stage.

• The Retail Stage determines the presence and level of L. monocytogenes in the two deli 

meat types (retail-sliced versus prepackaged).

• The Growth Stage uses an exponential growth rate modified to account for antimicrobial 

growth inhibitor usage to predict growth of L. monocytogenes in deli meat between retail

and consumption.

• The Consumption Stage predicts the L. monocytogenes exposure dose consumed in 

servings of deli meats, which is a consequence of serving size and the number of 

servings.

 

3 For our purpose, meat and poultry are considered together when discussing deli products (i.e., deli meat refers to 

any product containing beef, pork and/or poultry).
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• The Dose-response Stage predicts the probability of death from L. monocytogenes per 

serving by integrating the predicted exposure distribution with a dose-response 

relationship. 

The output of these four stages in this risk assessment is the annual number of illnesses and

deaths (and the corresponding risk of illness or death per serving) from L. monocytogenes in deli 

meat. While these four stages are updated in the 2003 FDA-FSIS risk assessment model for the 

deli meat food category, all other food categories remain as they were originally described. This 

risk assessment describes the analysis used to parameterize each of the four updated stages in 

more detail below.

2.1 STAGE I: PREVALENCE AND LEVEL OF L. MONOCYTOGENES IN RTE MEAT 

AND POULTRY DELI MEATS AT RETAIL

The prevalence and level of L. monocytogenes in RTE meat and poultry deli meats at retail 

establishments were determined using data from a study conducted by the NAFSS in which 6 of 

3,522 (0.17%) samples and 49 of 3,518 (1.39%) samples tested positive for L. monocytogenes 

from prepackaged and retail-sliced deli meats, respectively. This difference was statistically 

significant (p <0.05). Analyses of these data are described in depth in Appendix A. Of the six

positive samples from prepackaged deli meat, all had L. monocytogenes levels ≤0.3 MPN/gram.

Of the 49 positive samples from deli meat sliced and packaged at retail, L. monocytogenes levels 

ranged from <0.3 to ≥110 MPN/gram. Cumulative density plots, assuming a detection limit of 

0.008 MPN/gram (i.e. 1 MPN/125 g), are shown in Figure 3.
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Figure 3. Cumulative density functions for the amount of L. monocytogenes in prepackaged 

compared to retail-sliced RTE deli meats.

Data for the prevalence and level of L. monocytogenes in deli meat sampled at retail were fitted 

to probability distributions as inputs to the modified 2003 FDA-FSIS risk assessment model. 

Because there were few positive samples, distribution fits should be considered approximate. 

The survival analysis module of Number Cruncher Statistical Systems4

 

4 (NCSS: http://www.ncss.com/)

was used to fit an 

appropriate statistical distribution to prepackaged and retail-sliced deli meat separately. Survival 

analysis allows incorporation of left and right censoring into distribution fitting. Left censoring 

indicates that the true level of L. monocytogenes in deli meat is less than reported. Right 

censoring indicates that the true level of L. monocytogenes in deli meat is higher than reported. 

Interval censoring indicates that the true value is between two fixed values. To be conservative, 

all but one of the observed L. monocytogenes positive samples of deli meat with a level of ≤ 0.3 

MPN/gram were treated as having a level of 0.3 MPN/gram. The remaining positive sample was 

treated as an interval measurement between 0.008 MPN/gram and 0.3 MPN/gram. Negative 

samples were assumed to have L. monocytogenes levels ≤ 0.008 MPN/gram (i.e. ≤ 1 MPN/125 

gram). The inputs to the survival analysis are shown in Table 1. The comparison of maximum 

likelihood fit to various probability distributions is provided below for retail-sliced (Table 2) and 

-2 -1 0 1 2

0.985 0.990 0.995 1.000

log10(Lm concentration, MPN/g)

Cumulative Fraction

retail sliced

prepackaged
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prepackaged (Table 3) deli meat. The parameters for each distribution were determined by leastsquares regression fit to the corresponding probability plot. 

Table 1. Survival analysis input for statistical distribution fitting for the level of L. 

monocytogenes in deli meats at retail.

Retail (deli) sliced Prepackaged

No. 

Samples1

L. monocytogenes 

level 

(MPN/gram)1

Censor 

Type2

No. 

Sample

s

L. monocytogenes 

level (MPN/gram)

Censo

r 

Type2

3,469 ≤0.008 L 3,516 ≤ 0.008 L

1 Between 0.008 and 

0.3

I 1 Between 0.008 and 

0.3

I

29 0.3 F 5 0.3 F

3 0.92 F

1 0.93 F

1 0.94 F

3 2.3 F

1 15.3 F

1 24 F

1 46 F

3 ≥ 110 R

1 L. monocytogenes levels were not given for five positive retail-sliced deli meat samples. These 

data were thus not used in the distribution fitting. 2 Censor type refers to the censoring used by the survival analysis fit. L indicates left censoring 

(actual value is less than observed); I indicates interval censoring (actual value is between two 

known values). F indicates actual value is observed level. R indicates right censoring (actual 

value is greater than observed).

Table 2. Best fit maximum likelihood results and probability plot distribution parameters for 

retail-sliced deli meat.

Distribution Log Likelihood Shape1 Scale1

Weibull -315.606 NA2 NA2

Lognormal -316.634 -25.6314 9.309884

Lognormal10 -316.634 -11.1316 4.043231

Loglogistic -318.041 -19.0915 3.277907

Logistic -375.906 -13.795 3.168052

Extreme Value -396.146 -22.3905 15.19124

Exponential -13046.5 1 0.012057

Normal NA2 NA2 NA2

1 The interpretation of these parameters varies depending on the distribution. For most 

distributions, the shape is the mean of the distribution and the scale is the standard deviation.

2 The probability plot estimate could not be calculated for these parameters.
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Table 3. Best fit maximum likelihood results and probability plot distribution parameters 

for prepackaged deli meat.

Distribution Log Likelihood Shape1 Scale1

Extreme Value -43.1107 -2.38335 1.303234

Normal -43.2915 -1.80981 0.628146

Logistic -43.5157 -1.14005 0.183313

Weibull -49.6842 NA2 NA2

Lognormal10 -49.865 -27.3912 7.79663

Lognormal -49.865 -11.8958 3.386033

Loglogistic -50.0892 -19.078 2.275309

Exponential -715.407 1.00E+00 1.37E-03

1 The interpretation of these parameters varies depending on the distribution. For most 

distributions, the shape is the mean of the distribution and the scale is the standard deviation.

2 The probability plot estimate could not be calculated for these parameters.

Though the Weibull and extreme value distributions are suggested as best fitting these data 

(based on the maximum likelihood criterion), the lognormal distribution was selected as the most 

appropriate.

5

The lognormal fit to the distribution of the level of L. monocytogenes in retailsliced deli meat is statistically no different from the Weibull distribution. It is preferred that both 

retail-sliced and prepackaged distributions are modeled as the same type. Environmental 

contaminants such as bacterial levels are often fit to a lognormal distribution and this distribution 

has theoretical justification.

6

 

5 Note that the lognormal and lognormal (base 10) are equivalent in terms of fit.

The probability plots and the resulting fit for both retail-sliced and 

prepackaged deli meat are shown on the following page in Figure 4. The fitted cumulative 

density plots and observed data points are shown in Figure 5a. The fit for the retail-sliced deli 

meat appears adequate. The distribution fit for the prepackaged deli meat is uncertain because of 

only two data points. Figure 5b extrapolates the cumulative density curves to lower levels. The 

deli meat exposure assessment model uses levels as low as 10-8 MPN/gram as inputs.

6 See, for example, van Belle 2002.
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-toeat Meat and Poultry Deli Meats Report

11

Figure 4. Probability plots for fitted lognormal (base 10) distribution to observed levels for (a) 

retail-sliced and (b) prepackaged deli meat. Log10 (Conc)

Lognormal (base 10) Quantile

Lognormal Probability Plot: 

Retail Sliced Product

a) Retail-sliced

Lognormal (base 10) Quantile

Log10(Conc)

Lognormal Probability Plot: 

Plant Sliced Product

b) Prepackaged
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-toeat Meat and Poultry Deli Meats Report

12

Figure 5. Fitted cumulative distribution plots and observed retail data for L. monocytogenes 

levels in retail-sliced versus prepackaged deli meats. Illustration (a) is over the range of observed

L. monocytogenes levels. Illustration (b) is over the entire range of L. monocytogenes levels in 

deli meats used as an input into the deli meat exposure pathway of the 2003 FDA-FSIS risk 

assessment model.

-3 -2 -1 0 1 2 3

0.985 0.990 0.995 1.000

log10(Lm concentration, MPN/

Cumulative Fraction

retail sliced

prepackaged

(a)

-10 -5 0 5

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

log10(Lm concentration, MPN/

Cumulative Fraction

retail sliced

prepackaged

(b)
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-toeat Meat and Poultry Deli Meats Report

13

The quantile-quantile plot of the two fitted distributions is shown in Figure 6. Because the same 

distribution shape (lognormal) was selected for both retail-sliced and prepackaged, the quantilequantile plot is a straight line. The quantile-quantile line is below the 1:1 reference line, 

indicating, as expected, that for a given percentile, the prepackaged L. monocytogenes level is 

lower than the retail-sliced L. monocytogenes level over the range of interest. The quantilequantile line is not parallel to the reference line. The difference between the two distributions is 

greater at the extreme upper tails of the distributions.

Figure 6. Quantile-quantile plot of fitted distributions for L. monocytogenes levels from retailsliced and prepackaged deli meat. (A 1:1 reference line is included for visual comparison).

A fixed number of quantiles from the distribution of L. monocytogenes levels in deli meats serve 

as inputs to the 2003 FDA-FSIS exposure assessment model. Based on the fitted parameters 

shown in Table 2 and Table 3, the quantiles needed for the exposure assessment model were 
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-toeat Meat and Poultry Deli Meats Report

14

determined using the open source statistical software, R (R Development Core Team, 2007).

These quantiles are given in Table 4.

Table 4. Quantiles from fitted lognormal distributions for retail-sliced and prepackaged L. 

monocytogenes levels.

Cumulative Fraction

Retail-sliced L. 

monocytogenes level 

(MPN/gram)

Prepackaged L. 

monocytogenes level 

(MPN/gram)

0.8 1.87E-08 8.99E-10

0.85 1.15E-07 4.11E-09

0.9 1.12E-06 2.78E-08

0.95 3.30E-05 4.72E-07

0.99 1.88E-02 9.58E-05

0.995 1.92E-01 6.70E-04

0.999 2.31E+01 3.70E-02

0.9999 8.04E+03 4.98E+00

Max1 8.03E+06 6.16E+03

1

Based on simulation of 1,000,000 random numbers from the appropriate fitted distribution.

