Risk based optimization of the Danish pork Salmonella control program

H. Scott Hurd[1], Claes Enøe[2], Lene Sørensen[2], Henrik Wachmann[2], Steven M Corns[3], Kenneth M. Bryden[3], Helen H. Jensen[4]

[1] College of Veterinary Medicine, Iowa State University[2] Danish Meat Association, Copenhagen, Denmark

[3] Mechanical Engineering Department, Iowa State University[4] College of Agriculture, Dept of Economics, Iowa State University

AcknowledgmentsTine Hald, Matthias Greiner, Rene Bøker, Anne Bisp Lind

– Danish Institute for Food and Veterinary Research (DFVF)

Funded as a project of the International EpiLab – Guest Scientist at the EpiLab in Denmark, – Employed at the National Animal Disease Center:

USDA:ARS, Ames, Iowa, USA

Overview

• Evaluation of the on-farm pork Salmonella control program

• Has the last 10 years of investment significantly reduced human cases?

• Simulation/systems model with best data says……

U.S.

• 100.3 M pigs/year

• 71,000 producers

Iowa

• 144,700 km2

• 28.8 M pigs/year

Denmark

• 43,000 km2

• 23 M pigs/year

• 13,200 producers

Denmark is a small state with a UNIQUE form of pork production and marketing (Coop)

Context for this project

• “We are spending too much for on-farm surveillance”

• “Carcass decontamination is the cheapest way to control salmonella”

• “Washing all carcasses, not acceptable. Don’t want to eat feces”

• “The Level 3 herds are not important”• “Pressure on the L3 herds has improved all herds”• “Yeah, but has it improved public health?”• “The industry just cares about “cheaper”• “We are at a standstill in reducing carcass swab

prevalence”

BACKGROUNDThe current Salmonella herd

classification scheme

• Serological monitoring of all herds in the “coop” system – Form of a blood test (serum exudate, meat

juice)– (up to 60 pigs/herd/year)

• Assigned to 3 prevalence levels based on last 3 months of serology

The current Salmonella herd classification scheme

• High seroprevalence (~1.5%) assigned – to special hygienic slaughter conditions, – financial penalties, – and hot water decontamination

• Carcass swab (400cm2)– Pool of 5 samples– cultured for Salmonella– Similar to USDA:FSIS

Objectives

• Estimate the effect of various control interventions– On-farm (pre-harvest) vs.– Abattoir (post-harvest)

• Overtime – Previous 10 years (retrospective)– Next 10 years (prospective)

• Final outcome = human cases (unique).

Materials and Methods

• Stochastic systems model– Multiple Excel® spreadsheets

• Simulate the number of human Salmonella cases as a function of– Herd seroprevalence – Undefined abattoir interventions

• carcass swab culture status

– Human attribution model.

• Using best AVAILABLE data from Danish surveillance system

Systems model of pork attributable human cases (PAHC) of salmonellosis

Distribution of herds by

seroprevalence category and size

ProductionModule

Probability of carcass swab positive given

herd seroprevalence

category

Slaughter module

Attributes human cases as a function of

positive carcasses

AttributionModule

Pigs from each seroprevalence category

Human cases per year

Carcass swab positive pigs

0 5 15 2535 45 55 65 75 85 95

1995

1997

1999

2001

2003

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

Percentage

Seroprevalance Group

Year

Distribution of Herds by Average Annual Seroprevalence

1995

1996

1997

1998

1999

2000

2001

2002

2003

Changes in herd seroprevalence due to on-farm control program (actual data)

PRODUCTION module converts size distribution to number of pigs, using

stochastic version of average herd sizeOUTPUT --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Number of PIGS from each ave. annual seroprevalence category by herd size

Size category 0 5 15 25 35 45 55 65 75 85 95 Chk total1-100 4818 147 196 98 0 0 147 0 0 0 49 5457101-200 67147 6919 6564 3105 1508 710 1774 177 266 177 266 88613201-500 248058 206715 45695 19040 7616 7072 4896 3264 1088 0 544 543988501-1000 371424 600728 106292 51354 22692 19109 11943 3583 4777 1194 0 11930961001-2000 900033 1783336 344622 160601 76954 46842 23421 10038 0 0 0 33458462001-3000 718598 1959196 352520 193208 98299 37286 13558 13558 3390 0 0 33896133001-5000 1188339 3253784 611146 305573 198056 73564 28294 5659 0 0 0 56644145000-10000 1111645 3911092 1091310 494817 155901 6778 0 0 0 0 0 6771544>10000 136736 768552 170920 84871 16503 0 0 0 0 0 0 1177581Tot pigs by seroprev 4.6E+06 1.2E+07 2.6E+06 1.2E+06 5.6E+05 1.9E+05 8.4E+04 3.6E+04 9.5E+03 1.4E+03 8.6E+02 2.10E+07

Systems model of pork attributable human cases (PAHC) of salmonellosis

Distribution of herds by

seroprevalence category and size

ProductionModule

Probability of carcass swab positive given

herd seroprevalence

category

Slaughter module

Attributes human cases as a function of

positive carcasses

AttributionModule

Pigs from each seroprevalence category

Human cases per year

Carcass swab positive pigs

Slaughter module

• “Creates” carcass swab positive pigs• As a function of average annual

seroprevalence• Combines data from all Danish abattoirs in

the program• Regression parameter

– adjusted to match historical carcass swab results with the distribution of average annual seroprevalence for that year

Note how coefficient improves over time due to changes in abattoir processes

Table II: Historical (1995-2003) carcass Salmonella swab data and resulting coefficient used in the model for determining positive carcasses.

