Attributing Risk to Listeria monocytogenes Subgroups: Dose Response in Relation to Genetic Lineages

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Abstract
The objective of this study was to evaluate the hypothesis that the dose-response relationship for Listeria monocytogenes in humans varies with genotypic lineage or subtype. The linkages between molecular subtyping data and enumeration data for L. monocytogenes subtypes in foods consumed by the at-risk population were examined to test this hypothesis. We applied a conditional probability model to conduct a subtype-specific dose-response analysis, with the focus on invasive listeriosis. L. monocytogenes differed not only in the molecular subtype and lineage but also in the contamination level when isolates of the pathogen occurred in retail samples of ready-to-eat foods. Using the exponential model parameter r-value as a measure (essentially the probability of a single cell causing illness), we found that the virulence varied among L. monocytogenes lineages by several orders of magnitude. Under the assumptions made, for L. monocytogenes lineages I and II the consumption of a single cell would result in listeriosis with log average probabilities of −7.88 (equivalent to once in 107.78 times) and −10.3, respectively, as compared with −9.72 for L. monocytogenes independent of subtype. A greater difference in r-values was found for selected ribotypes. The uncertainty about the r-value estimates was small compared with the large differences in the virulence parameters themselves. Thus, for L. monocytogenes both subtype and the number of cells consumed matter, highlighting the usefulness of considering both exposure concentration and subtype prevalence in dose-response analysis. As advances are made in molecular subtyping and quantitative tools for dose-response analysis, further studies integrating genomic data into quantitative risk assessments will enable better attribution of disease risk to L. monocytogenes subtypes.