The risk assessment analysis used in this report assumed independence among samples. This 

assumption may not be met for these data, however, because the samples collected from the 

same retail location are likely to be correlated. Cross-contamination or poor hygienic conditions 

within a retail location may result in the clustering of positive L. monocytogenes findings by 

store; therefore, analyzing the data by store location may provide a more accurate estimate of the 

relative risk ratio for retail-sliced versus prepackaged products. However, due to the blinding 

process used during sample collection, individual store identifiers were removed. Without these 

store identifiers, store visits can only be estimated based on time and date of sample collection. 

Also, sample collection times were not provided for samples from Minnesota, so differentiating

individual store visits was not possible. As a result of these data limitations, all individual 

samples were treated as independent for this risk assessment analysis. A comparison of the 

results based on the assumption of independence of samples versus samples grouped by 

approximate store visit may be found in the Appendix.

2.2 STAGE II: GROWTH OF L. MONOCYTOGENES FROM RETAIL PURCHASE TO 

CONSUMPTION

To assess consumer exposures, the growth of L. monocytogenes from retail purchase to 

consumption was modeled. Given regulatory changes

7

 

7 FSIS’ interim final rule for the control of L. monocytogenes during processing (“Control of Listeria 

Monocytogenes in Ready-to-Eat Meat and Poultry Products,” 68 FR 34208; June 6, 2003 (revised January 1, 

2006); 9 CFR 430).

subsequent to the development of the 

2003 FDA-FSIS risk assessment, the model’s predicted growth for deli meats needed adjustment. 

The primary determinant of growth rate is whether or not the product used growth inhibitor. 

Total amount of growth depends on growth rate and consumer’s storage time and temperature. 
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-toeat Meat and Poultry Deli Meats Report

15

FSIS provides three alternatives for establishments producing certain RTE meat and poultry deli 

meats to control for L. monocytogenes (9 CFR 430, 2003): 

• Alternative 1: Employ both a post-lethality treatment and an antimicrobial growth 

inhibitor for Listeria monocytogenes on RTE deli meats. 

• Alternative 2: Employ either (a) a post-lethality treatment or (b) an antimicrobial growth 

inhibitor for the pathogen on RTE deli meats.

• Alternative 3: Employ sanitation measures only (uses neither a post-lethality treatment 

nor an antimicrobial growth inhibitor).

Deli meat that uses an antimicrobial growth inhibitor is expected to have lower growth rates of L. 

monocytogenes than deli meat that does not use antimicrobial growth inhibitor. Data on 

production volumes for each category were used to estimate the use of antimicrobial growth 

inhibitors in RTE deli meat, and current regulations were used to estimate conservative 

maximum allowable growth rates.

To qualify as using an antimicrobial growth inhibitor under the Interim Final Rule 9 CFR 430,

8

the growth of L. monocytogenes may not exceed two logs during the shelf life of the product. 

This information was used to modify the existing 2003 FDA-FSIS risk assessment model to 

account for different growth in deli meat with and without antimicrobial growth inhibitors. The 

comparison of retail-sliced versus prepackaged was calculated by splitting the deli meat category 

into four separate categories based on two factors: 1) where the deli meat was sliced and 2) 

whether antimicrobial growth inhibitor was used. Exponential growth rates for L. monocytogenes 

were calculated for product with and without antimicrobial growth inhibitors using data from the 

2003 FDA-FSIS risk assessment and the estimated fraction of deli meat in each category prior to 

the implementation of the Interim Final Rule. These older data were used for calculating growth 

rates because it better matched the period of reported growth rates in the 2003 FDA-FSIS risk 

assessment model. Once the growth rates were determined, this risk assessment used the more 

current manufacturer production volume data to calculate the fraction of deli meat in each 

category, i.e. the current best estimate of growth inhibitor use was used for all model runs. The 

older data was only used to calculate L. monocytogenes growth rates. 

Prior to the Interim Final Rule, fewer plants employed alternatives that used antimicrobial 

growth inhibitors therefore less product was formulated with an antimicrobial growth inhibitor

when compared to current conditions. The overall growth rate of L. monocytogenes should be 

lower after the implementation of the Interim Final Rule because the composition of the product 

is different – a greater fraction of product contains antimicrobial growth inhibitors. 

 

8 9 CFR 430 provides requirements for the FSIS’ interim final rule, “Control of Listeria Monocytogenes in Readyto-Eat Meat and Poultry Products,” (68 FR 34208; June 6, 2003 (revised January 1, 2006)). 
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-toeat Meat and Poultry Deli Meats Report

16

To calculate the relative growth rates for deli meat with and without antimicrobial growth 

inhibitor, the fraction of deli meat using antimicrobial growth inhibitor prior to the Interim Final 

Rule was needed. The number of establishments using each L. monocytogenes control alternative 

(1, 2 (a or b), or 3) was estimated by FSIS economists. This is shown in Table 5. The fraction of 

production was estimated by assuming that each plant within a Pathogen Reduction/Hazard 

Analysis and Critical Control Point (PR/HACCP) size category produced the same volume, and 

that the total fraction of production was 48%, 48%, and 4% for large, small, and very small 

plants (FSIS 2003) respectively.

Table 5. Plant distribution and estimated fraction of production prior to the Interim Final Rule.

Lm Control

Alternative

PR/HACCP Size Category1

Large Small Very Small Total

no.2 Fractio

n3

no.2 Fractio

n3

no.2 Fractio

n3

no.2 Fractio

n3

(1) Both post 

processing 

lethality and 

antimicrobial 

growth inhibitor

7 0.018 20 0.007 15 0.000 42 0.026

(2a) Post 

processing 

lethality only

15 0.039 79 0.029 49 0.001 143 0.068

(2b) 

Antimicrobial 

growth inhibitor 

only

40 0.104 122 0.044 65 0.001 227 0.149

(3) Neither post 

processing 

lethality nor 

antimicrobial 

growth inhibitor

123 0.319 1107 0.400 2072 0.038 3302 0.757

Total 185 0.480 1328 0.480 2201 0.040 3714 1.000

1 Based on PR/HACCP classification. 2 no. is the number of plants. 3 Fraction is the fraction of production = number of plants within size and alternative / total 

number of plants within size * total fraction by size.

Based on this analysis, it was estimated that 17.5% (2.6% + 14.9%) of deli meat used 

antimicrobial growth inhibitors prior to the implementation of the Interim Final Rule. The 
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-toeat Meat and Poultry Deli Meats Report

17

percentage of deli meat using antimicrobial growth inhibitors was assumed the same for 

prepackaged and retail-sliced deli meat.

The exposure assessment portion of the 2003 FDA-FSIS risk assessment model was adjusted to 

account for possible use of antimicrobial growth inhibitor by adjusting the exponential growth 

rate (EGR) of L. monocytogenes among RTE meat and poultry deli meats. The 2003 FDA-FSIS

risk assessment model estimated that the mean EGR at 5oC was 0.282 log10 CFU/gram/day. The 

model treats this as a stochastic parameter and adjusts for stochastic consumer storage time, 

temperature, and a correlation between the two. Appendix 8 in the 2003 FDA-FSIS risk 

assessment report lists the references used to calculate this value as 15 published articles with 23 

reported growth rates across a range of deli meat products. Most of these reference data are from 

the late 1980s to early 1990s, which is why the use of production data from prior to the 

implementation of the Interim Final Rule was deemed appropriate. Note that this older data was 

only used to calculate the growth rates. The current levels of growth inhibitor use were used in 

the risk assessment itself as described below.

FSIS L. monocytogenes Compliance Guidelines

9

state that to qualify as utilizing one of two most 

stringent alternative L. monocytogenes control options (Alternative 1 or 2) in the Interim Final 

Rule, no more than 2 log10 growth is allowed over the entire shelf life of the product. No 

temperature is specified during this shelf life, nor is the shelf life itself specified. If this standard 

is interpreted to be 2 log10 growth over 14 days at 5oC, the exponential growth rate is 2 log10

CFU/gram/14 days = 0.143 log10 CFU/gram/day. Using this calculation, as the product shelf life 

is reduced; the calculated EGR would increase because the same 2 log10 growth would occur in a 

shorter time. 

For comparison, consumer storage time is available based on an American Meat Institute (AMI 

2001) survey. Results of the survey suggest that approximately 40% of ready-to-eat product is 

stored for less than 3 days, and another 45% of product is stored from 4 to 7 days. A total of 96% 

of product is stored for less than 14 days. While consumer storage time is not the same as shelf 

life, the 14-day assumption appears reasonable. A sensitivity analysis of this shelf life 

assumption is provided in Section 3.3.2 below.

If the EGR for product with antimicrobial growth inhibitor (GI) is based on the regulation, then 

to calculate the EGR for product without GI:

fGI x EGRwith + (1 - fGI) x EGRwithout = EGRFDA

0.175 x 0.143 log10 CFU/gram/d + 0.825 x EGRwithout = 0.282 log10 CFU/gram/day

EGRwithout =0.311 log10 CFU/gram/day

where fGI = fraction of product with growth inhibitor (prior to Interim Final Rule)

 

9 (http://www.fsis.usda.gov/OPPDE/rdad/FRPubs/97-

013F/Lm_Rule_Compliance_Guidelines_May_2006.pdf )
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-toeat Meat and Poultry Deli Meats Report

18

EGRFDA = EGR used in 2003 FDA-FSIS risk ranking

EGRwith = EGR for product with growth inhibitor

EGRwithout = EGR for product without growth inhibitor

The EGR for both product with and without antimicrobial growth inhibitor are within the 

observed range for the 23 literature values noted previously and within one standard deviation of 

the mean EGR. 

The maximum L. monocytogenes level that can occur in product can also be adjusted. As there 

are no additional data for this parameter, it was left unchanged from the original 2003 FDA-FSIS

risk assessment model. 

To adjust the growth rates in the deli meat exposure pathway of the 2003 FDA-FSIS risk 

assessment model, an additional multiplier based on adjusting the mean EGR was added. If the 

product did not have GI, the stochastic EGR for each iteration was multiplied by 0.311/0.282 = 

1.104. If the product did have GI, the stochastic EGR for each iteration was multiplied by 

0.143/0.282 = 0.507. Note that the EGR for product with GI is calculated based on FSIS 

regulation, not on actual industry performance, which may be different.

2.3 STAGE III: DELI MEAT CONSUMPTION

2.3.1 Consumer Storage Time and Temperature

The comparative risk analysis used consumer storage time/temperature analysis from a national 

survey of U.S. adults using a Web-enabled panel survey approach. The survey was conducted by 

RTI International, Tennessee State University, and Kansas State University. The purpose of the 

survey was to characterize home storage and refrigeration practices for a variety of refrigerated 

ready-to-eat (RTE) foods and consumers' knowledge including use of open date statements 

among pregnant women, seniors, and the remaining population. A description of the survey and 

an analysis of the data are given by Cates et al. (2006) and Kosa et al. (2007). The study design, 

survey questionnaire (PDF format), data dictionary, respondent and survey data (Microsoft Excel 

format) are available online10. Note that the survey asked consumers how long the package was 

stored until the product was consumed. The reported storage times represent the time for the last 

serving, but some product would normally be consumed prior to this. Because the same question 

was used for both prepackaged and retail sliced product, and because longer storage times 

represent the greater risk, the reported storage times for the last serving were used to compare 

retail-sliced versus prepackaged product. The results are reported in Pouillot et al. (2010).

