The incubation period for RMSF is about seven days, with a range of 2 to 14 days after the tick bite. Patients usually present with an abrupt onset of severe headache, fever, chills, weakness, and myalgias. Later, a nonproductive cough may develop. Gastrointestinal manifestations are very common (eg, nausea, vomiting, loss of appetite). Meningitis and meningoencephalitis may seldom occur.
The rash appears two to four days after the onset of symptoms. Most patients seek care before the rash develops, so a strong suspicion of RMSF is required. Treatment with doxycycline should be started before a definitive diagnosis is made. The rash begins as small, blanching macules on the wrists, ankles, or forearms. It evolves to maculopapules and spreads to the entire body, including the palms and soles. 50% of rashes become petechial and hemorrhagic, which is associated with more severe disease. 20% of rashes may be atypical. Nonspecific laboratory findings include:
- Normal WBC count with a left shift
- Decreased platelets
- Slightly elevated transaminases and other liver enzymes
- CSF examination may demonstrate neutrophils or lymphocytes, an elevated protein, and normal glucose
75% of infected patients are hospitalized and have peak temperatures greater than 104° F (40° C) and almost 50% have peak temperatures greater than 40.5° C. 25% develop delirium, ataxia, stupor, or coma. Death is uncommon, but when it occurs, it is 1-2 weeks after the onset of symptoms and in the 4-8% of cases not treated with an effective antimicrobial, such as doxycycline.