Rocky Mountain Spotted Fever

Daniel J. Sexton, M.D., F.A.C.P.
Duke University School of Medicine

Definition/Key Clinical Features
Differential Diagnosis
Best Tests
Best Therapy
Best References

Definition/ Key Clinical Features

• An acute illness caused by Rickettsia rickettsii and transmitted by tick bite or through contact with infected tick tissues
• Abrupt onset
• Incubation period after exposure: 2–12 days
• Nonspecific symptoms of early phase include fever, headache, malaise and myalgias, and nausea and vomiting
• Prominent abdominal pain, especially in children
• Jaundice due to hepatic involvement may be present in severe cases
• Macular or maculopapular skin eruption that develops 3–5 days after exposure and evolves into a generalized petechial rash; rash may be delayed or absent in some patients
• Generalized ecchymosis or gangrene of the digits, genitals, ears, and nose may rarely occur in severe cases
• Neurologic symptoms, including confusion, seizures, and encephalopathy
• Uncommonly, periorbital edema, especially in children, and cough

Differential Diagnosis

• Bacterial infection
• Measles
• Staphylococcal bacteremia
• Hepatitis
• Leptospirosis
• Meningococcemia
• Infectious mononucleosis
• Ehrlichiosis (distinguished from Rocky Mountain spotted fever [RMSF] by presence of severe leukopenia and absence of rash)
• Drug eruption (especially if antibiotics such as ß-lactams are used as empirical therapy before the appearance of a typical skin rash)

Best Tests

Clinical Features

• Diagnosis must be based on clinical features and epidemiologic setting; there is no completely reliable diagnostic test for RMSF in the early phases of illness; therapy must begin before laboratory confirmation is obtained

Laboratory Studies

• Complete blood count
• White blood cell count is usually normal
• Thrombocytopenia occurs in most cases but may be absent early in illness
• Aminotransferases, bilirubin, and creatinine measurement: blood levels will be elevated in most patients, particularly those with severe illness
• Serologic testing: no role in the initial diagnosis of acutely ill patients with suspected RMSF
• Diagnostic proof of RMSF can be obtained by direct immunofluorescent or immunoenzyme staining of skin biopsy samples or, in the convalescent phase, by detection of characteristic antibodies
• Antibodies do not appear before eighth to 10th day of illness
• Blood culture requires specialized laboratory facilities available only in a few centers
• Skin biopsy may reveal R. rickettsii and typical mononuclear perivascular infiltrate if special fluorescent conjugate or immunoperoxidase stains are used
• Polymerase chain reaction diagnosis is not sensitive or widely available
• Lumbar puncture in patients with neurologic symptoms often shows a lymphocytic pleocytosis in the cerebrospinal fluid