The data were filtered to consider storage time-temperature for deli meats only. For storage 

time, there was a statistically significant difference between retail-sliced versus prepackaged 

product. Both storage time distributions could be fit by Weibull distributions as shown in Table 

6. 

 

10 http://www.foodrisk.org/exclusives/CSPRRTEF/index.cfm
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-toeat Meat and Poultry Deli Meats Report

19

Table 6. Fitted Weibull distributions according to the deli meat category

Deli Meat Category n Shape Scale

Retail-sliced 443 1.830 7.777

Prepackaged 387 1.137 18.390

These different distributions are shown graphically in Figure 7. Note the long tail for the 

prepackaged product storage time indicating that the prepackaged product was held for longer 

times than the retail-sliced product.

Figure 7. Relative frequency of storage time for retail-sliced versus prepackaged product.

These distributions were used to calculate the storage time for the cumulative probabilities used 

in the 2003 FDA-FSIS risk assessment model. The results, along with the existing FDA-FSIS 

times, are shown in Table 7 and Figure 8.
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-toeat Meat and Poultry Deli Meats Report

20

Table 7. Storage times for retail-sliced and prepackaged product.

Cumulative 

Probability

Consumer Storage Time (d)

FDA-FSIS Retail-Sliced Prepackaged

0 0.0 0.00 0.00

0.39 2.0 5.29 9.89

0.84 5.5 10.83 31.33

0.91 9.0 12.57 39.83

0.96 12.5 14.73 51.42

0.97 18.0 15.44 55.44

0.99 26.0 17.92 70.46

0.999 45.0 22.36 100.64

Figure 8. Storage times for retail-sliced and prepackaged product.

Two points stand out from this analysis. The first is that storage time for retail-sliced product is 

significantly shorter than for prepackaged product. The median storage time for retail-sliced 

product is 6.4 days, while for prepackaged product is 13.3 days. The second point is that both 

storage distributions are longer than the distribution used in the original 2003 FDA-FSIS risk 

assessment model except for the extreme tail of the retail-sliced distribution. 
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-toeat Meat and Poultry Deli Meats Report

21

The same survey data were used to analyze storage temperatures. These data were fit by a 

logistic distribution with location parameter of 40.15oF and scale parameter of 3.193. The sample 

size was 2,037. The 2003 FDA-FSIS risk assessment model uses 939 temperature measurements 

as inputs, not a probability distribution. The temperature for a given run is then sampled from 

these values. To run the model with the new temperature distribution, 939 random numbers were 

generated from the fitted logistic distribution. The relative frequencies are shown in Figure 9. 

The newer logistic distribution is slightly less peaked and has a longer tail toward higher 

temperatures than the original 2003 FDA-FSIS data. Storage temperatures were assumed the 

same regardless of whether the product was retail-sliced or prepackaged.

Figure 9. Relative frequencies for consumer storage temperature.

2.3.2 Serving Sizes and Categories

Serving sizes and total number of deli meat servings annually consumed estimated for the 2003 

FDA-FSIS risk assessment were used for this analysis. Nevertheless, total servings of deli meats 

needed to be proportioned among (i) prepackaged deli meats with antimicrobial growth 

inhibitors; (ii) prepackaged deli meats without antimicrobial growth inhibitors; (iii) retail-sliced 

deli meats with antimicrobial growth inhibitors; and (iv) retail-sliced deli meats without 

antimicrobial growth inhibitors.
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-toeat Meat and Poultry Deli Meats Report

22

Therefore, the fraction of servings for each of the four deli meat categories was estimated from 

industry survey data from USDA/FSIS Form 10,240-1, Production Information on Post-Lethality 

Exposed Ready-to-Eat Products, gathered in July 2007 in accordance with 9 CFR 430.4(d). For 

example, 32.2% of servings were calculated to be prepackaged product with antimicrobial 

growth inhibitor. Overall, approximately 47% of deli meat is sold prepackaged, and 53% is 

retail-sliced (Table 8).

Table 8. Percent of deli meat production by slicing location and antimicrobial growth inhibitor

use during July 2007.

Alternative Prepackaged 

(sliced at plant)

Retail-Sliced Total

With antimicrobial growth 

inhibitor

32.2% 26.7% 58.9%

Without antimicrobial 

growth inhibitor

14.4% 26.7% 41.1%

Total 46.6% 53.4% 100%

The current version of the exposure analysis model does not incorporate a lag time prior to 

growth. There is evidence that product with growth inhibitor has a longer lag time than product 

without inhibitor (Legan et al., 2004; Pradhan et al., 2009), and this should be considered in 

future modeling. However, the growth inhibitor usage affects both retail-sliced and prepackaged 

products. This difference might increase the estimated relative risk because a slightly higher 

fraction of prepackaged product uses growth inhibitor (Table 8).

2.4 STAGE IV: L. MONOCYTOGENES DOSE-RESPONSE RELATIONSHIP

In the 2003 FDA-FSIS risk assessment, there are three age-specific dose-response relationships 

that have been developed – one for those 60 years of age or older (referred to as “elderly” in the 

2003 FDA-FSIS risk assessment), those who are more than 30 days old to 60 years of age 

(“intermediate” age population), and fetuses and neonates from 16 weeks after fertilization to 30 

days old (“neonatal” population). The methods used in the 2003 FDA-FSIS risk assessment are

the same as those used here.

The dose-response model was run in calibrated mode. In calibrated mode, a scaling factor was

used for each of the 4,000 simulations to adjust the dose-response curve from the mouse doseresponse model to meet a specified number of deaths in humans. For this analysis, the 

comparative risk assessment was calibrated to the number of deaths attributed to deli meats, 

based on data from the Centers for Disease Control and Prevention, used in the 2003 FDA-FSIS

risk assessment. Given the increased implementation of Listeria monocytogenes control 

procedures at the processing plant and antimicrobial growth inhibitor use in the product; these 

values are likely to overstate estimated deaths under current conditions. Thus, the estimated 

deaths are meant for comparative purposes only. A sensitivity analysis that modifies the total 

number of deaths is described in Section 3.3.3.
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-toeat Meat and Poultry Deli Meats Report

23

3.0 Results

3.1 ESTIMATED DEATHS AND ILLNESSES BY SLICING LOCATION AND 

GROWTH INHIBITOR USE

The original deli meat category in the 2003 FDA-FSIS Listeria risk assessment was split into 

four separate categories such that exposure distributions were estimated (using that model) for (i) 

prepackaged deli meats with antimicrobial growth inhibitors; (ii) prepackaged deli meats without 

antimicrobial growth inhibitors; (iii) retail-sliced deli meats with antimicrobial growth inhibitors; 

and (iv) retail-sliced deli meats without antimicrobial growth inhibitors. These exposure 

distributions were generated from the two contamination distributions at retail (i.e., one for 

prepackaged deli meat and another for retail-sliced deli meat). The growth predictions applied to 

each of these distributions predicted the effects of variable storage times but similar temperatures 

on the number of L. monocytogenes per gram of deli meat depending on whether it included 

antimicrobial growth inhibitors or not. The exposure distribution finally determined the 

variability in dose per serving by incorporating the distribution of serving size of deli meat in 

grams. In the 2003 FDA-FSIS risk assessment model, these exposure distributions were 

integrated with the FDA-FSIS L. monocytogenes dose-response models (one for each of the three 

age-specific subpopulations) to predict the annual number of deaths attributed to each of the four 

categories. The estimated mean numbers of deaths per year are given in Table 9 below. Clearly,

the use of antimicrobial growth inhibitors reduces the number of estimated deaths. This is most 

notable for the retail-sliced product, which starts with a higher concentration of L.monocytogenes

level at retail. Also notable is the impact that the lower L. monocytogenes starting distribution 

has on lowering the number of deaths from prepackaged products. 

The estimated mean number of deaths per year associated with prepackaged product was 34.1, 

and the estimated mean number of deaths per year associated with retail-sliced product was 

166.9, with an estimated total annual number of deaths equal to 201.0 (Table 9). Seventeen 

percent of the estimated per annum deaths (34.1/201.0 = 16.96%) are attributable to prepackaged 

product, while the remaining 83% are attributable to retail-sliced product (166.9/201.0 = 

83.03%). The relative risk on a per annum basis for deli meats sliced at retail versus those sliced 

in plants is thus 166.9/34.1 = 4.89. These results are almost identical to the findings of the 

preliminary analysis from the 2003 FDA-FSIS L. monocytogenes risk assessment model, which 

used NFPA retail data (Gombas et al., 2003).

A similar analysis was conducted for illnesses. The 2003 FDA-FSIS risk assessment model 

assumes a constant illness to mortality ratio by age group of 3.7, 11.3, and 12.7 for elderly, 

intermediate, and neonatal age groups respectively. Corresponding results for estimated illnesses 

are in Table 10. Because the illnesses are calculated directly from estimated deaths, the 

attributions between prepackaged and retail-sliced product did not change appreciably.
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-to-eat Meat and Poultry Deli 

Meats Report

24

Table 9. Estimated mean number of deaths per year from L. monocytogenes in deli meat among three populations stratified by age and 

four deli meat categories using the storage time that differed between prepackaged and retail-sliced product.

Deli Meat Category Elderly

(95% CI)

Intermediate Age

(95% CI)

Neonatal

(95% CI)

All Age Groups 

(95% CI)

Prepackaged with growth 

inhibitor

8.1

(7.9, 8.3)

1.9

(1.9, 2.0)

0.5

(0.5, 0.5)

10.5

(10.3, 10.8)

Prepackaged without growth 

inhibitor 

18.1

(17.7, 18.6)

4.4

(4.3, 4.5)

1.1

(1.1, 1.1)

23.6

(23.0, 24.2)

Retail-sliced with growth 

inhibitor

20.4

(19.9, 20.9)

4.9

(4.7, 5.0)

1.3

(1.2, 1.3)

26.5

(25.9, 27.2)

Retail-sliced without growth 

inhibitor

108.2

(106.4, 109.9)

25.4

(25.0, 25.8)

6.7

(6.7, 6.8)

140.3

(138.1, 142.6)

Subtotal: Prepackaged 26.2

(25.7, 26.8)

6.3

(6.2, 6.4)

1.6

(1.6, 1.6)

34.1

(33.4, 34.9)

Subtotal: Retail-sliced 128.6

(126.7, 130.5)

30.3

(29.9, 30.7)

8.0

(7.9, 8.1)

166.9

(164.5, 169.3)

Subtotal: With growth inhibitor 28.5

(27.9, 29.1)

6.8

(6.6, 6.9)

1.8

(1.7, 1.8)

37.1

(36.3, 37.8)

Subtotal: Without growth 

inhibitor

126.3

(124.4, 128.1)

29.8

(29.4, 30.3)

7.8

(7.7, 7.9)

163.9

(161.6, 166.3)

Total 154.8

(152.7, 156.9)

36.6

(36.2, 37.1)

9.6

(9.5, 9.7)

201.0

(198.4, 203.6)
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-to-eat Meat and Poultry Deli 

Meats Report

25

Table 10. Estimated mean number of illnesses per year from L. monocytogenes in deli meat among three populations stratified by age 

and four deli meat categories.

