Risk Analysis, Vol. 29, No. 7, 2009 DOI: 10.1111/j.1539-6924.2009.01224.x Source Attribution of Food-Borne Zoonoses in New Zealand: A Modified Hald Model Petra Mullner, 1,2, ∗ Geoff Jones, 3 Alasdair Noble, 3 Simon E. F. Spencer, 1,4 Steve Hathaway, 2 and Nigel Peter French 1 A Bayesian approach was developed by Hald et al. (1) to estimate the contribution of different food sources to the burden of human salmonellosis in Denmark. This article describes the de- velopment of several modifications that can be used to adapt the model to different countries and pathogens. Our modified Hald model has several advantages over the original approach, which include the introduction of uncertainty in the estimates of source prevalence and an improved strategy for identifiability. We have applied our modified model to the two major food-borne zoonoses in New Zealand, namely, campylobacteriosis and salmonellosis. Major challenges were the data quality for salmonellosis and the inclusion of environmental sources of campylobacteriosis. We conclude that by modifying the Hald model we have improved its identifiability, made it more applicable to countries with less intensive surveillance, and fea- sible for other pathogens, in particular with respect to the inclusion of nonfood sources. The wider application and better understanding of this approach is of particular importance due to the value of the model for decision making and risk management. KEY WORDS: Campylobacter; food-borne zoonoses; microbial risk assessment; risk attribution; Salmonella 1. INTRODUCTION Source attribution is the process of determining what proportion of a particular disease is acquired from a given source (e.g., poultry) and through a given pathway (e.g., food, water, and person-to- person transmission). This capacity to attribute cases of human disease to a food vehicle or another source 1 Molecular Epidemiology and Veterinary Public Health, Hopkirk Institute, Massey University, Palmerston North, New Zealand. 2 New Zealand Food Safety Authority, Wellington, New Zealand. 3 Institute of Fundamental Sciences, Massey University, Palmer- ston North, New Zealand. 4 Current affiliation: School of Mathematical Sciences, University of Nottingham, Nottingham, UK. ∗ Address correspondence to Petra Mullner, Massey University, Palmerston North 4442, New Zealand; tel: +64(6)350-5270; fax: +64(6)350-5716; p.mullner@massey.ac.nz, petra.mullner@ nzfsa.govt.nz. responsible for illness is critical for the identifica- tion and prioritization of food safety interventions, and a variety of approaches are used worldwide. (2) Most quantitative risk assessments commonly deal with one pathogen occurring in a single food com- modity and are targeted to identify options for prevention, intervention, and control. In contrast, source attribution models provide information about the public health impact of all important sources and pathways. (3) These methods are intended to provide decisionmakers with a set of tools for priority set- ting to achieve a more targeted control of diseases. (3) Overall, these tools are now vital components for the prioritization of hazards and interventions in food systems. (4) Classical approaches to source attribution in- clude full risk assessments, analysis and extrap- olation of surveillance or outbreak data, and analytical epidemiological studies. (5–8) However, 970 0272-4332/09/0100-0970$22.00/1 C  2009 Society for Risk Analysis