Rickettsia rickettsii - Rocky Mountain Spotted Fever

Diagnosis

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

Background

Epidemiology

• Incidence – 3-5/million
• Age – peak incidence 5-9 years
• Transmission
  • Dermacentor (D. variabilis, D. andersoni) and Rhipicephalus sanguineus ticks in the U.S.
  • Humans are accidental hosts
  • 95% of cases occur April through September

Organism

Gram-negative obligate intracellular coccobacilli of the Rickettsiaceae family

Risk Factors

• Bite from an infected tick
• Residence in a wooded area
• Residence in or travel to Central, mid-Atlantic, and Southern U.S. states

Clinical Presentation

• Symptom onset – 2-14 days after tick bite (incubation period)
• Difficult to differentiate from viral illness
• Nonspecific signs and symptoms
  • Classic triad – fever, headache, and rash
  • Rash typically appears on second or third day of illness
    • Begins as small, pink macules on wrists, palms, ankles, and soles of feet – might include petechiae
    • Spreads to trunk
    • Usually follows systemic symptoms – absence should not rule out possible rickettsial etiology
• Other symptoms – malaise, myalgia, abdominal pain, vomiting, and photophobia
• Untreated disease can lead to major organ dysfunction
• Mortality – extremely rare
  • Dependent on cardiac and central nervous system involvement or delay in treatment

Pediatrics

Epidemiology

Fatality rate higher in children than adults, especially in children <10 years.

Clinical Presentation

• Nausea, vomiting, fever, rash, loss of appetite, abdominal pain
• May have swelling of hands