Deli Meat Category Elderly

(95% CI)

Intermediate Age

(95% CI)

Neonatal

(95% CI)

All Age Groups

(95% CI)

Prepackaged with growth 

inhibitor

30.0

(29.2, 30.8)

21.7

(21.1, 22.3)

6.5

(6.4, 6.6)

58.2

(56.7, 59.7)

Prepackaged without growth 

inhibitor 

67.1

(65.4, 68.7)

49.6

(48.4, 50.9)

13.7

(13.4, 13.9)

130.4

(127.3, 133.5)

Retail-sliced with growth 

inhibitor

75.5

(73.7, 77.3)

54.9

(53.6, 56.3)

16.0

(15.7, 16.2)

146.4

(143.0, 149.8)

Retail-sliced without growth 

inhibitor

400.2

(393.6, 406.8)

287.5

(282.8, 292.1)

85.6

(84.5, 86.6)

773.2

(761.1, 785.4)

Subtotal: Prepackaged 97.1

(95.1, 99.1)

71.3

(69.8, 72.9)

20.2

(19.8, 20.5)

188.6

(184.7, 192.4)

Subtotal: Retail-sliced 475.7

(468.7, 482.7)

342.4

(337.6, 347.2)

101.5

(100.4, 102.6)

919.6

(906.8, 932.4)

Subtotal: With growth inhibitor 105.5

(103.4, 107.7)

76.6

(75.0, 78.2)

22.5

(22.1, 22.8)

204.6

(200.5, 208.6)

Subtotal: Without growth 

inhibitor

467.3

(460.3, 474.2)

337.1

(332.3, 341.9)

99.2

(98.1, 100.3)

903.6

(890.9, 916.4)

Total 572.8

(565.1, 580.5)

413.7

(408.5, 418.9)

121.7

(120.5, 122.9)

1108.2

(1094.4, 1122.1)
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

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26

The estimated number of deaths was summed across each age group for each simulation. 

A histogram and cumulative density plot of the estimated number of deaths between 

retail-sliced and prepackaged product are shown Figure 10 and 11, respectively. Both 

figures clearly indicate the public health impact of retail-slicing of deli meats.

Figure 10. Estimated number of listeriosis deaths per annum for retail-sliced and 

prepackaged product based on the 4,000 dose-response simulations.
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

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Figure 11. Cumulative density plots estimated number of deaths per annum for retailsliced and prepackaged product based on the 4,000 dose-response simulations.

Table 11 summarizes the interaction between slicing location and growth inhibitor use. 

Note that among the 4 categories, 69.8% of deaths are attributed to retail-sliced product 

that does not contain growth inhibitor.

Table 11. Summary of estimated listeriosis death (%) by deli meat product category.

Category

% of predicted listeriosis deaths per 

year

Prepackaged

Retailsliced TOTAL

Used growth inhibitor 5.2% 13.2% 18.4%

Did not use growth 

inhibitor

11.7% 69.8% 81.5%

TOTAL 16.9% 83.0% 100.0%
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To evaluate better if the estimated mean number of deaths among the different scenarios 

were statistically different, a bootstrap analysis comparing the means of the scenarios was 

undertaken. One hundred thousand samples (with replacement) were sampled from the 

4,000 simulations of each specified scenario. The mean of each of these 100,000 samples

was then calculated. This process was repeated 100,000 times to generate a distribution 

of means. The mean and 95% confidence interval from this distribution was then 

obtained. Sensitivity analysis indicated that even the 2.5th and 97.5th % quantiles had 

stabilized with 100,000 runs. Recall that these simulations were based on the starting L. 

monocytogenes distributions at retail for either the retail-sliced or prepackaged. 

Uncertainty about these distributions was not included. Thus, the resulting confidence 

intervals are narrower and will find statistical differences compared to when the initial 

distributions included uncertainty as well (Table 12). 

Table 12. Statistical comparison of mean number of estimated listeriosis deaths by deli 

meat type.

Scenario Mean LCL1 (2.5%) UCL2

(97.5%)

Prepackaged 34.1 33.4 34.8

Retail-sliced 166.8 164.4 169.1

Difference in means (retail-sliced –

prepackaged)

132.6 130.2 135.2

1 LCL=lower confidence level about the mean.

2 UCL=upper confidence level about the mean.

The 95% confidence interval for the difference in means does not include 0. Thus, the 

difference in means is statistically significant from 0 at 95% confidence. Using the 

fractions of each product in Table 8 and an annual number of deli meat servings of 

2.84x109

, 1.78x1010, and 5.95x106 for elderly, intermediate, and neonatal, the estimated 

deaths per serving are shown in Table 13. The number of servings is taken from FDAFSIS (2003). The neonatal values are based on the intermediate number of servings 

corrected for a pregnancy rate of 0.0174 and an exposure period of between 1 and 30 

days with the most likely value of 7 days based on triangular distribution.

Table 13. Estimated mean number of deaths per serving among the three age groups and 

four deli meat categories.

Food Category Elderly Intermediate Neonatal

Prepackaged with antimicrobial growth 

inhibitor 8.87 x10-9 3.34 x10-10 2.67 x10-7

Prepackaged without antimicrobial growth 

inhibitor 4.44 x10-8 1.71 x10-9 1.26 x10-6

Retail-sliced with antimicrobial growth 

inhibitor 2.69 x10-8 1.02 x10-9 7.92 x10-7

Retail-sliced without antimicrobial growth 

inhibitor 1.42 x10-7 5.34 x10-9 4.24 x10-6
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

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29

3.2 COMPARISON WITH OTHER FOOD GROUPS

The prevalence of L. monocytogenes at both federally inspected processing plants and at 

retail have decreased. The evidence for this at the plant is from FSIS’s monitoring 

program results (see Figure 1; prevalence is number of positive results obtained 

compared to total number of samples collected expressed as a percentage) and a 

comparison of the NFPA versus NAFSS datasets shown below in Table 14. (The NFPA 

data is shown for comparison, and was not used as part of the risk assessment.)

Table 14. Comparison of NFPA versus NAFSS L. monocytogenes Prevalence Data

Product

Prevalence

NFPA1 NAFSS2

Retail-sliced 2.7% 1.4%

Prepackaged 0.4% 0.2%

n 9199 7040

Sampling dates 2000-2001 2005-2006

Sample size 25 g 125 g

1 National Food Processors Association (Gombas et al., 2003)

2 National Alliance for Food Safety and Security (Draughon, 2006)

The data were collected approximately five years apart and indicate that the more recent 

prevalences at retail are approximately half of the earlier values. The changes are 

actually greater because the more recent NAFSS study used a larger sample size, and so 

would have been more likely to find low levels of L. monocytogenes as positives and thus 

increase the prevalence.

This reduction in the contamination rate for L. monocytogenes was incorporated through 

the measured NAFSS retail data. In other words, the comparative risk assessment was 

conducted with the most recent retail data available. 

A full risk ranking analysis of multiple food categories as done for the 2003 FDA-FSIS 

risk assessment was beyond the scope of this project. However, a section that analyzes 

the results across the 23 or 26 food categories (depending on whether deli meats were 

considered as a single category or stratified into four categories) is given below. 

Because only data for deli meats in the 2003 FDA-FSIS risk assessment was updated, this 

may impact the results. For example, improvements by other industries, e.g. changes in 

milk pasteurization processes, were not evaluated. So the analysis in effect compares the 

Listeria monocytogenes contamination in deli meat associated with industry practices in 

place around 2006 with the other RTE foods reflecting industry practices prior to 2003.

The results show that, when treating deli meat as four categories, the greatest number of 

deaths per annum and the greatest risk per serving is attributed to retail-sliced deli meat 

without growth inhibitors. Treating deli meat as one combined category indicates that 

deli meats are associated with the greatest number of deaths per annum, and rank third for 

the highest risk per serving behind raw frankfurters and pâté (Figure 12).
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Median estimated deaths per annum

0 50 100 150 200

Median estimated deaths per serving

0.0

5.0e-9

1.0e-8

1.5e-8

2.0e-8

2.5e-8

Median estimated deaths per serving

0.0

5.0e-9

1.0e-8

1.5e-8

2.0e-8

2.5e-8

3.0e-8

retail-sliced deli meat 

without growth inhibitors

milk

raw 

frankfurters

raw 

frankfurter

milk

deli 

meats

pate

A. Combined deli 

 meat categories

B. Individual deli 

 meat categories

pate

Figure 12. Comparison of estimated deaths per annum and deaths per serving across the 

various food groups. (a) deli meat categories combined into one category. (b) deli meat 

treated as 4 separate categories. Note that all data are from the 2003 FDA-FSIS risk 

assessment except those of deli meats.
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31

3.3 SENSITIVITY ANALYSES

3.3.1 Consumer Storage Times / Temperatures

A sensitivity analysis was conducted to better evaluate the impact that different consumer 

storage time distributions between retail-sliced and prepackaged had on the comparative 

risk. The storage time and temperatures from the original 2003 FDA-FSIS risk 

assessment model were used and the storage time for the retail-sliced product was 

modified to be some fraction of the prepackaged product (which remained unchanged 

throughout). Thus for the first run of the sensitivity analysis, the consumer storage times 

were assumed to be the same for retail-sliced and for prepackaged product. Consumer 

storage times used in the 2003 FDA-FSIS exposure assessment model were taken from a 

consumer survey conducted by the American Meat Institute (AMI) (2001). Results of the 

survey suggest that approximately 40% of ready-to-eat product is stored for less than 3 

days, and another 45% of product is stored from 4 to 7 days. A total of 96% of product is 

stored for less than 14 days.

Results of this sensitivity analysis are shown in Table 15. The estimated mean number of 

deaths per year associated with prepackaged product was 13.8 (4.4+9.4), and the 

estimated mean number of deaths per annum associated with retail-sliced product was 

125.5 (23.5+102.0), with an estimated total annual number of deaths equal to 139.3.11

All of these values are lower than the corresponding numbers for the original analysis 

because the consumer storage times were shorter (see Figure 8). There were 139.3 deaths 

estimated for the original analysis. Ten percent of the estimated per annum deaths 

(13.8/139.3 = 9.89%) are attributable to prepackaged product, while the remaining 90% 

are attributable to retail-sliced product (125.6/139.3 = 90.11%). The relative risk on a per 

annum basis for deli meats sliced at retail versus sliced at plant is thus 125.6/13.8 = 9.1. 

