Transmission and Epidemiology

Rocky Mountain spotted fever (RMSF) is a tickborne disease caused by the intracellular bacterium Rickettsia rickettsii.


Tick Bites

Figure 1: Estimated geographic distribution of American dog ticks (above) and Rocky Mountain wood ticks (below)

2 maps of the United States.  The top map shows where in the U.S. is the American Dog Tick located.  The entire eastern half of the country as well as California are highlighted.  The bottom map shows where the Rocky Mountain Wood Tick can be found.  The North West part of the U.S. is highlighted.

Primarily, through tick bites.

Blood transfusion or organ transplant

  • Transmission of R. rickettsii through blood transfusion is extremely rare.
  • Organ transplant acquired RMSF has not been documented, but is physiologically possible.


  • Geographic range: RMSF is one of several diseases reported under the category “Spotted fever rickettsiosis (SFR).” SFR are reported in each of the lower 48 states, but more than 60% of the cases are reported out of five states (North Carolina, Oklahoma, Arkansas, Tennessee, and Missouri).
  • Peak transmission: Although cases are reported in every month of the year, most cases occur during May–August.
  • Different epidemiology for Arizona and Northern Mexico: Transmission of R. rickettsii by the brown dog tick (Rhipicephalus sanguineus) in these areas results in distinct differences in epidemiology.
    • Figure 2: Estimated geographic distribution of brown dog ticks

      Map of the United States highlighting in yellow where the Brown Dog Tick can be found.  The entire map is highlighted in yellow.

      This tick species is found throughout the world and spends the majority of its life on domestic dogs or in areas where dogs may be found (including kennels, yards, and even inside homes).

    • RMSF cases in these areas occur year-round and tick exposure occurs in and around the home.
    • Unlike other areas of the United States, cases in areas Arizona and Northern Mexico are characterized by unusually high incidence and case fatality rates, particularly among children.