Rocky Mountain spotted fever is the most common rickettsial illness in the United States, occurring primarily in the eastern coastal, southeastern, and western states. Most cases occur from May to October after outdoor activity in wooded areas, with peak incidence among children 1 to 14 years of age. Approximately 40% of infected persons are unable to recall a tick bite.
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The incubation period of Rocky Mountain spotted fever is 2 to 14 days, with an average of 7 days. The onset is nonspecific with headache, malaise, and fever. A pale, rose-red macular or maculopapular rash appears in 90% of cases. The rash begins peripherally and spreads to involve the entire body, including palms and soles. The early rash blanches on pressure and is accentuated by warmth. It progresses over hours or days to a petechial and purpuric eruption that appears first on the feet and ankles, then the wrists and hands, and progresses centripetally to the trunk and head. Myalgias, especially of the lower extremities, and intractable headaches are common. Severe cases progress with splenomegaly, myocarditis, renal impairment, pneumonitis, and shock.
Thrombocytopenia (usually <100,000 cells/mm3), anemia, hyponatremia, and elevated hepatic transaminase levels are common laboratory findings. Organisms can be detected in skin biopsy specimens by fluorescent antibodies or polymerase chain reaction, although this test is not widely available. Serologic testing is used to confirm the diagnosis, although treatment is begun before confirmation.
The differential diagnosis includes meningococcemia, bacterial sepsis, toxic shock syndrome, leptospirosis, ehrlichiosis, measles, enteroviruses, infectious mononucleosis, collagen vascular diseases, Henoch-Schönlein purpura, and idiopathic thrombocytopenic purpura. The diagnosis of Rocky Mountain spotted fever should be suspected with fever and petechial rash, especially with a history of a tick bite or outdoor activities during spring and summer in endemic regions. Fever, headache, and myalgias lasting more than 1 week in patients in endemic areas indicate Rocky Mountain spotted fever. Delayed diagnosis and late treatment usually result from atypical initial symptoms and late appearance of the rash.
Therapy for suspected Rocky Mountain spotted fever should not be postponed pending results of diagnostic tests. Doxycycline is the drug of choice, even for young children, despite the theoretical risk of dental staining in children younger than 9 years of age. Fluoroquinolones also are active against R. rickettsii and may be an alternative treatment.
In severe infections, capillary leakage results in noncardiogenic pulmonary edema (acute respiratory distress syndrome), hypotension, disseminated intravascular coagulation, circulatory collapse, and multiple organ failure, including encephalitis, myocarditis, hepatitis, and renal failure.
Untreated illness may persist for 3 weeks before progressing to multisystem involvement. The mortality rate is 25% without treatment, which is reduced to 3.4% with treatment. Permanent sequelae are common after severe disease.
Preventive measures to avoid tick-borne infections and careful removal of ticks are recommended.
The term ehrlichiosis often is used to refer to all forms of infection with Ehrlichia. Human monocytic ehrlichiosis is caused by
E. chaffeensis, which infects predominantly monocytic cells andis transmitted by the tick A. americanum. Disease may also be caused by Ehrlichia ewingii. Human anaplasmosis is caused by
A. phagocytophilum and is transmitted by the tick I. scapularis.