To better understand the transmission and epidemiology of human listeriosis, 647 Listeria monocytogenes isolates obtained from human listeriosis cases in four U.S. locations (Michigan, Ohio, New York State, and New York City) over 61 months (1998 to 2003) were characterized by automated EcoRI ribotyping. A total of 65 ribotypes were differentiated among the characterized isolates; 393, 227, and 24 isolates were classified into lineages I, II, and III, respectively, and 3 isolates were not classified to lineage. The three most common ribotypes (responsible for 39% of all cases) represented L. monocytogenes epidemic clones, each of which had previously been linked to at least two human listeriosis outbreaks. Categorical analyses revealed that ribotypes and lineages were nonrandomly distributed among the four locations. Temporal cluster analysis of cases identified 13 statistically significant temporal subtype clusters, which represented 26% of all cases. Three of these clusters matched previously described human listeriosis outbreaks. Isolates involved in clusters belonged to nine ribotypes. Four, eight, and one cluster were caused by lineages I, II, and III, respectively. The two largest clusters were both caused by the epidemic clone representing ribotype DUP-1044A. Categorical analyses revealed no significant associations between lineage or ribotype and clinical manifestation (central nervous system infection, septicemia, fetal infection, or other infection) or disease outcome (fatal or not fatal). Although human listeriosis cases are caused by isolates belonging to a diversity of EcoRI ribotypes, specific lineage I epidemic clones cause a large number of human listeriosis cases. Many human listeriosis cases can be grouped into statistically significant temporal clusters, including widely distributed and region-specific clusters associated with isolates of various ribotypes. L. monocytogenes lineages and EcoRI ribotypes do not appear to differ in their likelihood of causing different clinical manifestations or mortality.

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