MICROBIOLOGY

• Obligate intracellular, small Gram-negative coccobacillus [Fig 1], member of Rickettsiaceae family, the agent of Rocky Mountain spotted fever (RMSF).
• Tick-transmitted, late spring to early fall most commonly but may be year-round in warmer states (e.g., Southeastern U.S.).
  
  • The common American brown dog tick most common vector, other tick species also described.
    • American dog tick (Dermacentor variabilis) and Rocky Mountain wood tick (Dermacentor andersoni) primary arthropods vectors in the U.S.
    • Brown dog tick (Rhipicephalus sanguineus) also reported as a vector in Arizona, Mexico. RMSF emerging in Southwestern U.S.
    • Amblyomma cajennense or A. aureolatum may carry R. rickettsii in Central and South America.
    • Amblyomma americanum (Lone Star tick) described as a vector in North Carolina.
  • Infections described in Arizona and West Coast, also in Central and South America (there known as Brazilian spotted fever, Tobia fever or fiebre manchada).
• Due to likely serologic crossreactivity and mistaken cases of other Rickettsial spp. (e.g., R. parkeri diagnosed as RMSF, cases in the U.S. reported by CDC since 2010 as Spotted Fever Group Rickettsia (SPGR) to reflect the situation that some cases are due to other organisms than R. rickettsii) actual numbers of RMSF cases reported likely overstated.