Rocky Mountain spotted fever is a severe, acute, rickettsial disease transmitted by ticks and limited to the Western Hemisphere. Its major symptoms are similar to those of epidemic typhus, but its rash covers the entire body, including the face, the palms of the hands, and the soles of the feet. Between 20 and 25 percent of untreated victims die, making Rocky Mountain spotted fever the most severe rickettsial infection in the Americas. First identified in the Rocky Mountain region of the United States, this place name has never been dislodged, even though it is inaccurate and even misleading.

Etiology and Epidemiology

The severity with which Rocky Mountain spotted fever treats its victims underscores its natural existence as an infection of ticks and their mammalian hosts. The microbial cause of the disease, Rickettsia rickettsii, normally inhabits ixodid, or hard shell, ticks, apparently causing little harm to the host. Although small mammals are susceptible to a mild infection with R. rickettsii an may transmit it to uninfected ticks, the principal means by which the organism is maintained in nature is from one generation to the next in the eggs of the female tick.

The epidemiology of Rocky Mountain spotted fever is linked to areas favorable for the habitation of the vector ticks. The Rocky Mountain wood tick, Dermacentor andersoni, and the American dog tick, Dermacentor variabilis, are the most common vectors in the United States, although the Lone Star tick, Amblyomma americanum, also transmits the disease in the south central and southeastern parts of the United States.