Rickettsia rickettsii is a tick-borne illness (zoonosis) among the spotted fever group rickettsioses and is the etiologic agent of Rocky Mountain Spotted Fever (RMSF).
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
- IgG and IgM testing
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
ARUP Laboratory Tests
Preferred test for acute or convalescent phase of disease
Acute and convalescent titers often necessary
Low-positive results suggest past exposure or infection, whereas high-positive results may indicate recent or past infection but are inconclusive for diagnosis
Semi-Quantitative Indirect Fluorescent Antibody
Detect antibodies during convalescent phase
Semi-Quantitative Indirect Fluorescent Antibody
Detect antibodies during acute phase; paired concurrent specimen with IgG reduces false-positive rate
Convalescent sera may be required for diagnosis
Semi-Quantitative Indirect Fluorescent Antibody
Medical Experts
Couturier

References
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Biggs HM, Behravesh CBarton, Bradley KK, et al. Diagnosis and Management of Tickborne Rickettsial Diseases: Rocky Mountain Spotted Fever and Other Spotted Fever Group Rickettsioses, Ehrlichioses, and Anaplasmosis - United States. MMWR Recomm Rep. 2016;65(2):1–44.
17236897
CDC - Rocky Mountain Spotted Fever (RMSF)
U.S. Department of Health and Human Services, Centers for Disease Control and Prevention. Rocky Mountain spotted fever (RMSF). [Last reviewed: May 2019; Accessed: Feb 2020]
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Chen LF, Sexton DJ. What's new in Rocky Mountain spotted fever? Infect Dis Clin North Am. 2008;22(3):415-viii.