Rickettsia rickettsii and Other Spotted Fever Group Rickettsiae (Rocky Mountain Spotted Fever and Other Spotted Fevers)

Spotted fever group (SFG) rickettsiae are small, obligately intracellular gram-negative bacteria that cause tick-, mite-, and flea-borne infections. Rickettsia rickettsii causes Rocky Mountain spotted fever (RMSF). In the United States, it is transmitted by Dermacentor variabilis, Dermacentor andersoni, and Rhipicephalus sanguineus ticks in the eastern two-thirds, western, and southwestern United States, respectively. The disease is also endemic to Central and South America. Other SFG rickettsiae cause human illness with a broad geographic distribution. RMSF manifests with fever, headache, and myalgias. Nausea, vomiting, and abdominal pain may also occur. Rash is common but may not be present in the first days of illness. Rash typically starts on the wrists and ankles before appearing proximally. Involvement of the palms and soles occurs in 36% to 82% of cases. Skin necrosis, gangrenous digits, neurologic complications, azotemia, pulmonary edema, and acute respiratory distress syndrome are manifestations of severe infection. The untreated case-fatality rate of RMSF is 23% and up to 4% despite appropriate antimicrobials. Other SFG rickettsioses are generally less severe than RMSF and often have an associated inoculation eschar. The indirect immunofluorescence assay is the serologic method of choice. Seroconversion or a fourfold rise in immunoglobulin G from acute illness to convalescence confirms the diagnosis. Immunohistochemical detection of SFG rickettsiae or detection of rickettsial nucleic acid—both from skin or eschar biopsy specimens—can establish the diagnosis during acute infection. Doxycycline, 100 mg twice daily for 7 to 10 days, is the treatment of choice for all SFG rickettsioses.