Rickettsia rickettsii - Rocky Mountain Spotted Fever

Indications for Testing

• Tick bite exposure and relevant epidemiologic setting
• Fever, headache, myalgia, nausea, vomiting; rash 2-5 days after fever

Laboratory Testing

• Diagnosis and laboratory confirmation recommendations (CDC)
• ​Antibiotic therapy should be initiated in the event of clinical suspicion, before testing, and not discontinued even if results are negative
• Diagnosis
  • IgG and IgM testing
    • Significant increase in immunoglobulins in samples taken during both acute and convalescent stages using indirect fluorescent antibody (IFA) testing confirms diagnosis (retrospective confirmation; should not be used to make treatment decisions)
    • IgM antibody does not usually appear until 7-14 days after disease onset and is less specific than IgG
    • Cannot distinguish between cross-reacting antibodies generated against other spotted fever group Rickettsia spp
  • Other testing
    • CBC may show thrombocytopenia
    • Sodium levels may be decreased in more severe disease
    • Alanine​ aminotransferase (ALT) and aspartate aminotransferase (AST) may be slightly elevated

Histology

• Immunohistochemistry – direct immunofluorescence or immunoperoxidase tests on skin biopsies
  • Sensitivity ~70%; specificity 100%
  • Testing not widely available
  • Negative result does not rule out disease

Differential Diagnosis

• Rash
  • Consider patient’s locale and recent travel, as some diagnoses can be excluded on geographic basis