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. have a geographic distribution.
    • R. typhi (murine or endemic typhus), worldwide.
  • Divided into the spotted fever group (SFG) and typhus group (TG).
• Organisms infect endothelial cells, whose dysfunction leads to severe manifestations of the 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).
        • See separate Rickettsia rickettsii module for details on this pathogen.
      • R. parkeri (American Boutonneuse fever): Gulf Coast tick (Amblyomma maculatum) or Lone Star tick (A. americanum).
        • Febrile illness often with an eschar at the site of the tick bite.
      • R. philipiii (previously known as Rickettsial spp. 364D): transmitted by Dermacentor occidentalis (Pacific Coast tick)
        
        • Cases to date only describe in California but tick vector ranges the West Coast.
    • Worldwide: list not comprehensive, consider in returning travelers to the U.S.
      • R. akari (Rickettsialpox): mite (Lioponyssoides sanguineus, ectoparasite of house mice)
        • Seen in the U.S. and former states of the USSR.
        • Urban disease
        • Closely related, but no longer considered part of the SFGR.
      • R. australis (Australian tick typhus): I. holochyclus and I. tasmani host but only I. holochyclus aka the Australian paralysis tick, 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)
        • Seen most commonly in the Mediterranean, India, Asia (Southwest) and Africa.
        • Subspecies:
          • 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): ticks (Amblyomma hebraeum, A. variegatum)
        • Mostly Southern Africa.
        • Most common rickettsial infection seen in travelers to Africa.
      • R. japonica (oriental spotted fever): At least six tick species implicated.
        • Japan
      • R. raoulti: dog tick (Dermacentor reticulatus)
        • Described in Europe, Eastern Europe.
        • May cause SENLAT syndrome (scalp eschar, neck lymphadenopathy after tick bite)
      • R. sibirica (Siberian tick typhus, North Asian tick typhus): multiple tick vectors
        • Siberia, Northern China, Mongolia
      • R. slovaca: transmitted by dog ticks, Dermacentor spp.
        • Associated with tick-borne lymphadenopathy (TIBOLA)
  • Typhus Group:
    
    • R. prowazekii (epidemic typhus): lice-transmitted.
      • Body louse (Pediculus humanus var. corporis)
      • Worldwide
      • Also, occasionally transmitted in North America by flying squirrel (Glaucomys volans)
        • Sporadic, 39 cases in the US (1976-2001)
    • 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, aka scrub typhus): transmitted by chiggers
      • Southeast Asia.
      • Can be a severe illness, similar to RMSF.
• Rickettsial susceptibility testing not routinely performed given the difficulty of culture.
  • The reference standard is the dilution method using a plaque assay system (cell culture).