Rickettsia species is a topic covered in the Johns Hopkins ABX Guide.

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MICROBIOLOGY

  • Obligate intracellular, small Gram-negative rod; genus Rickettsia
    • An expanding number of species cause human infection.
    • Transmitted by a variety of hematophagous arthropods: ticks, mites, lice, chiggers, fleas.
      • Most Rickettsia spp. are geographic in distribution.
      • R. typhi (murine or endemic typhus), worldwide.
    • Divided into spotted fever group (SFG) and typhus group (TG).
  • Organisms infect endothelial cells, whose dysfunction leads to severe manifestations of disease.
  • Rickettsial species and associated arthropod vectors: list continues to grow, >15 known species causing human infection (number below by no means complete).
    • Spotted Fever Group Rickettsia (SPGR):
      • North American species of interest:
        • R. rickettsii (Rocky Mt spotted fever, RMSF): tick, Western hemisphere (Dermacentor spp., dog and wood ticks and others).
        • R. parkeri (spotted fever syndrome in United States), Gulf Coast tick (Amblyomma maculatum), febrile illness often with eschar at site of tick bite. 
        • Rickettsia species 364D transmitted by Dermacentor occidentalis (Pacific Coast tick)
          • Cases to date only describe in California but tick vector ranges the West Coast.
      • World-wide: list not comprehensive, consider in returning travelers to U.S.
        • R. akari (Rickettsialpox): mite (Lioponyssoides sanguineus, ectoparasite of house mice), seen in U.S. and former states of USSR.
        • R. australis: Australian tick typhus, I. holocyclus and I. tasmani host but only I. holochyclus believed to frequently bite humans.
        • R. felis: flea-borne spotted fever, increasingly common and described in the Americas, Europe, Africa, SE Asia and Australia.
        • R. conorii (Mediterranean spotted fever, MSF): dog tick (Rhipicephalus and Haemaphysalis genera), Mediterranean, India, Asia (Southwest) and Africa.
          • R. conorii conorii: MSF
          • R. conorii caspia: endemic in Astrakhan region near Caspian sea
          • R. conorii indica: endemic in India, eschar rare
          • R. conorii isrealensis: eschar less frequent than MSF.
        • R. africae (African tick-bite fever): tick (Amblyomma hebraeum, A. variegatum), mostly Southern Africa.
        • R. japonica (oriental spotted fever): Japan. At least six tick species implicated.
        • R. raoulti: may cause SENLAT syndrome (scalp eschar, neck lymphadenopathy after tickbite)
        • R. sibirica (Siberian tick typhus, North Asian tick typhus): Siberia, Northern China, Mongolia; multiple tick vectors.
        • R. slovaca: associated with tick-borne lymphadenopathy (TIBOLA)
    • Typhus Group:
      • R. prowazekii (epidemic typhus): lice-transmitted.
        • Body louse (Pediculus humanus var. corporis)
        • Worldwide
      • R. typhi (murine/endemic typhus): global distribution, transmitted by fleas.
        • Rat flea (Xenopsylla cheopsis) and cat flea (Ctenocephalides felis).
        • Usually tropical, subtropical in distribution.
  • Scrub Typhus Group: reclassified from Rickettsia to Orientia.
    • Orientia tsutsugamushi (formerly R. tsutsugamushi, Tsutsugamushi disease): scrub typhus, transmitted by chiggers; southeast Asia.
  • Rickettsial susceptibility testing not routinely performed given difficulty of culture. Reference standard is dilution method using plaque assay system (cell culture).

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Last updated: September 3, 2016