A mean of 698.0 illnesses were attributed to retail-sliced product and a mean of 76.8 

illnesses were attributed to prepackaged product, for a relative risk ratio of 9.1. In other 

words, if retail-sliced product is held by the consumer for similar lengths of time to 

prepackaged product, the risk ratio increases (The risk ratio when storage times differ for 

both prepackaged and retail-sliced product is 4.89, as shown in Section 3.1).

 

11 The risk assessment calibration mode used was set to 390 deaths across all food groups. This number 

may be lower today given the increased use of post-processing lethality and antimicrobial growth 

inhibitors compared to when the original 2003 FDA-FSIS risk assessment model was developed.
FSIS Comparative Risk Assessment for Listeria monocytogenes in Ready-to-eat Meat and Poultry Deli 

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32

Table 15. Estimated mean number of listeriosis deaths per year from deli meat among three populations stratified by age and four deli 

meat categories using the same storage time for both prepackaged and retail-sliced product.

Deli Meat Category Elderly

(95% CI)

Intermediate Age

(95% CI)

Neonatal

(95% CI)

All Age 

Groups (95% 

CI)

Prepackaged with growth 

inhibitor

3.4

(3.3 - 3.5)

0.8

(0.8 - 0.8)

0.2

(0.2 - 0.2)

4.4

(4.3 - 4.5)

Prepackaged without growth 

inhibitor 

7.2

(7.0 - 7.3)

1.8

(1.7 - 1.8)

0.5

(0.4 - 0.5)

9.4

(9.1 - 9.6)

Retail-sliced with growth 

inhibitor

18.0

(17.5 - 18.4)

4.4

(4.3 - 4.6)

1.1

(1.1 - 1.2)

23.5

(23.0 - 24.1)

Retail-sliced without growth 

inhibitor

78.0

(76.5 - 79.6)

18.9

(18.5 - 19.3)

5.1

(5.0 - 5.2)

102.0

(100.1 - 104.0)

Subtotal: Prepackaged 10.5

(10.3 - 10.8)

2.6

(2.5 - 2.6)

0.7

(0.7 - 0.7)

13.8

(13.5 - 14.1)

Subtotal: Retail-sliced 96.0

(94.3 - 97.7)

23.3

(22.9 - 23.7)

6.2

(6.1 - 6.3)

125.6

(123.4 - 127.7)

Subtotal: With growth inhibitor 21.3

(20.8 - 21.8)

5.3

(5.1 - 5.4)

1.4

(1.3 - 1.4)

27.9

(27.3 - 28.6)

Subtotal: Without growth 

inhibitor

85.2

(83.6 - 86.8)

20.7

(20.3 - 21.0)

5.6

(5.5 - 5.6)

111.4

(109.4 - 113.4)

Total 106.5

(104.7 - 108.3)

25.9

(25.5 - 26.3)

6.9

(6.8 - 7.0)

139.3

(137.1 - 141.6)
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33

The summary interaction analysis between slicing location and growth inhibitor use is 

shown in Table 16. The public health impact has worsened slightly compared to when 

both prepackaged and retail-sliced product had different storage time as shown in Table 

11. Since the retail-sliced product is more likely to be contaminated, and have higher L. 

monocytogenes concentrations when it is contaminated, the longer retail sliced product is 

held by the consumer the greater the potential for bacterial growth. 

Table 16. Summary of estimated, percent of listeriosis deaths by deli meat category.

Category

Percent of predicted deaths per year

Prepackaged retail-sliced TOTAL

Used growth inhibitor 3.2% 16.9% 20.1%

Did not use growth 

inhibitor

6.7% 73.2% 79.9%

TOTAL 9.9% 90.1% 100.0%

This sensitivity analysis used the same storage time distribution for both retail-sliced and 

prepackaged product. Consumers most likely store retail-sliced deli meats for shorter 

periods than prepackaged deli meats. Thus, to assess the effect of a reduced consumer 

storage time, the storage time distribution in the retail exposure model was adjusted by 

arbitrary factors of 0.25, 0.50, and 0.75. The results in terms of the number of deaths and 

illnesses are shown in Table 17. The ratio of deaths caused by retail-sliced versus 

prepackaged product is shown in Figure 13. The comparative risk ratio decreased as the 

consumer storage times for the retail-sliced meats decreased; however, retail-sliced 

product is estimated to cause 1.7 times more deaths than prepackaged product even when 

stored for a quarter of the time. All else being equal, if consumers store retail sliced deli 

meat for only 25% of the time that they store prepackaged deli meat, retail sliced product 

still causes a greater number of deaths than prepackaged product.

Table 17. Estimated mean number of listeriosis deaths and illnesses per annum by 

fraction of consumer storage time between retail-sliced and prepackaged product.

Storage Time Fraction 25% 50% 75% 100%

Deaths 70.7 105.5 127.1 139.3

Illnesses 397.8 589.9 708.0 774.7

Ratio of Deaths, Retail-sliced: Prepackaged 1.7 3.7 5.4 9.1
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34

Figure 13. Relative ratio of listeriosis deaths with differing consumer storage times 

between retail-sliced and prepackaged product.

3.3.2 Shelf Life

To evaluate the effect that different shelf life assumptions had on the results, the shelf 

life was varied among 10 days, 14 days (base run already reported), 21 days, 40 days, and 

80 days. Recall that shelf life is not directly incorporated into the model. Rather, based 

on the requirements to qualify for growth inhibitor use under the Interim Final Rule, it is 

used to calculate the exponential growth rates for product with and without growth 

inhibitor. In effect, for this model the shelf life is a measure of the effectiveness of the 

growth inhibitor. Longer shelf lives imply more effective growth inhibitors.

Table 18 summarizes the growth rates and growth multipliers for each of the shelf lives 

considered. Note that as the shelf life increases, growth inhibitors are assumed to be 

more effective (the multiplier for the growth inhibitor product decreases). To keep the 
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

Ready-to-eat Meat and Poultry Deli Meats Report

35

same overall log mean observed growth rate of 0.282 log10 CFU/gram/day, the multiplier 

for the non growth inhibitor product must increase.

Table 18. EGR for product with and without growth inhibitor by shelf life.

Shelf Life 10 day 14 day 21 day 40 day 80 day

With growth inhibitor 0.20 0.14 0.10 0.05 0.025

Without growth inhibitor 0.30 0.31 0.32 0.33 0.34

Multiplier for product with 

growth inhibitor

0.709 0.506 0.338 0.177 0.089

Multiplier for product 

without growth inhibitor

1.062 1.104 1.140 1.174 1.193

An increase in the shelf life decreased the total estimated deaths from 218 at a 10 day 

shelf life to approximately 190 at an 80 day shelf life. The percent of deaths attributed to 

retail-sliced product increased slightly, from 80.1% at a 10 day shelf life to 86.0% at a 80 

day shelf life. The risk ratio (the number of deaths attributed to retail-sliced product / the 

number of deaths attributed to prepackaged product) showed the greatest change, from 

4.0 at a 10 day shelf life to 6.2 at an 80 day shelf life. This change is due to an ever

smaller denominator. Figure 14 summarizes these results. 

Table 19 summarizes interaction between retail-sliced and prepackaged product for the 

80 day shelf life scenario. The key message from this table is that proper use of effective 

growth inhibitors significantly reduces deaths. None of the predicted deaths are 

associated with growth inhibitor product. Note that this assumes consistent and effective 

use of growth inhibitors throughout the industry, which is yet to be verified. 
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Total estimated deaths

0

50

100

150

200

250

Fraction of total deaths 

from retail-sliced product

0.0

0.2

0.4

0.6

0.8

Shelf Life (d)

10 14 21 40 80

Risk ratio 

retail-sliced/prepackaged

0

2

4

6

(a)

(b)

(c)

Figure 14. Impact of varying shelf life for a) total estimated deaths, b) fraction of deaths 

from retail-sliced product, and c) risk ratio.
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

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Table 19. Estimated percent listeriosis deaths by deli meat category for 80 day shelf lif

Category

e.

Percent of predicted deaths per year

prepackaged retail-sliced TOTAL

Used growth inhibitor 0.0% 0.0% 0.0%

Did not use growth 

inhibitor

14.0% 86.0% 100.0%

TOTAL 14.0% 86.0% 100.0%

The overall conclusion from the shelf life sensitivity is that the estimated percentage of 

deaths attributed to retail-sliced product is not sensitive to the shelf life assumption. The 

14-day shelf life assumption found that 83% of deaths from deli meats are attributable to 

retail-sliced product, i.e. retail-sliced products are 4.88 times riskier than prepackaged. 

The sensitivity analysis found that if the shelf life varied from 10 to 80 days, the 

percentage of deaths attributed to retail-sliced product varied from 80% to 86%. Thus, 

the shelf life is not a sensitive variable here for the major conclusions of the risk 

assessment.

3.3.3 Total Number of Deaths

This risk assessment kept the same total number of deaths in each age group across all 

food categories the same as the 2003 FDA-FSIS risk assessment and this value may well 

have changed since the 2003 report. However, the total number of deaths applies to all 

food categories combined, and the percent of deaths attributed to any specific food 

category is not impacted by the specific value used for the total number of deaths. A

sensitivity analysis was conducted by arbitrarily reducing the number of deaths in each 

age group across all 26 food categories12 by 50% (Table 20). While, as expected, the 

predicted mean total number of deaths dropped from 201 to 99.5 per year, the percent 

breakdown among the four deli meat categories (prepackaged with growth inhibitor, 

retail-sliced with growth inhibitor, prepackaged without growth inhibitor and retail-sliced 

without growth inhibitor) remained largely unchanged – 83% of deaths attributed to deli 

meats are from retail-sliced product.

Table 20. Estimated percent listeriosis deaths by deli meat category when total deaths 

across all food groups are reduced by 50%.

