Rocky Mountain spotted fever (RMSF) is caused by the gram-negative bacterium Rickettsia rickettsii. It is the most severe rickettsial illness of humans and is difficult to diagnose due to its nonspecific findings. Without treatment, the case fatality rate is 20%-30% with a median time to death of 8 days. Otherwise healthy adults and children have been known to die from the disease.

The disease is spread by ticks (the American dog and Rocky Mountain ticks) via a tick bite or by crushing a tick and transmitting its fecal matter via a mucosal surface (eg, rubbing the eyes). RMSF occurs over a wide distribution throughout the contiguous United States, but cases are most commonly reported from Arkansas, Missouri, North Carolina, Oklahoma, and Tennessee. Over 90% of cases occur during April through September. The disease is more frequent in males; age-specific incidence is highest in children. Overall, the incidence of RMSF has been increasing from 300-800 to 2000 cases per year, but the case fatality rates (at least in the United States) have been decreasing (attributed to enhanced recognition and early treatment).

Early clinical manifestations of RMSF include high fever, severe headache, myalgia, and nausea and vomiting. Later manifestations include rash, photophobia, confusion, ataxia, seizures, cough, dyspnea, arrhythmias, jaundice, and severe abdominal pain. Thrombocytopenia or hyponatremia may be seen. A serious complication of RMSF is rhabdomyolysis. Long-term sequelae include central nervous system deficits and amputations.