The rash of RMSF usually occurs between the third and forth day of fever, but may occur later. The rash often begins on the wrists and ankles, and then spreads centripetally to the legs, arms, face, and abdomen usually sparing the mucous membranes. Early lesions blanch with pressure, but later become petechial. Biopsy of the rash shows a perivascular infiltration with round cells. The diagnosis of RMSF requires a high level of suspicion.
Most patients have a history of exposure to ticks or tick infested areas.
Because survival is linked to early diagnosis, and rash may not be present in the early stages, it is important to consider RMSF before a rash is evident. The spinal fluid is usually normal, but an elevated spinal fluid protein and a few mononuclear cells may be present in comatose patients. The white cell count is normal or low, and thrombocytopenia and hyponatremia are common. Immunofluorescence staining of skin or rash biopsies can provide a rapid diagnosis. The indirect immunofluorescent antibody test is superior to the old Weil-Felix reactions, but serologies do not provide a rapid diagnosis.