Investigation of the Hald model as a method to improve foodborne illness source attribution estimates Antonio Vieira, DVM, MPH, PhD Enteric Diseases Epidemiology Branch January 31, 2012

Investigation of the Hald Model Overview  Background  Purpose  Project Description  Deliverables  Timeline 2

Investigation of the Hald Model Background  Hald model is a mathematical approach (Bayesian) to estimate the contributions of different food sources to reported foodborne illnesses (Hald et al., Risk Analysis 2004;24: 255–269)  Model is only method that links human illness to food contamination at different points in the food chain  Model has been used in several countries to estimate foodborne illness source attribution  Denmark has used it to monitor progress toward reducing salmonellosis 3

Background  Hald model links the number of reported foodborne illnesses caused by Salmonella to  Measured levels of contamination in food products  Amount of food products consumed  Model assumes that the number of human cases caused by specific food commodities is influenced by  Food source factors: the different food processing and preparation practices associated with each food commodity  Pathogen factors: the severity of illness caused by each type (serotype) of Salmonella Investigation of the Hald Model 4

Background  CDC and the Food Safety Inspection Service (FSIS) previously adapted the Hald model using US data sources  Hald model relied on data sources specific to Denmark  FSIS and CDC work included: Identifying appropriate data sources in the US and determining gaps Changing the model slightly to accommodate different data sources  US model attributed Salmonella illnesses to food commodities using contamination data from food products in processing plants Different from outbreak methods that use information about foods at the time of consumption 5

Investigation of the Hald Model Results of Initial Work Estimated source attribution over time Estimated commodity consumption over time Guo et al. Foodborne Path Dis (

Investigation of the Hald Model Purpose  Expand the model to incorporate additional food sources of Salmonella  Explore whether Hald model can provide reasonable estimates and measures of uncertainty for foodborne illness source attribution in the US 7

Estimate the expected number of human illnesses attributable to specific food products using human illness data, food consumption data, and pathogen isolation data from food products Investigation of the Hald Model Project Description 8

InputOutput Observed human cases per Salmonella serotype ( ) Human cases per serotype per food source Amount of Salmonella serotypes in each food product (p) Amount of food products consumed (M) Ability of subtypes to cause infection (q) Ability of food product to cause infection (a) Project Description 9

Model uses Bayesian techniques and Markov Chain Monte Carlo (MCMC) simulation to estimate model outputs:  The “a” and “q” variables are estimated  The number of cases attributable to each food product in the model ( λ ) is calculated for each year of the study using the estimated “a” and “q” values p*M*a*q = λ Investigation of the Hald Model Project Description 10

Investigation of the Hald Model Possible Data Sources  Human Salmonella illness National Salmonella Surveillance System (NSSS) Foodborne Diseases Active Surveillance Network (FoodNet) Foodborne Disease Outbreak Surveillance System (FDOSS)  Food product Salmonella isolate data From slaughter plants (FSIS) From trace back of contaminated products and other sources (FDA) From samples of produce (USDA) From food industry testing  US food consumption Food Consumption Data System (USDA Economic Research Service) National Health and Nutrition Examination Survey (NHANES) 11

Investigation of the Hald Model Deliverables  Evaluation of a method of source attribution in the United States for Salmonella that has been successfully used in other countries  Opportunity to develop an approach to foodborne illness source attribution that may allow us to evaluate differences in source attribution over time  A source attribution method that provides something no other current model does  A mechanism to evaluate the relationships between food contamination, consumption, and human illness  A method to estimate source attribution at a point in the food chain other than at consumption 12

Investigation of the Hald Model Timeline  Project team is collecting and evaluating potential data sources  Project team working to refine model  Salmonella source attribution estimates for specific FSIS- and FDA- regulated food commodities – 11/2012  Model will meet long-term plan to develop foodborne illness source attribution models using a variety of data sources 13

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