Category

Percent of predicted deaths per year

prepackaged retail-sliced TOTAL

Used growth inhibitor 5.2% 13.2% 18.4%

Did not use growth inhibitor

11.8% 69.8% 81.6%

TOTAL 17.0% 83.0% 100.0%

 

12 In the 2003 FDA-FSIS risk assessment, 23 food categories were ranked according to their inherent risk. 

In that risk assessment, deli meat was considered as one category. However, for the purpose of this risk 

assessment, deli meat was split into 4 categories based on slicing location and whether it contained 

growth inhibitor or not. Therefore, this risk assessment considered 26 food categories rather than 23.
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

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3.4 RELATIVE IMPACTS OF MODEL VARIABLES

Using the original comparative risk assessment model results, statistical analyses were 

conducted to elucidate the relative importance of model inputs. A recursive partitioning 

and regression tree was generated in R to determine which factor (age, slicing location, or 

growth inhibitor use) had the greatest effect on the number of resulting deaths (Figure 

15). The first division in the tree indicates that age is the most important factor and that 

the elderly are more likely to die from listeriosis than either the neonatal or the 

intermediate population. Following the tree along the elderly branch, the next division is 

by slicing location. The tree indicates that retail-sliced product is at greater risk for 

causing listeriosis than prepackaged product. Finally, the retail-sliced product is divided 

according to growth inhibitor use. Because the slicing location split always occurs prior 

to the growth inhibitor use split, this indicates that slicing location is a better predictor of 

the number of deaths than growth inhibitor use.

Age = 

neonatal & 

Intermediate

mean deaths = 16.7

n = 48000

Age = Elderly

Sliced = 

prepackaged

Sliced = retail

GI = yes GI = no

mean deaths = 38.7

n = 16000

mean deaths = 5.8

n = 32000

mean deaths = 1.9

n = 16000

mean deaths = 9.6

n = 16000

mean deaths = 3.1

n = 8000

mean deaths = 16.1

n = 8000

Age = neonatal Age = intermediate

mean deaths = 6.7

n = 4000

mean deaths = 25.3

n = 4000

Sliced = 

prepackaged Sliced = retail

mean deaths = 13.2

n = 8000

mean deaths = 64.2

n = 8000

GI = yes GI = no

mean deaths = 20.4

n = 4000

mean deaths = 108.1

n = 4000

Figure 15. Recursive partitioning and regression tree for comparing importance of 

slicing location versus growth inhibitor usage. Consumer storage time temperature data 

from RTI.

Using the data from all 4,000 simulations, box plots were generated for each deli meat 

category by age group (Figure 16). As seen in the box plots, each of the four deli meat 

categories follows a similar trend, with the elderly age group at the highest risk for death. 

Both retail categories consistently had higher medians than either plant-sliced category, 

e.g. retail-sliced product with growth inhibitors was associated with slightly more deaths 

than prepackaged product without growth inhibitors. The significant increase in 
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Ready-to-eat Meat and Poultry Deli Meats Report

39

estimated deaths for retail-sliced product without growth inhibitors compared to those 

with growth inhibitors is apparent. 

Category

with GIwithout GIwith GIwithout GIwith GIwithout GIwith GIwithout GIwith GIwithout GIwith GIwithout GI

Estimated Deaths

0.1

1

10

100

1000

Prepack Retail Prepack Retail Prepack Retail

Elderly Intermediate Neonatal

Figure 16. Box plots for each deli meat category by age group. Prepack = prepackaged, 

Retail = retail-sliced, GI = growth inhibitor.

An interaction plot for the elderly age group was created to compare the effect of growth 

inhibitor use and product slicing location on the mean number of deaths (Figure 17). 

This illustrates once again the interaction of slicing location and growth inhibitor use. 

There is a significant difference between the mean number of deaths resulting from retailsliced product when compared to prepackaged product (Figure 17a). While the use of 

growth inhibitor greatly decreased the mean number of deaths resulting from retail-sliced 

product, prepackaged product without growth inhibitor results in fewer deaths than retailsliced product with growth inhibitor (Figure 17b). 
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

Ready-to-eat Meat and Poultry Deli Meats Report

40

Figure 17. Interaction plots comparing the effect of growth inhibitor (GI) use and slicing 

location on the mean number of deaths from listeriosis.
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

Ready-to-eat Meat and Poultry Deli Meats Report

41

4.0 Conclusions

Based on this analysis, RTE meat and poultry products sliced at retail are approximately

4.88 times more risky on an annual basis than prepackaged product in terms of deaths 

from listeriosis. Retail-sliced products are associated with 83% of all L. monocytogenes

deaths from deli meats. This percentage is largely unaffected by consumer storage time, 

product shelf life, or total number of L. monocytogenes associated deaths. 

This risk assessment shows that the interaction of high L. monocytogenes prevalence and 

concentrations in retail-sliced product together with the lack of growth inhibitors for 

some product are the primary drivers of the risk of death from listeriosis. Potentially, 

retail delis have two options available to lower this risk. First, retail delis may wish to 

consider using product that incorporates growth inhibitors, when available. Second, retail 

delis can exert controls on the transmission and cross-contamination of L. monocytogenes

within the retail environment. 

Finally, in order to proffer effective and efficient mitigation strategies to reduce the risk 

of contracting listeriosis from retail-sliced products, it is very important to understand the 

events, actions and occurrences of the retail deli and its environment. Such understanding 

will study the acts of deli workers while handling, slicing and packaging retail deli 

products and must be able to pinpoint risk areas and opportunities for reducing the 

inherent risks.
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

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Appendix A: L. monocytogenes in Ready-to-eat Meat and Poultry Deli meat

The presence and level of L. monocytogenes in RTE meat and poultry products were

determined using data from a study conducted by the National Alliance for Food Safety 

and Security (NAFSS) (Draughon, 2006).

A-1 DATA COLLECTION METHODS

The sampling group was comprised of four designated sites in the Foodborne Disease 

Active Surveillance Network (FoodNet). These were Northern California (CA), Georgia 

(GA), Minnesota (MN), and Tennessee (TN). Sampling was weighted by the populations 

in counties (http://www.census.gov) so that exposure could be estimated. Approximately 

75% of shopping is done at major supermarket chains and 25% is done at other grocers, 

such as independent retailers (Gombas et al. 2003). The number of samples collected 

from supermarkets versus independent retailers was weighted accordingly. Also, >50% of 

consumers purchase RTE meat products that are sliced at delicatessens with the 

remainder purchasing sliced prepackaged products. USDA data suggest that 

approximately 47% of RTE deli meat is sliced at the processing plant and prepackaged.

13

The relative number of samples between prepackaged and retail-sliced was therefore kept 

approximately equal as part of the sampling design. Sample data were encoded by the 

researchers to prevent identification of the store. 

Approximately 2,000 samples (125 grams each) were analyzed from each of the four 

designated sites, with approximately equal numbers of samples sliced at retail versus 

sliced at a processing establishment and a smaller number of intact product samples. The 

sampling protocol was designed to allow for statistically valid comparisons among sites, 

RTE products type, and retail-sliced versus prepackaged, assuming an α = 0.05 and a 

90% power of detecting a difference of 2% in the comparison of binomial proportions. 

The following product types were sampled: cured poultry, uncured poultry, pork, and 

beef. Analysis of approximately 1,000 samples of each product type was done to support 

conclusions at the desired level of certainty. Use of any antimicrobial or growth 

inhibiting agents was noted at the time of sample collection. 

Intact samples were collected by purchasing whole, intact hams, roast beefs, turkey rolls, 

etc. These large pieces of cooked meat are commonly referred to as “logs” or “chubs.”

Each chub tends to weigh between 10 and 20 pounds. A random number table was used 

to choose five 35-gram core samples from each intact chub. The core samples were then 

tested for presence and level of L. monocytogenes.

 

13 Estimated based on industry survey data collected with USDA/FSIS Form 10,240-1, Production 

Information on Post-Lethality Exposed Ready-to-Eat Products, gathered in July 2007 in accordance 

with 9 CFR 430.4(d). (Table 8)
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43

Specific instructions were provided for sample collectors, including the product category, 

the number of samples of each type of product to be obtained, size of the sample to be 

purchased, and how to choose, collect, hold and transport the sample. Sample collection 

was standardized to maintain consistency. 

Sampling and laboratory analyses followed standard laboratory practices. This included 

temperature monitoring during shipment, chain of custody documentation, aseptic 

transfer and handling within the laboratory, and initiating analyses within 24 hours of 

receipt of sample. The laboratories were instructed to discard any sample with package 

damage such that the microbiological integrity of the sample was not compromised. 

Samples not meeting quality control requirements were noted and discarded. The FSIS 

standard laboratory method for L. monocytogenes detection was implemented by the 

laboratories for use in this study. Presence/absence for L. monocytogenes was determined 

by inoculation in UVM broth followed by Fraser broth then modified Oxford (MOX)

agar. Positive samples were quantified using a FSIS protocol 9-tube Most Probable 

Number (MPN) method with a reported detection limit of 0.3 MPN/gram. Typical 

colonies appearing on MOX plates were spot inoculated onto RAPID’ L. mono (BioRad, 

Hercules, CA) for species identification. Additionally, MOX plates characterized by 

esculin hydrolysis were screened for genetic confirmation using Gene-Trak assay 

(Neogen Corporation, Lansing, MI). 

NAFSS research laboratories, as approved by FSIS, were experienced in detecting L. 

monocytogenes in food. Samples were assigned codes and the following product 

information recorded: sampling location (FoodNet site along with producer information, 

retailer’s name, and location of purchase), date of receipt at the laboratory, whether the 

sample appeared to be packaged in-store or prepackaged, and the use-by or sell-by date. 

Any store information or identifiers were removed prior to transfer to FSIS.

A-2 STATISTICAL ANALYSES

Statistical analyses were performed using Number Cruncher Statistical Systems (NCSS) 

2001 (Hintz, 2001) and R version 2.6.1 (R Development Core Team, 2007). For 

statistical tests, p values less than 0.05 were considered statistically significant, and p 

values between 0.05 and 0.10 were considered marginally significant.

Data were analyzed in a variety of ways. The prevalence of L. monocytogenes among 

product samples sliced at retail and those that were prepackaged were analyzed by 

sampling site, product type, store type, time of day (morning or afternoon), and quarter of 

the year using tests of proportions. The null hypothesis for this test is that all the 

prevalences are equal. The alternative hypothesis is that at least one prevalence differs 

from some other. This type of statistical test assumes independence among the samples, 

an assumption that is not likely met for these data. Because multiple samples were 

collected at the same store, multiple positive L. monocytogenes findings are likely to be 

correlated because of cross-contamination and poor hygienic conditions at the store. 

Statistical tests with correlated positive samples would, on average, claim to find 

statistically significant results more commonly than intended. 
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Tests of proportions were also conducted at the retail store level. A store was considered 

positive for retail-sliced or prepackaged if any of the samples for that category were 

found positive for L. monocytogenes. Stores are much more likely to be independent, but 

serious problems arise from this approach as well. Store identifiers (even arbitrary labels) 

were removed from data provided prior to submittal to FSIS as part of the data encoding 

and blinding process. Store visits were therefore estimated based on date and time of 

sampling collection. A second problem was that sample collection times were not 

provided for samples from Minnesota, thus the number of stores available was much 

smaller than the number of samples. Statistical tests based on only a few hundred samples 

lack sufficient statistical power and are unlikely to detect small differences in prevalence 

rates at reasonable levels of confidence. Finally, this approach does not directly 

incorporate the number of samples collected at each store.

