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

This article focuses on the taxonomic classification of the bacterium responsible for Pacific Coast tick fever (PCTF), a tickborne illness resembling Rocky Mountain spotted fever (RMSF). Originally isolated in 1966 from the Pacific Coast tick (Dermacentor occidentalis) in California and previously referred to as Rickettsia 364D or Rickettsia philipii, whole-genome sequencing and integrative analyses have demonstrated that this pathogen is a distinct subspecies of Rickettsia rickettsii, the agent of RMSF. The authors propose the formal name Rickettsia rickettsii subsp. californica for this subspecies, which differs genetically, antigenically, ecologically, and clinically from R. rickettsii subsp. rickettsii. R. rickettsii subsp. californica is maintained exclusively in D. occidentalis ticks in California and causes milder human disease characterized by eschars and infrequent rash, contrasting with the more severe RMSF caused by R. rickettsii subsp. rickettsii.