Table 1.

Strengths and Limitations of Source Attribution (SA) Methods

SA approach	Strengths	Limitations
Subtyping approaches Microbial subtyping	Identifies the most important reservoirs of the zoonotic agent, and therefore: is useful to prioritize interventions at production level. reduces uncertainty due to cross-contamination and the risk of attributing to an “accidental” source. is able to follow trends over time.	Limited to pathogens that are heterogeneously distributed among the reservoirs. No information on different transmission pathways from reservoirs to humans. Data intensive, requiring a collection of representative isolates from all (major) sources. Standardized subtyping methods with appropriate level of discrimination are required (methods are changing over time, and new methods are potentially too discriminatory for SA models).
Comparative exposure assessment	Attributes illness to sources taking into account the different transmission routes from the same reservoir. Once a model is developed, new data can be easily included.	Often limited by lack of data, which results in large uncertainties around the estimates.
Epidemiological studies Case–control studies (including systematic review)	Valuable tool to identify relevant risk factors for human infections, including sources of exposure, predisposing, behavioral or seasonal factors. A systematic review of published case–control studies can provide an overview of the relevant exposures and risk factors for that infection, and may detect temporal and geographical variations. Can identify a wide range of known and unknown risk factors.	Misclassification due to immunity may reduce attributable risk or even suggest protection. Most studies only explain a small fraction of all cases. Cases may reflect a mixture of possible sources of exposure, and it may be difficult to distinguish between these exposures. Statistical power to determine the importance of common exposures often requires enrollment of many participants. Misclassification of exposures due to lack of accuracy of recall may lead to an underestimation of the burden of illness attributed to specific exposures.
Analysis of data from outbreaks	Documentation that a specific pathogen was transmitted to humans via a specific food item can be available. Data may capture the effect of contamination at multiple points from the farm-to-consumption chain A wide variety of food vehicles are represented, including less frequently identified food items. Data from outbreak investigations may be the most readily available source of information for source attribution in some countries or regions.	Quality of evidence varies and food classification schemes are not harmonized. Large outbreaks, outbreaks associated with point sources, outbreaks that have short incubation periods, and outbreaks that cause serious illness are more likely to be investigated. Illnesses included in data from outbreak investigations may not be representative of all foodborne illnesses. Certain food vehicles are more likely to be associated with reported outbreaks than others, which can lead to an overestimation of the proportion of human illnesses attributed to a specific food.
Intervention studies	Allows for a direct measure of the impact of a given source on the number of human cases of infection, avoiding the account for the effect of external sources or risk factors.	Interpretation of data from “large-scale” interventions is difficult, since usually several interventions are implemented at the same time. Complex and resource demanding studies.
Expert elicitations	Useful tool when data are lacking. For some pathogens, may be the only available method for SA.	Conclusions are based on the individual experts’ judgment, which may be misinformed or biased.