Year Prevalence in Cuts1

Observed Carcass Swab Prevalence2

Resulting Coefficient3

1995 1.30% 2.47% 0.1207

1996 1.30% 2.47% 0.1100

1997 1.10% 2.09% 0.0971

1998 1.10% 2.09% 0.1467

1999 n/a 1.92% 0.1275

2000 n/a 1.43% 0.0570

2001 n/a 1.37% 0.0490

2002 n/a 1.38% 0.0530

2003 n/a 1.43% 0.0550

1Inflation factor for cut to carcass swap prevalence = 1.927 2From National Data of Danish Meat Association 3Estimated from the observed distribution of herds within average seroprevalence (Table 1) and the observed carcass swab prevalence

Systems model of pork attributable human cases (PAHC) of salmonellosis

Distribution of herds by

seroprevalence category and size

ProductionModule

Probability of carcass swab positive given

herd seroprevalence

category

Slaughter module

Attributes human cases as a function of

positive carcasses

AttributionModule

Pigs from each seroprevalence category

Human cases per year

Carcass swab positive pigs

Attribution module

• Uses attribution data 1999-2003– Methods shown in Hald et al. 2004. Risk Analysis

• Connects positive carcasses – (servings) produced that year with number of pork

attributed human Salmonella cases• Attribution factor

– pert distribution, • Same factor is used in retrospective (historical) and

prospective (future) simulations

Simulation • 5,000 iterations in @Risk• Outputs of interests

– pork attributable salmonellosis cases

• Retrospective (historical) simulations– (1995-2003)– Uses available historical trend data on

• On-farm seroprevalence or• Abattoir regression parameter

• Prospective (future) simulations– (2004-2013)

Systems model of pork attributable human cases (PAHC) of salmonellosis

Distribution of herds by

seroprevalence category and size

ProductionModule

Probability of carcass swab positive given

herd seroprevalence

category

Slaughter module

Attributes human cases as a function of

positive carcasses

AttributionModule

Pigs from each seroprevalence category

Human cases per year

Carcass swab positive pigs

Retrospective simulations(3 scenarios)

• Both combined (Historical)• Abattoir parameter improvements only

• Post-harvest• Farm at 1995 seroprevalence distribution• Farm at 2003 seroprevalence distribution

• Changes in herd seroprevalence over time• Pre-harvest• Abattoir 1995 regression parameter• Abattoir 2003 regression parameter

Historical: Combined effect of on-farm (pre-harvest) changes and abattoir (post-harvest)

Simulated pork attributable human cases

050

100150200250300350400450

Tota

l cas

es

Historical - Farm &Abattoir

Lower Historical

Upper Historical

Significant (P <0.05) changes in risk

occurred 1998-2000

Changes in herd seroprevalence over time: Abattoir parameter at 1995 level

= Effect of on-farm (pre-harvest) program only

Simulated pork attributable human cases

0

100

200

300

400

500

1995

1996

1997

1998

1999

2000

2001

2002

2003

To

tal

case

s

Historical - Farm &Abattoir

Lower Historical

Upper Historical

Abattoir95

19% decrease, 69 cases (P>0.05)

Abattoir (post-harvest) improvements only:

If no on-farm control program? (i.e. seroprevalence stayed at 1995 level)Simulated pork attributable human cases

0

100

200

300

400

500

1995

1996

1997

1998

1999

2000

2001

2002

2003

To

tal

case

s

Historical - Farm &Abattoir

Lower Historical

Upper Historical

Farm 1995

A difference of 81 cases (P>0.05). Due to abattoir

improvements only

Where do we go from here?Prospective simulations (2004-2013)

• Compare on-farm to abattoir– Improvements on-farm at same rate as the

past– Improvements in abattoir at same rate as

the past

• Carcass decontamination in slaughter– Reduced Pr (CS+) by 90%

Improve abattoir only,

40 cases

Improve farm only = no change

Human cases based on holding on-farm seroprevalence levels at 2003 levels and improving abattoir methods (Farm2003ImpAb); reverting on-farm to 1995 values while keeping abattoir values constant (RevF); and continuing to improve on-farm while keeping abattoir values constant (ImpF).

Reduce all herds to <25% ave. annual seroprevalence

0 5 15 25 35 45 55 65 75 85 95

2004

2006

2008

2010

2012

0.0%

5.0%

10.0%

15.0%

20.0%

25.0%

30.0%

35.0%

40.0%

45.0%

50.0%

Percentage

Seroprevalance Group

Year

Distribution of herds by average annual seroprevalence

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

Decrease cases by ~10 to 190/yr

Cases while steadily improving abattoir methods for all on-farm values set at 25% or less (AllFarm25); reverting on-farm to 1995 values while keeping abattoir values constant (RevF); and continuing to improve on-farm while keeping abattoir values constant (ImpF).

Still end up with 110 cases/yr

What else can be done?Carcass decontamination

Number of pork attributable human cases

Percentage of the national herd decontaminated

Equivalent annual seroprevalence

cuttoff Lower Mean Upper

0% NA 136 152 168

1.50% Level 3, >45% 131 147 163

10.00% > 25% 110 126 142

100.00% all 14 15 16

Summary of results - Retrospective

• On-farm program reduced risk in early years (‘95-’98)

• Improvements since ‘98 due to unspecified abattoir improvements

• On-farm program ALONE would have only reduced average annual cases by ~69

Summary of results - Prospective

• Massive on-farm improvements needed to further reduce risk (new technology?)

• Don’t go back (on-farm seroprevalence)• Steady improvement in general slaughter

hygiene will make minimal reduction in risk

• Some form of carcass decontamination has the most significant impact on risk

Interesting points to ponder

• Denmark is a health policy “lab”• Lessons on the economics of food

safety• Use of “attribution” model• Shows what happens when “connect all

the dots”• Systems models provide a tool for

policy and research decision making

Future work: Add economics to a U.S model

Contaminated servings or potential infectious servings