title: Vectors as Epidemiological Sentinels: Patterns of Within-Tick Borrelia burgdorferi Diversity.
abstract: Hosts including humans, other vertebrates, and arthropods, are frequently infected with heterogeneous populations of pathogens. Within-host pathogen diversity has major implications for human health, epidemiology, and pathogen evolution. However, pathogen diversity within-hosts is difficult to characterize and little is known about the levels and sources of within-host diversity maintained in natural populations of disease vectors. Here, we examine genomic variation of the Lyme disease bacteria, Borrelia burgdorferi (Bb), in 98 individual field-collected tick vectors as a model for study of within-host processes. Deep population sequencing reveals extensive and previously undocumented levels of Bb variation: the majority (~70%) of ticks harbor mixed strain infections, which we define as levels Bb diversity pre-existing in a diverse inoculum. Within-tick diversity is thus a sample of the variation present within vertebrate hosts. Within individual ticks, we detect signatures of positive selection. Genes most commonly under positive selection across ticks include those involved in dissemination in vertebrate hosts and evasion of the vertebrate immune complement. By focusing on tick-borne Bb, we show that vectors can serve as epidemiological and evolutionary sentinels: within-vector pathogen diversity can be a useful and unbiased way to survey circulating pathogen diversity and identify evolutionary processes occurring in natural transmission cycles.
authors.affiliation: Department of Epidemiology of Microbial Diseases, Yale School of Public Health, New Haven, Connecticut, United States of America.
authors.forename: Katharine S
authors.initials: KS
authors.lastname: Walter
authors.affiliation: Department of Epidemiology of Microbial Diseases, Yale School of Public Health, New Haven, Connecticut, United States of America.
authors.forename: Giovanna
authors.initials: G
authors.lastname: Carpi
authors.affiliation: Department of Ecology and Evolutionary Biology, Yale University, New Haven, Connecticut, United States of America.
authors.forename: Benjamin R
authors.initials: BR
authors.lastname: Evans
authors.affiliation: Department of Ecology and Evolutionary Biology, Yale University, New Haven, Connecticut, United States of America.
authors.forename: Adalgisa
authors.initials: A
authors.lastname: Caccone
authors.affiliation: Department of Ecology, Evolution, and Environmental Biology, Columbia University, New York, New York, United States of America.
authors.forename: Maria A
authors.initials: MA
authors.lastname: Diuk-Wasser
mesh_terms: D000818:Animals; D025065:Borrelia burgdorferi / Q000235:genetics*; D014644:Genetic Variation; D007303:Insect Vectors / Q000235:genetics*; D018884:Ixodes / Q000469:parasitology*; D008193:Lyme Disease / Q000453:epidemiology*