Rocky Mountain Spotted Fever

Rocky Mountain spotted fever (RMSF) is the most severe and most often reported rickettsial illness in the United States. It is caused by Rickettsia rickettsii, a species of bacteria that is spread to humans by ixodid (hard) ticks (Figure 16-6).

Initial signs and symptoms include sudden onset of fever, headache, and muscle pain, followed by development of rash. The disease can be difficult to diagnose in the early stage. RMSF is most common among males and children. Risk factors are frequent exposure to dogs and living near wooded areas or areas with high grass. The presentation of RSMF is nonspecific, following an incubation of about 5 to 10 days after a tick bite. Initial symptoms can include fever, nausea, vomiting, severe headache, muscle pain, and lack of appetite. Later signs and symptoms include rash, abdominal pain, joint pain, and diarrhea. The rash first appears 2 to 5 days after the onset of fever. Most often it begins as small, flat, pink, nonitchy spots on the wrists, forearms, and ankles. The characteristic red spotted rash of RMSF is usually not seen until the sixth day or later after onset of symptoms. As many as 10% to 15% of patients never develop a rash (Figure 16-7). No widely available laboratory assay provides rapid confirmation of early RMSF, although commercial PCR testing is available. Therefore, treatment decisions should be based on epidemiologic and clinical clues. Treatment should never be delayed while waiting for confirmation by laboratory results. Routine clinical laboratory findings suggestive of RMSF include normal WBC count, thrombocytopenia, hyponatre-mia, and elevated liver enzyme levels. Serologic assays are the most often used methods for confirming cases of RMSF.

Doxycycline is the drug of choice for patients with RMSF. Therapy is continued for at least 3 days after fever subsides and until there is unequivocal evidence of clinical improvement, generally for a minimum total course of 5 to 10 days. Tetracyclines are usually not the preferred drug for use in pregnant women. Whereas chloramphenicol is typically the preferred treatment for RMSF during pregnancy, care must be used when administering chloramphenicol late during the third trimester of pregnancy because of risks associated with gray baby syndrome.

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