Salmonella Source Attribution in Japan by a Microbiological Subtyping Approach

In order to estimate the number of human Salmonella infections attributable to each of major animal-food source, and help identifying the best Salmonella intervention strategies, a microbial subtyping approach for source attribution was applied. We adapted a Bayesian model that attributes illnesses to specific sources and allows for the estimation of the differences in the ability of Salmonella subtypes and food types to result in reported salmonellosis. The number of human cases caused by different Salmonella subtypes is estimated as a function of the prevalence of these subtypes in the animal-food sources, subtype-related factors, and source-related factors. National-surveillance serotyping data from 1998 to 2007 were applied to the model. Results suggested that the relative contribution of the sources to salmonellosis varied during the 10 year period, and that eggs are the most important source of disease, being responsible for over 50 % of the cases in most years. Broilers followed in importance in 1999, 2000, 2001, 2002 and 2005, while swine was the second most important source in 2000, 2004 and 2007. Salmonella was seldom isolated from cattle and few cases were attributed to this source. The proportion of cases attributed to an unknown source varied substantially between years. We conclude that this is valid approach to attribute salmonellosis in Japan, and that and improved dataset would substantially improve results. This is the first indication of the relative contribution of different foods for human salmonellosis, and results may be used for further research, risk management and public health strategies.