Clinical Overview of Transmission and Epidemiology

At a glance

Rocky Mountain spotted fever (RMSF) is a tickborne disease caused by the intracellular bacterium Rickettisa rickettsii. RMSF is transmitted through bites of infected ticks, mostly American dog ticks, Rocky Mountain wood ticks, and brown dog ticks.

Magnified image of an American Dog Tick on a blade of grass

How it spreads

Primarily, through tick bites:

Map of the continental United States showing where in the U.S. the American Dog Tick is located. The entire eastern half of the country is highlighted.
Estimated geographic distribution of American dog ticks.
Map of the continental United States showing the approximate distribution of the Rocky mountain wood tick. The area effected is the Northwestern part of the country.
Estimated geographic distribution of Rocky Mountain wood ticks
Map of the continental United States highlighting in yellow where the Brown Dog Tick can be found.  The entire map is highlighted in yellow.
Estimated geographic distribution of brown dog ticks.

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

Epidemiology

RMSF is one of several diseases reported under the category “Spotted fever rickettsiosis (SFR).” SFR are reported in each of the lower 48 states. More than 60% of cases are reported in North Carolina, Oklahoma, Arkansas, Tennessee, and Missouri. For more information, see geography.

Although cases are reported in every month of the year, most cases occur during May–August. For more information, see seasonality.

Transmission of R. rickettsii by the brown dog tick (Rhipicephalus sanguineus) in Arizona and Northern Mexico results in distinct differences in epidemiology. This tick species is found throughout the world and is often found on or near domestic dogs. This includes kennels, yards, and 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.