Synchronous phenology of juvenile Ixodes scapularis, vertebrate host relationships, and associated patterns of Borrelia burgdorferi ribotypes in the midwestern United States

To elucidate features of enzootic maintenance of the Lyme disease bacterium that affect human risk of infection, we conducted a longitudinal study of the phenology of the vector tick, Ixodes scapularis, at a newly invaded site in the north-central United States. Surveys for questing ticks and ticks parasitizing white-footed mice and eastern chipmunks revealed that I. scapularis nymphal and larval activity peaked synchronously in June and exhibited an atypical, unimodal seasonality. Adult seasonal activity was bimodal and distributed evenly in spring and fall. We discuss implications of these phenology data for the duration of the I. scapularis life cycle. Densities of Borrelia burgdorferi-infected, questing nymphs were comparable to those found in endemic areas elsewhere in the midwestern and northeastern U.S. Molecular genetic diversity of B. burgdorferi infecting these ticks and rodents was assessed by analysis of the ribosomal spacer types (RSTs). RST 1, a clade that includes strains with highly pathogenic properties, was relatively uncommon (3.4%) in contrast to the northeastern U.S., whereas less pathogenic ribotypes of the RST 2 and 3 clades were more common. These features of the ecology of this midwestern Lyme disease system likely contribute to the lower incidence of Lyme disease in humans in the Upper Midwest compared with that of the Northeast owing to reduced exposure to pathogenic strains of B. burgdorferi.