Rickettsia rickettsii subsp californica subsp nov, the Etiologic Agent of Pacific Coast Tick Fever

The etiologic agent of Pacific Coast tick fever, a moderately severe tickborne illness that resembles Rocky Mountain spotted fever (RMSF), was first isolated in 1966 from specimens of Dermacentor occidentalis (the Pacific Coast tick) obtained in California. For several decades, this bacterium was identified ambiguously as the unclassified spotted fever group Rickettsia species 364-D, Rickettsia 364, or Rickettsia philipii. However, none of these epithets satisfied criteria of formal bacterial nomenclature. Data developed from mouse serotyping studies performed >45 years ago, and multilocus sequence typing several decades later, indicated that this bacterium was similar to, but distinct from, isolates of Rickettsia rickettsii, the etiological agent of RMSF. We applied an integrative taxonomic approach, combining phenotypic, ecological, and clinical data with whole-genome sequencing of 11 contemporary isolates of this pathogen to identify it as a distinct subspecies of R. rickettsii, and propose the name Rickettsia rickettsii subsp californica subsp nov.