The final approach used was a logistic regression that predicts the store prevalence for 

retail-sliced and prepackaged product as a function of indicator variables: where the 

product was sliced, the store type, and the time of day the sample was collected. Because 

it is based on store prevalence, this approach is not subject to the correlation problem. 

The regression was weighted by the number of samples taken at the store, and evaluated 

more than one explanatory variable simultaneously. 

A-3 STUDY RESULTS

A-3.1 Prevalence and Number of Samples

Fifty-seven samples were found to be positive for L. monocytogenes resulting in an 

overall prevalence rate of 0.76%. Two of these positives were found in chub samples, six 

were found in prepackaged samples, and the remaining 49 positives were found in retailsliced samples. The number of prepackaged and retail samples across the four FoodNet 

sites is shown in Table A-1. 

Table A-1. Prevalence of product samples1 and stores visited based on sampling 

locations.

Category

Sampling Locations (Site)

CA GA MN TN

Number of product samples2 0.74%

(10/1360)

0.60%

(12/2000)

0.95%

(16/1685)

0.85%

(17/1995)

Estimated number of stores 

sampled3

6.98%

(6/86)

4.93%

(7/142)

n/a3 10.23%

(9/88)

1

 Product samples from each store include those sliced and packaged at retail and those 

sliced and packaged by the manufacturer.

2

 Chub data are not included. Number of positive samples and total number of samples 

are given in parentheses.

3 Store visit estimated based on similar sampling date and time. No sample times were 

provided for MN, so estimate of stores sampled was not available.
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Slightly fewer product samples were taken in CA than other sites. More stores were 

sampled in GA than other sites. In addition to prepackaged and retail-sliced product 

samples, 105 and 300 additional chub samples were collected in MN and TN 

respectively. Assuming independence, a test of proportions indicated no statistically 

significant difference for the prevalence within product samples among the four sites (p = 

0.75). Neither was there any statistical difference for the store prevalence across the sites 

(p = 0.31). This allowed for pooling of the data for purposes of discussing total 

prevalence.

The number and prevalence for retail-sliced and prepackaged samples by quarter of the 

year is shown in Table A-2. More product samples and more stores were visited in the 

3rd quarter than in other quarters. Assuming independence, a test of proportions indicated 

a statistically significant difference for the prevalence within product samples (p = 0.01) 

but not store prevalence (p = 0.31).

Table A-2. Prevalence of product samples (retail-sliced, prepackaged) and stores visited 

based on quarter of year.

Category

Quarter of Year

1st 2nd 3rd 4th

Number of product samples 0.16%

(2/1275)

0.74%

(13/1746)

1.15%

(28/2430)

0.76%

(12/1589)

Estimated number of stores 

sampled1

2.63%

(2/76)

7.37%

(7/95)

5.34%

(7/131)

10.00%

(6/60)

1 Store visit estimated based on similar sampling date and time. No sample times were 

provided for MN, so product samples include MN but stores sampled do not. Chub data 

are not included.

The number and prevalence for retail-sliced and prepackaged samples by time of day is 

shown in Table A-3. Slightly more product samples and stores were sampled in the 

afternoon. Assuming independence, a test of proportions indicated a statistically 

significant difference for the prevalence within product samples (p = 0.04) but not store 

prevalence (p = 0.75).

Table A-3. Prevalence of product samples (retail-sliced, prepackaged) and stores visited 

based on time of day (AM versus PM).

Category

Time of Day

AM PM

Number of product samples1 0.51%

(13/2540)

1.04%

(32/3060)

Estimated number of stores sampled1 5.42%

(9/166)

6.81%

(13/191)

1 Store visit estimated based on similar sampling date and time. No sample times were 

provided for MN, so neither product samples nor stores sampled include MN. Chub data 

are not included.
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The more interesting time of day analysis looked solely at retail-sliced product as shown 

in Table A-4. Retail-sliced product samples collected in the afternoon were more than 

twice as likely to test positive for L. monocytogenes – 1.92% versus 0.92%. Assuming 

independence, this difference was statistically significant (p = 0.04). While the store 

prevalences were also higher in the afternoon (7.83% versus 5.80%), the differences were 

not statistically significant (p = 0.64).

Table A-4. Prevalence of only retail-sliced product and stores visited based on time of 

day (AM versus PM).

Category

Time of Day

AM PM

Number of product samples1 0.92%

(12/1307)

1.92%

(31/1612)

Estimated number of stores sampled1 5.80%

(8/138)

7.83%

(13/166)

1 Store visit estimated based on similar sampling date and time. No sample times were 

provided for MN, so neither product samples nor stores sampled include MN.

The number and prevalence for retail-sliced and prepackaged samples is shown in Table 

A-5. As designed, more product samples were collected at major grocery chains. 

Assuming independence, a test of proportions found a marginal statistically significant 

difference for the prevalence within product samples (p = 0.07) but not store prevalence 

(p = 0.82).

Table A-5. Prevalence of product samples (retail-sliced, prepackaged, and chubs) and 

stores visited based on store type (major grocery chain versus other grocers).

Category

Store Type2

A B

Number of product samples1 0.64%

(31/4801)

1.10%

(24/2186)

Estimated number of stores sampled1 5.58%

(11/197)

6.71%

(11/164)

1 Store visit estimated based on similar sampling date and time. No sample times were 

provided for MN, so product samples include MN but stores sampled do not.

2 A represents major grocery chains. B represents other grocers.

Product samples were collected from prepackaged product, from product sliced at retail 

delis, and a limited number from intact chubs collected at retail. The number of RTE

product samples by location of slicing is shown in Figure A-1. A total of 3,518 retailsliced samples, 3,522 prepackaged samples, and 405 chub samples were collected. A 

given chub may have been sampled multiple times making the number of unique chubs 

uncertain. 
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Figure A-1. Number of RTE samples by location of slicing (source).

The data also indicate that deli meat sliced at retail is more likely to be contaminated than 

prepackaged deli meat (1.39% versus 0.17%). The results are shown in Figure A-2. 

Assuming independence, a test of proportions between retail and prepackaged prevalence 

indicated retail-sliced deli meat had a statistically significant higher prevalence (p < 

0.0001).

14

 

14 Chub data were not included in this test of proportions.

405

3518 3522

0

500

1000

1500

2000

2500

3000

3500

4000

chub retail sliced prepackaged

Source

Total Number of Samples
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Figure A-2. Prevalence of L. monocytogenes in deli meat by location of slicing.

The site and slicing location results for sliced deli meat only are shown in Table A-6. 

Chub results are not included. The striking difference in prevalence between retail-sliced

versus prepackaged is evident at all sites. Differences among the sites are relatively 

minor.

Table A-6. Prevalence of L. monocytogenes in retail-sliced and prepackaged deli meat by 

site.

Site

(number of positive samples and the total number of 

samples are shown in parentheses)

CA GA MN TN Overall

Processing

Retail-sliced

1.3%

(12/929)

1.4%

(10/731)

1.4%

(12/841)

1.5%

(15/1017)

1.4%

(49/3518)

Prepackaged

0.0%

(0/1071)

0.0%

(0/629)

0.5%

(4/844)

0.2%

(2/978)

0.2%

(6/3522)

Overall

0.6%

(12/2000)

0.7%

(10/1360)

0.9%

(16/1685)

0.9%

(17/1995)

0.8%

(55/7040)

Note: Chub data are not included.

0.0%

0.2%

0.4%

0.6%

0.8%

1.0%

1.2%

1.4%

1.6%

chub retail sliced prepackaged

Prevalence (%)

49 / 3518

6 / 3522

2 / 405
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For the 362 stores identified across the three sites available (CA, GA, TN) retail-sliced 

deli meat was sampled at 308 stores and prepackaged deli meat was sampled at 313 

stores. For most stores, both types of deli meat was collected – 259 of these stores had 

both retail-sliced and prepackaged samples collected, 49 had only retail-sliced samples 

collected, and 54 had only prepackaged sliced samples collected. The testing results 

showed that only one store had positives samples for both retail-sliced and prepackaged 

deli meat. An additional 20 of the stores had positive retailed-sliced samples, and one 

store had positive prepackaged deli meat only.

Histograms of the number of retail-sliced and prepackaged deli meat samples taken at 

each store are shown in Figure A-3. For retail-sliced deli meat, the number of deli meat

samples per store ranged from 1 to 30, with a median of 8. The 25th and 75th% quantiles 

were 6 and 10 respectively. For prepackaged deli meat, the number of deli meat samples 

ranged from 1 to 24, with a median of 9. The 25th and 75th% quantiles were 6 and 11,

respectively.
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Figure A-3. Number of deli meat samples collected per store. MN data are not included 

because stores could not be identified.

Some differences existed among the different sites for labeling types of deli meats. After 

correcting for obvious misspellings and accounting for multiple orderings, the types of 

deli meats listed in the data were: beef, beef/chicken/pork, beef/chicken/turkey, 

beef/pork, beef/pork/turkey, bologna, chicken, chicken/pork, chicken/turkey/pork, ham, 

mixed, pork, pork/turkey, poultry, poultry (chicken), poultry (chicken/pork), poultry 

(chicken/pork/beef), poultry (turkey), poultry (turkey/pork), and roast beef.

Many categories of deli meat types had very few samples. For purposes of this analysis, 

these categories were combined into 5: beef, bologna, pork, poultry, or mixed. Deli meat

labeled as “bologna” was classified into different product types. If labeled by the sampler 

as “beef bologna,” it was categorized as beef. If labeled with mixed components, it was 

categorized as mixed. If labeled simply as bologna, it was categorized as bologna. Deli 
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meat listed as poultry but containing mixed components was categorized as mixed. For 

example, the samples labeled “poultry (chicken/pork)” were categorized as mixed. Based 

on this categorization, the counts by product type are given in Figure A-4.

Figure A-4. Number of RTE samples by deli meat type. Chub data are not included. One 

sample (not shown) did not include any listing for deli meat type.

The prevalence of L. monocytogenes across the different deli meat types is shown in 

Figure A-5. Although it appears that beef has a slightly higher prevalence, the differences 

were not statistically significant based on a test of proportions (p = 0.22) among the five

different deli meat types (beef, bologna, mixed, pork, poultry). The corresponding L. 

monocytogenes prevalence for beef, bologna, mixed meat, pork, poultry deli meats were 

1.28%, 0.31%, 0.44%, 0.87%, and 0.65%, respectively. There does not appear to be any 

difference in the prevalence of L. monocytogenes based on whether the deli meat was 

cured or uncured. A similar test was conducted for retail-sliced only deli meat samples 

with similar results. Overall, there was no statistically significant difference in the 

prevalence of L. monocytogenes among the different deli meat types (p = 0.43).

