Phylogenetic Differentiation of Rickettsia parkeri Reveals Broad Dispersal and Distinct Clustering within North American Strains.

The tick-borne pathogen Rickettsia parkeri causes a mild rickettsiosis, with cases reported from several countries to its known distribution in the Americas. Molecular analyses have identified a clear distinction between strains of (s. s.) and (s. l.) as well as separation between North American and South American s. s. strains. To expand on this previous work, we developed a multilocus sequence typing analysis with two aims: first, to investigate the genetic diversity within strains of North American s. s., and second, to further the understanding of the genetic relationships between s. s. and s. l. Sixty-four isolates and 12 -positive tick lysates were analyzed using a novel typing scheme consisting of four coding regions and two intergenic regions. A concatenated Bayesian phylogeny that identified eight clades was constructed: three represent the s. l. strains, and five represent the s. s. strains. The clades appear to be generally phylogeographically organized and associated with specific tick vectors. However, while one of the four s. s. North American clades appears to be limited to the southwestern United States, the other North American clades exhibit broad dispersal, most notably seen in the largest group, which includes representative samples extending from northern Mexico to Delaware. This work highlights the increasingly recognized geographic range of in the Americas and suggests a potential public health risk for these areas. Since 1937, when Rickettsia parkeri was originally identified in Amblyomma maculatum group ticks, the recognized range and associated vectors for this pathogen have expanded significantly. In recent years, has been identified in 12 tick species from seven countries in the Americas. Herein, we provide evidence that the greatest genetic diversity within exists in North America, where one and four genotypes are present. While one distinct genotype exists only in the southwestern United States, three genotypes are broadly distributed in the eastern United States, with the largest of these found across the known range of in North America. In contrast, the South American samples represent a single genotype and are completely clonal at the loci analyzed, irrespective of their country of origin.