MICROBIOLOGY

• Obligate intracellular, small Gram-negative coccobacillus [Fig 1], member of Rickettsiaceae family, the agent of Rocky Mountain spotted fever (RMSF).
• Tick-transmitted, it occurs most commonly in late spring to early fall but may occur year-round in warmer states (e.g., the Southeastern U.S.).
  
  • The common American brown dog tick is the most common vector; other tick species are also described.
    
    • American dog ticks (Dermacentor variabilis) and Rocky Mountain wood ticks (Dermacentor andersoni) are the primary arthropod vectors in the U.S.
    • Brown dog tick (Rhipicephalus sanguineus) was 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) is described as a vector in North Carolina.
  • Infections are described in Arizona and the West Coast, as well as 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 (SFGR) to reflect the situation that some cases are due to other organisms than R. rickettsii) actual numbers of RMSF cases reported likely overstated.