1093

321 229

1727

3669

0

500

1000

1500

2000

2500

3000

3500

4000

beef bologna mixed pork poultry

Product Type

Number of Product Samples
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Figure A-5. Prevalence of L. monocytogenes in RTE deli meats by deli meat type. Chubs 

were not included.

Samplers were asked to identify if the sample included an antimicrobial formulation. Of 

the 7,446 samples, 51 were identified as using an antimicrobial agent, 1,008 did not use 

an antimicrobial agent, and 6,387 were blank. Antimicrobial agents listed included 

potassium lactate, sodium diacetate, calcium lactate, sodium phosphate, and sodium 

benzoate. Of the 57 samples positive for L. monocytogenes, 1 listed sodium 

lactate/sodium diacetate use, 6 listed sodium erythorbate (not considered a growth 

inhibitor), and 50 were blank.

15

There is an indication that positive retail-sliced samples were clustered by store when 

positive L. monocytogenes results were found. Figure A-6 illustrates the deli meat sample 

prevalence for retail-sliced deli meat among the 21 stores with at least one positive result. 

Three of these stores had 50% or greater prevalence, and six of these stores had greater 

than 30% prevalence. Of the 308 identified stores sampled for retail-sliced deli meat, 37 

 

15 Because of the large number of blanks, this antimicrobial formulation data was not used as part of the 

risk assessment described below. Instead, USDA data on current industry practices were used to estimate 

the fraction of product with antimicrobial usage.

0.00%

0.20%

0.40%

0.60%

0.80%

1.00%

1.20%

1.40%

beef bologna mixed pork poultry Grand

Total

Product Type

Prevalence of Product Samples (%)

14 / 1093

1 / 321

1 / 229

15 / 1727

24 / 3669

55 / 7040
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L. monocytogenes positive deli meat samples were found among 22 stores. The remaining 

positive samples were from MN, where individual stores could not be identified. Six of 

these stores accounted for 21 of the 37 positive samples found. Thus, it appears that a few 

retail stores accounted for most of the positive deli meat samples found. This finding is 

indicative of cross contamination at the retail establishment. It is also the reason that the 

independence assumption of the test of proportions for deli meat samples is likely not 

completely valid.

Figure A-6. Prevalence of L. monocytogenes in RTE deli meats samples sliced at retail.

The estimated store visit was based on similar sampling date and time. No sample times 

were provided for MN; thus, MN data not included. Thirty-seven total deli meat samples 

are shown.

A-3.2 Logistic Regression

To overcome the limitations with the test of proportions used above (non-independence 

for deli meat samples and small sample size for store samples), a logistic regression was 

performed. Logistic regression is appropriate when the dependent variable represents a 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

A B C D E F G H I J K L M N O P Q R S T U

Store ID

Product Sample Prevalence (%)

2/2

5/7

1/2

4/10

3/8

3/94/14

1/5 1/51/5

2/11
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proportion of positive results such as the deli meat prevalence for retail-sliced deli meat

at an individual store. The assumptions for standard linear regression are not valid here: 

the dependent variable is bounded to fall between 0 and 1, the errors are not normally 

distributed, and the regression must be weighted by the sample size used to calculate the 

prevalence. Logistic regression transforms the prevalence to a scale more suitable for 

regression. The analysis was performed in R using the generalized linear model (glm). In 

the language of R, a binomial family was specified which used the logit transformation as 

the link function.

The prevalence of retail-sliced and prepackaged deli meat was calculated separately for 

each store. This prevalence was regressed against several indicator variables: processing 

type (retail-sliced versus prepackaged), time of day, and store type. Retail-sliced and 

prepackaged prevalences from the same store were treated as independent. Given that 

only one store had both processing types found positive, this seemed a reasonable 

approach. The number of samples of each type was used to weight the regression. (Thus, 

store prevalences with only one sample received less weight than store samples with 30 

samples.) The logistic regression approach also had the advantage that all three 

explanatory variables were included simultaneously.

The regression function was

logit(prevalence) = β0 + β1 ⋅ processing type + β2 ⋅store type + β3 ⋅ time of day

where: logit() = the logit transformation function; prevalence = the deli meat sample 

prevalence for each store and processing type (retail-sliced versus prepackaged); 

processing type = 0/1 indicator variable with 0 for prepackaged and 1 for retail-sliced; 

store type = 0/1 indicator variable with 0 for type A stores (major grocery chains) and 1 

for type B stores (other grocery stores); and time of day = 0/1 indicator variable with 0 

for AM and 1 for PM. 

The number of data points used in the regression was 613. This is less than twice the 

number of individual stores sampled (2*362=724) because not all stores had both retailsliced and prepackaged samples collected.

The results for the parameter estimates are given in Table A-7. The variables processing 

type and store type are statistically significant. The time of day the sample was collected 

is marginally significant.

Table A-7. Results of logistic regression for store prevalence as function of processing 

type, store type, and time of day indicator variables. Data for MN not included. N=613.

Parameter Estimate Standard 

Error

Z value p

Intercept -7.96 0.76 -10.39 <0.0001

Processing type 2.90 0.73 4.00 <0.0001

Store type 0.99 0.33 3.03 0.002

Time of day 0.59 0.35 1.68 0.093
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As expected from examining the data, whether the sample was prepackaged versus retailsliced was strongly statistically significant. This is consistent with the test of proportions 

for deli meat samples. The result for time of day is consistent with the deli meat sample 

test of proportions for time of day. Both results indicate marginal statistical significance.

Figure A-7 illustrates the results using a logistic regressions based on one explanatory 

variable at a time as the explanatory variable. Because the vast majority of points had 0 

prevalence and only two values (0/1) were used for the explanatory variables, a small 

random number was added to the (x,y) coordinate for each point in order to better 

illustrate the density of points at 0 prevalences.

16

Figure A-7. Graphical display of logistic regression results using deli meat sample 

prevalence at individual stores as the dependent variable. MN data not included.

 

16 The statistical term for this is jitter.

Process Type

Store Prevalence

0.0 0.2 0.4 0.6 0.8 1.0

prepackaged retail-sliced

Store Type

Store Prevalence

0.0 0.2 0.4 0.6 0.8 1.0

A B

Time of Day

Store Prevalence

0.0 0.2 0.4 0.6 0.8 1.0

AM PM
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A-3.3 Comparison of Findings of the National Alliance for Food Safety and Security 

with those of the Food Processors’ Association

A comparison of NAFSS retail contamination findings with those of the National Food 

Processors Association (now Food Products Association) (Gombas et al. 2003) is 

enlightening, although keep in mind that sample collection methods, sample sizes and 

analyses methods differed and these can all affect the results. The total number of deli 

meat samples was roughly equivalent: Gombas et al. sampled approximately 9,000 deli 

meat samples compared to about 7,000 (excluding chubs) for this research. The split 

between retail-sliced and prepackaged was somewhat different however. Approximately 

77% of the samples from Gombas et al. were prepackaged, versus approximately 50% for 

this work. USDA/FSIS data suggest that approximately 47% of RTE deli meat is sliced at 

the processing plant and prepackaged.

17

Gombas et al. found retail-sliced and prepackaged prevalences of 2.7% and 0.4% 

respectively. This research found prevalences lower by about a factor of 2: 1.4% and 

0.2% respectively. This may indicate improvements in deli meat handling, increased use 

of post-processing lethality and antimicrobial growth inhibitor, or other improvements at 

the processing plant or retail between when the studies were conducted. 

The earlier research found a difference in prevalence between their two sampled sites.

Table A-8 below shows the derived results. Compare these data to the corresponding 

Table 6 above for the more recent data. Whereas this work found a consistent prevalence 

across all sites and a significant difference between retail-sliced versus prepackaged, the 

earlier work found no difference in processing type at one site and a statistically 

significant difference at another. 

Table A-8. Prevalence of L. monocytogenes in sliced deli meat by site and processing 

type from the Food Products Association (Gombas et al. 2003).

Site

CA MD Overall

Processing1

Retail-sliced 0.70% 4.2% 2.7%

Prepackaged 0.55% 0.19% 0.4%

Overall 0.6%

(28/4600)

1.2%

(54/4599)

0.9%

(82/9199)

1 The number of positive samples and the total number of samples are shown in 

parentheses where available.

 

17 Estimated based on industry survey data collected with USDA/FSIS Form 10,240-1, 

Production Information on Post-Lethality Exposed Ready-to-Eat Products, gathered in 

July 2007 in accordance with 9 CFR 430.4(d). See Table 8 above for details
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Gombas et al. also found that the prevalence was higher for retail-sliced deli meat, but 

that the levels for positives were actually higher for prepackaged deli meat. This current 

work found consistently that both the prevalence and levels were higher for retail-sliced

deli meat compared to prepackaged. 

A-4 CONCLUSIONS

Table A-9 summarizes the results of all the statistical testing. RTE deli meat is more 

contaminated with L. monocytogenes, both in terms of prevalence and level, when sliced 

at retail than when prepackaged. The marginal statistical link between positive results and 

time of day as well as the clustering according to the store where the sample was 

collected is an indication that cross contamination within retail establishments is 

occurring. There was no significant difference in prevalence of L. monocytogenes among 

the various four FoodNet sites.

Table A-9. Overall results of statistical tests for prevalence of L. monocytogenes on RTE

meat and poultry deli meats by location, season, time of day for slicing at retail, and by 

deli meat type.

Variable

Statistical Test1

Deli meat

samples2

Stores3 Logistic 

regression4

Geographic location N

(p=0.75)

N

(p=0.31)

Quarter of year Y

(p=0.01)

N

(p=0.31)

Time of Day Y

(p=0.04)

N

(p=0.75)

M

(p=0.093)

Time of day (retail-sliced only) Y

(p=0.04)

N

(p=0.64)

Store Type M

(p=0.07)

N

(p=0.82)

Y

(p=0.002)

Prepackaged versus retailslicedY

(p<0.0001)

Y

(p<0.0001)

Deli meat Type N

(p=0.22)

Deli meat Type (retail-sliced 

only)

N

(p=0.43)

1 Chub data were not included in any of the analyses. Statistical test results were 

considered statistically significant if α < 0.05 and marginal if 0.05 ≤ α ≤ 0.10. A “Y” 

indicates the differences were statistically significant; an “N” indicates that they were 
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

Ready-to-eat Meat and Poultry Deli Meats Report

58

not; an “M” indicates that the differences were marginally significant. The exact p values 

for the test result are given in parentheses below.

2 Deli meat samples were assumed independent for the purposes of the test of 

proportions. In practice, because multiple samples were collected from the same store, 

samples were not independent. Thus, the test of proportions is more likely to erroneously 

claim a statistically significant result than the choice of α would indicate.

3 A store was considered positive if at least one of the deli meat samples collected at the 

store was positive for L. monocytogenes.

4 All three explanatory variables were included simultaneously.
FSIS Comparative Risk Assessment for Listeria monocytogenes in 

Ready-to-eat Meat and Poultry Deli Meats Report

59

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