Rickettsia rickettsii - Rocky Mountain Spotted Fever

  • Diagnosis
  • Background
  • Lab Tests
  • References
  • Related Topics

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
    • Thrombocytopenia and hyponatremia may occur
  • 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

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 small, pink macules on the wrists, palms, ankles, and soles of feet
        • Might include petechiae
      • Rash finally spreads to the trunk
      • Rash 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
  • Neurologic
  • Cardiovascular
    • Myocarditis
    • Pericarditis
  • Mortality is dependent on cardiac and central nervous system involvement or delay in treatment
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.

Rickettsia rickettsii (Rocky Mountain Spotted Fever) Antibodies, IgG & IgM by IFA 0050371
Method: Semi-Quantitative Indirect Fluorescent Antibody


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


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


Chapman AS, Bakken JS, Folk SM, Paddock CD, Bloch KC, Krusell A, Sexton DJ, Buckingham SC, Marshall GS, Storch GA, Dasch GA, McQuiston JH, Swerdlow DL, Dumler SJ, Nicholson WL, Walker DH, Eremeeva ME, Ohl CA, Tickborne Rickettsial Diseases Working Group, CDC. Diagnosis and management of tickborne rickettsial diseases: Rocky Mountain spotted fever, ehrlichioses, and anaplasmosis--United States: a practical guide for physicians and other health-care and public health professionals. MMWR Recomm Rep. 2006; 55(RR-4): 1-27. PubMed

Huntzinger A. Guidelines for the Diagnosis and Treatment of Tick-Borne Rickettsial Diseases. American Cancer Society. Leawood, KS [Accessed: Jun 2015]

General References

Buckingham SC, Marshall GS, Schutze GE, Woods CR, Jackson MA, Patterson LE, Jacobs RF, Tick-borne Infections in Children Study Group. Clinical and laboratory features, hospital course, and outcome of Rocky Mountain spotted fever in children. J Pediatr. 2007; 150(2): 180-4, 184.e1. PubMed

Chen LF, Sexton DJ. What's new in Rocky Mountain spotted fever? Infect Dis Clin North Am. 2008; 22(3): 415-32, vii-viii. PubMed

Dantas-Torres F. Rocky Mountain spotted fever. Lancet Infect Dis. 2007; 7(11): 724-32. PubMed

Delord M, Socolovschi C, Parola P. Rickettsioses and Q fever in travelers (2004-2013). Travel Med Infect Dis. 2014; 12(5): 443-58. PubMed

Minniear TD, Buckingham SC. Managing Rocky Mountain spotted fever. Expert Rev Anti Infect Ther. 2009; 7(9): 1131-7. PubMed

Walker DH, Paddock CD, Dumler S. Emerging and re-emerging tick-transmitted rickettsial and ehrlichial infections. Med Clin North Am. 2008; 92(6): 1345-61, x. PubMed

Medical Reviewers

Last Update: August 2016