Rickettsia rickettsii - Rocky Mountain Spotted Fever


Indications for Testing

  • Flu-like illness with or without rash in association with tick bite exposure and proper epidemiologic setting

Laboratory Testing

  • Diagnosis and laboratory confirmation recommendations (CDC)
  • Diagnosis is made based upon clinical grounds with history of tick exposure
    • Definitive diagnosis made only retrospectively using serology
  • Serology
    • The best evidence for infection is a significant change in 2 appropriately timed specimens where both tests are done in the same laboratory at the same time
      • Appearance of an IgM antibody response by IFA or ELISA normally occurs 7-14 days after the onset of disease
        • Not usually detectable by day 5 (when fatality starts to rise dramatically)
      • Negative initial IgM does not exclude the disease
      • Cannot distinguish between cross-reacting Rickettsiae
    • Consider concurrent testing for similar illnesses – human monotropic ehrlichiosis, Neisseria species, Borrelia burgdorferi
  • Febrile antibody testing
    • More specific than Weil-Felix but still has cross-reactivity with Brucella and Salmonella
    • Must be used in conjunction with clinical presentation
  • Weil-Felix – lacks sensitivity and specificity; do not use
  • Blood culture – research labs only
    • Highly sensitive and specific


  • 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

Clinical Background

Rickettsia rickettsii is a tick-borne illness (zoonosis) and the etiologic agent of Rocky Mountain Spotted Fever (RMSF).


  • Incidence – 3-5/1,000,000
  • Age – peak incidence 5-9 years
  • Transmission
    • Via Dermacentor (variabilis, andersoni), Amblyomma, and Rhipicephalus sanguineus ticks in the U.S.
      • Geographical distribution is restricted to the western hemisphere
    • Humans are accidental hosts
    • 95% of the cases occur April through September


  • Gram-negative coccobacilli of the Rickettsiaceae family – obligate intracellular organisms
  • Characteristic feature of the Rickettsiae – life cycle requires multiplying in an arthropod
  • Invertebrate hosts are both reservoirs and vectors
  • Rickettsia are part of a family of organisms responsible for the following rickettsial diseases
    • Spotted fever and typhus (vector: tick, louse, flea, or gamasid mite)
    • Scrub typhus (vector: chigger)
    • Ehrlichiosis (vector: tick)
    • Neorickettsiosis
    • Q-fever

Risk Factors

  • Dog exposure to ticks
  • Residence in a wooded area
  • Residence in Central and Mid-Atlantic states
  • Male sex

Clinical Presentation

  • The incubation period between tick bite and onset of symptoms is 2-14 days
  • RMSF is difficult to differentiate from viral illness
  • Nonspecific signs and symptoms
    • Classic triad – fever, headache, and rash
    • Rash typically appears on the second or third day of illness
      • Rash begins as macules on the wrists, palms, ankles, and soles of feet, which then results in petechiae form
      • Rash finally spreads to the trunk
      • Rash is hallmark of infection but usually follows systemic symptoms; its absence should not rule out a possible rickettsial etiology
  • Other symptoms include malaise, myalgias, vomiting, and photophobia
    • Mild pulmonary involvement, manifested by cough and infiltrates, is found in about one-third of patients with RMSF
  • Neurologic (25% of patients)
  • Cardiovascular
    • Myocarditis
    • Pericarditis
  • Mortality is dependent on cardiac and central nervous system involvement or delay in treatment


  • Initiation of early antibiotic therapy is necessary to reduce mortality
    • Decision to treat should not be delayed until lab confirmation of organism

Indications for Laboratory Testing

  • Tests generally appear in the order most useful for common clinical situations
  • Click on number for test-specific information in the ARUP Laboratory Test Directory
Test Name and Number Recommended Use Limitations Follow Up
Rickettsia rickettsii (Rocky Mountain Spotted Fever) Antibodies, IgG & IgM by IFA 0050371
Method: Semi-Quantitative Indirect Fluorescent Antibody

Confirm infection with Rickettsia rickettsii (Rocky Mountain Spotted Fever)

Acute and convalescent titers often necessary

Low-positive results suggest past exposure or infection while high-positive results may indicate recent or past infection but are inconclusive for diagnosis

Initial testing may not be helpful; determine treatment from clinical and other laboratory assessments

Any antibody reactivity to R. rickettsii should also be considered group reactive for the spotted fever group (R. conorii, R. honei, R. akari, R. japonica, R. australis, and R. sibirica)

Febrile Antibodies Identification Panel 2010805
Method: Semi-Quantitative Agglutination/Semi-Quantitative Indirect Fluorescent Antibody/Qualitative Immunoblot

Aids in confirming presence of Rocky Mountain Spotted Fever; not recommended for initial testing

Includes Brucella Antibody (Total) by Agglutination; Rickettsia rickettsii Antibody, IgM; Rickettsia rickettsii antibody, IgG; Rickettsia typhi Antibody, IgG by IFA; Rickettsia typhi Antibody, IgM by IFA, and Salmonella typhi and paratyphi Antibodies

Initial testing may not be helpful; determine treatment from clinical and other laboratory assessments

Additional Tests Available
Click the plus sign to expand the table of additional tests.
Test Name and NumberComments
Rickettsia rickettsii (Rocky Mountain Spotted Fever) Antibody, IgG 0050369
Method: Semi-Quantitative Indirect Fluorescent Antibody
Rickettsia rickettsii (Rocky Mountain Spotted Fever) Antibody, IgM 0050372
Method: Semi-Quantitative Indirect Fluorescent Antibody