Rocky Mountain Spotted Fever

Characteristic rash of late-stage Rocky Mountain spotted fever on legs of a patient, ca. 1946 (photo provided courtesy of Rocky Mountain Laboratories, NIAID, NIH, Hamilton, Montana)

Rocky Mountain spotted fever is the most severe and most frequently reported rickettsial illness in the United States. It also occurs in Mexico and in Central and South America. The disease is caused by Rickettsia rickettsii, a species of bacteria that is spread to humans by ixodid (hard) ticks. Initial signs and symptoms of the disease include sudden onset of fever, headache, and muscle pain, followed by development of rash. The disease can be difficult to diagnose in the early stages, and without prompt and appropriate treatment it can be fatal.