Genetic Diversity of Rickettsia parkeri in sympatric tick populations at a focal site in northern Virginia: Implications for pathogen dispersal and spotted fever group rickettsiosis.
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Rickettsia parkeri is emerging as a tick-borne pathogen of public health importance, particularly for its previously unrecognized role as a potential pathogen responsible for rapidly increasing cases of spotted fever group rickettsiosis in the United States. The primary vectors of R. parkeri, Amblyomma maculatum ticks, are undergoing a rapid population expansion from their historic range along the Gulf Coast. In 2010, a highly infected (>40%) population of A. maculatum was discovered at a landfill in Fairfax County, Virginia. Infected D. variabilis were collected the following year, representing a potential spillover event, and ongoing surveillance confirms sustained circulation in this species. Multilocus sequence typing of ompA, ompB, sca4, gltA, and the 17 kDa surface antigen precursor gene was used to assess the genetic variability of R. parkeri across tick species over time. Analysis confirmed complete sequence identity between R. parkeri from D. variabilis and A. maculatum ticks at the landfill site, while five single nucleotide changes in four of the five gene targets differentiate R. parkeri in Fairfax County, Virginia from all sequences available in GenBank. Phylogenetic analysis suggests that R. parkeri infection in D. variabilis results from the introduction and dispersal of a single R. parkeri variant in the region. This study presents novel insight into the transmission dynamics and genetic diversity of R. parkeri along the A. maculatum expansion front, highlighting the potential for spillover in regions where multiple tick species exist in sympatry.