Rocky Mountain spotted fever.

Rickettsia rickettsii, a bacterial organism spread to humans by the bite of ixodid (hard) ticks, is the etiologic agent of RMSF. The two major vectors of RMSF in the U.S. are the American dog tick, Dermacentor variabilis and the Rocky Mountain wood tick, Dermacentor andersoni. Other domestic tick species have been shown to be infected with Rickettsia rickettsia, or have been identified as experimental vectors in laboratory studies. Some domestic ticks have no role in transmission in the U.S. but are considered important vectors in Central and South America. Although the vector of RMSF is the tick, exposure to ticks or tick-infested habitats is only reported in 60% of the cases.