Using outbreak data for source attribution of human salmonellosis and campylobacteriosis in Europe

Salmonella and Campylobacter are the most important bacterial causes of foodborne illness in Europe. To identify and prioritize food safety interventions, it is important to quantify the burden of human foodborne illness attributable to specific sources. Data from outbreak investigations are observed at the public health endpoint and can therefore be a direct measure of attribution at the point of exposure. An analysis or summary of outbreak investigations is useful for attributing illnesses to foods, but often the implicated foods in reported outbreaks are complex foods, containing several food items, many of which could be the specific source of the infection. We describe a method that is able to attribute human cases to specific food items contained in complex foods. The model is based on data from investigations of Salmonella and Campylobacter outbreaks in the European Union in 2005 and 2006. The reporting of the causative vehicles for the outbreaks was not harmonized between and within countries. Consequently, we organized the implicated foods in mutually exclusive food categories. We estimated that the most important food sources for salmonellosis cases were eggs (32%) and meat and poultry-meat (15%), and that the majority of the cases of campylobacteriosis were attributed to chicken (10%). For both pathogens, a large proportion of cases could not be linked to any source. Among illnesses that could be attributed to a source, 58% of salmonellosis cases were attributed to eggs, and 29% of campylobacteriosis cases were attributed to chicken. Results also revealed regional differences in the relative importance of specific sources. We assessed the method to be of limited value to attribute human campylobacteriosis due to the limited number of outbreaks. Nevertheless, the presented source attribution approach can be applied to other foodborne pathogens, and is easily adaptable to countries having an appropriate number of reported outbreaks.