B01017: Review of US Food Safety and Inspection Service (FSIS) Salmonella enteritidis risk assessment model.

Study Duration: November 1999 to August 2000

Contractor: HVR Consulting Services Ltd

The FSIS farm-to-table risk assessment model for Salmonella enteritidis in eggs and egg products comprises the following modules: production, shell eggs processing and distribution, egg products processing and distribution, preparation and consumption, and public health outcomes. These modules are represented by a number of worksheets and tables within the spreadsheet model and there is not necessarily a direct mapping between modules and worksheets. The model comprises the following worksheets:

• Summary – a series of tables provide the user with key model outputs and allows input of key assumptions.
• ProdModel – represents the production model.
• Model – a set of tables representing the shell eggs processing and distribution, egg products processing, distribution, preparation and consumption modules and the mitigation of risk for each of these modules.
• PublicHealth – represents the Public Health Outcomes modules.
• Paths – summarises data for the preparation and consumption module in terms of possible egg preparation and use.

The immense size and complexity of both the model and its associated worksheets and tables means that it is not easy for a user unfamiliar with the structure of the model to navigate from one area to another.

The overall structure of the model and the input data required to run it reflects the US requirement to establish the risk of foodborne illness for Salmonella enteritidis infected eggs and strategies to mitigate these risks. The model concentrates solely on the internal contamination of the egg via ovarian transmission, hence excludes consideration of shell-borne organisms. Therefore the current US model does not take into account contamination of egg contents by shell fragments during breaking, or cross-contamination due to, for example, poor hygiene or the use of contaminated utensils during food preparation. It also appears that the US model does not consider imported eggs.

The mitigation parameters in the model allow a simple representation of risk mitigation, modifying both results calculated by the baseline model tables and relevant input data with the intention of reducing the number of human illness outcomes. It is not possible to use the model to examine or compare the cost/effectiveness of any specific mitigation strategy.

This study aims to review the US FSIS farm-to-table risk assessment model for Salmonella enteritidis in eggs and egg products in order to address the following questions:

• What are the key strengthens and weaknesses of this analysis?
• How appropriate is the use of Monte Carlo simulation and a highly detailed spreadsheet?
• What alternative methodologies, if any, would be appropriate?
• Which aspects of the model would not be readily transferable to the UK context, and why not?
• What would be required to apply this methodological approach to the UK, and what are the resource requirements likely to be?

The outputs of this study will inform the work being undertaken in a risk assessment by a working group of the Advisory Committee on the Microbiological Safety of Food (ACMSF).

This study concluded that:

• The use of the Monte Carlo simulation in the US Food Safety and Inspection Service (FSIS) model is appropriate given the supporting data sets.
• The use of Monte Carlo simulation in any comparable UK model would be determined by the availability, quality and quantity of appropriate supporting data sets.
• The current US model is not sufficient for UK use due to its sole focus on ovarian transmission of Salmonella enteritidis and its exclusion of issues relating to shell-borne Salmonella enteritidis organisms and cross-contamination.
• The current US model does not take into account imported eggs.
• Shell-borne Salmonella enteritidis contamination, cross-contamination and imported egg modules will have to be developed for the UK model.
• The current US model does not take into account different egg production methods.
• The mechanisms by which different egg production methods are incorporated into the UK model are dependent on the availability, quality, quantity and degree of detail of UK data.
• The data structures of the original US model would provide a useful template for any UK data collection and estimation exercise.
• Any resulting gaps in the UK data set could be addressed by subject matter expert estimation.
• The risk mitigation structures employed in the US model do not allow meaningful comparison of different mitigation strategies in terms of their cost-effectiveness.
• The employment of the variation reduction method in the US model is poorly documented; it is not applicable to a UK model as currently proposed and so could be removed.
• The simplified model, derived from the US model, generates results comparable to the original US model but with significant reductions in both run-time and model complexity but does not take into account shell-borne Salmonella enteritidis organisms, cross-contamination or imported eggs.
• The exemplar, simplified model provides a starting point for the development of shell-borne and cross-contamination modules, bearing in mind that this would require bacterial growth/survival equations describing growth/survival on the shell surface and in egg mixtures after the egg is broken for use.
• The exemplar, simplified model does not take into account imported eggs.
• The use of a spreadsheet methodology complicates any detailed analysis of dynamic variables and relationships such as a shift from one production method to another over time or the impact of high-level policy decisions.
• Other Operational Research (OR) methodologies such as System Dynamics (SD) would be more suitable for the analysis of dynamic variables and relationships.

Final report is available from the Agency's Information centre.
To obtain a copy, please contact the Enquiry Desk, Information Services, Food Standards Agency (tel: 020 7276 8181/8182 or email: infocentre@foodstandards.gsi.gov.uk).

Contact: For any enquiries concerning this research project, please contact the relevant programme contact or email: science@foodstandards.gsi.gov.uk