Francisella tularensis in the United States

Abstract

The causative agent of tularemia, Francisella tularensis, is a formidable biologic agent that occurs naturally throughout North America. We examined genetic and spatial diversity patterns among 161 US F. tularensis isolates by using a 24-marker multiple-locus variable-number tandem repeat analysis (MLVA) system. MLVA identified 126 unique genotypes. Phylogenetic analyses showed patterns similar to recently reported global-scale analyses. We observed clustering by subspecies, low genetic diversity within F. tularensis subsp. holarctica, and division of F. tularensis subsp. tularensis into 2 distinct subpopulations: A.I. and A.II. The 2 F. tularensis subsp. tularensis subpopulations also represent geographically distinct groups; A.I. occurs primarily in the central United States, and A.II. occurs primarily in the western United States. These spatial distributions are correlated with geographic ranges of particular vectors, hosts of tularemia, and abiotic factors. These correlates provide testable hypotheses regarding ecologic factors associated with maintaining tularemia foci.

Figure 1

Genetic relationships among 48 North American Francisella tularensis subsp. tularensis A.I. subpopulation isolates based upon allelic differences at 24 variable number tandem repeat (VNTR) markers. County, state, and year of isolation are specified to the right of each branch or clade. G indicates number of distinct VNTR marker genotypes, dots indicate host-linked isolates, boxed designation indicates prominent F. tularensis subsp. tularensis laboratory strain SCHU S4, and asterisks indicate isolates with an unknown year of isolation.

Figure 5

Phylogenetic relationships among subgroups A.I., A.II., B type, and Francisella tularensis subsp. novicida at 24 variable number tandem repeat markers. Scale bar represents genetic distance.

Figure 4

Genetic relationships among 6 North American Francisella tularensis subsp. novicida isolates based upon allelic differences at 24 variable number tandem repeat (VNTR) markers. County, state, and year of isolation are specified to the right of each branch or clade. G indicates number of distinct VNTR marker genotypes, and boxed designation indicates F. tularensis subsp. novicida type strain Utah 112 (U112).

Figure 2

Genetic relationships among 35 North American Francisella tularensis subsp. tularensis A.II. subpopulation isolates based upon allelic differences at 24 variable number tandem repeat (VNTR) markers. County, state, and year of isolation are specified to the right of each branch or clade. G indicates number of distinct VNTR marker genotypes, triangle indicates epidemiologically linked isolate, asterisk indicates isolate with an unknown year of isolation, boxed designation indicates F. tularensis type strain B-38, and square indicates a set of genetically identical but epidemiologically unlinked isolates.

Figure 3

Genetic relationships among 72 North American Francisella tularensis holarctica B type isolates based upon allelic differences at 24 variable number tandem repeat (VNTR) markers. County, state, and year of isolation are specified to the right of each branch or clade. G indicates number of distinct VNTR marker genotypes, squares indicate genetically identical but epidemiologically unlinked isolates, asterisk indicates isolate with an unknown year of isolation, dot indicates a host-linked isolate, and triangles indicate epidemiologically linked isolates.

Figure 6

Spatial distribution of 125 Francisella tularensis isolates for which information on originating county was available. Locations (colored circles) correspond to county centroids. More than 1 subspecies was isolated from some counties in California (Alameda, Contra Costa, Los Angeles, San Luis Obispo, and Santa Cruz) and Wyoming (Natrona) (see Figures 1–3). In some cases, a single circle may represent instances where >1 sample of a given subspecies or genotypic group was isolated from a single county. Two isolates with county information, 1 from northern British Columbia and 1 from Alaska, are not shown.

Figure 7

Spatial distributions of isolates from the A.I. and A.II. subpopulations of Francisella tularensis subsp. tularensis relative to A) distribution of tularemia vectors Dermacentor variabilis, D. andersoni, Amblyomma americanum, and Chrysops discalis; and B) distribution of tularemia hosts Sylvilagus nuttallii and S. floridanus.

Figure 8

Genetic and spatial data of the A.I and A.II subpopulations of Francisella tularensis subsp. tularensis in the United States. A) Ancestral status of these 2 subpopulations is unclear; either could have founded the other, or a third unknown subpopulation could have been the ancestor. B) Highly restricted bacterial-endemic regions could now be breaking down because of human-mediated dispersal of the pathogen across the country. The small circles indicate the spatial distribution of the A.I and A.II isolates, as shown in Figure 6.