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Rocky Mountain Spotted Fever: Differential Diagnoses & Workup

Author: Burke A Cunha, MD, Professor of Medicine, State University of New York School of Medicine at Stony Brook; Chief, Infectious Disease Division, Winthrop-University Hospital
Coauthor(s): Richard H Snyder, MD, Vice-Chair, Program Director, Department of Medicine, Norfolk General Hospital; Clinical Associate Professor, Department of Internal Medicine, East Virginia Medical School; Marie Spevak O'Brien, DO, Assistant Clinical Professor of Medicine, Arthritis and Rheumatology, Lehigh Valley Physician Group
Contributor Information and Disclosures

Updated: Oct 19, 2009

Differential Diagnoses

Babesiosis
Q Fever
Dengue Fever
Rickettsialpox
Ehrlichiosis
Streptococcus Group A Infections
Enteroviruses
Streptococcus Group B Infections
Hepatitis, Viral
Streptococcus Group D Infections
Infectious Mononucleosis
Syphilis
Leptospirosis
Thrombotic Thrombocytopenic Purpura
Lyme Disease
Toxic Shock Syndrome
Malaria
Tularemia
Mediterranean Spotted Fever
Typhus
Meningitis
Meningococcemia

Other Problems to Be Considered

Acute surgical abdomen
Allergic vasculitis
Brill-Zinsser disease
Drug hypersensitivity
Atypical measles
Murine typhus
Rubeola
Undifferentiated viral illness
Drug reactions
Other acute rickettsial infections (eg, Mediterranean spotted fever, North Asian tick typhus, Siberian tick typhus, Astrakhan fever, African tick bite fever, Japanese spotted fever, Queensland tick typhus, Flinders Island spotted fever, Israeli tick typhus, Marseilles tick bite fever)

Workup

Laboratory Studies

  • Rocky Mountain spotted fever (RMSF) diagnosis relies on clinical (fever, rash, myalgia) and epidemiologic (tick exposure) criteria.
  • After exposure to vector ticks, patients who develop fever, petechial rash, and vomiting require antibiotic therapy. Antibiotic therapy should be initiated before laboratory confirmation is available.
  • A clinical diagnosis of RMSF is difficult to establish, and laboratory findings are nonspecific. Immunologic methods for detection of rickettsiae are unavailable in most clinics.
  • White blood cell count  
    • Leukopenia is present initially, then mild leukocytosis.
    • Patients usually have a normal WBC count.
  • Platelets  
    • Thrombocytopenia (<150,000 cells/µL) occurs in 32-52% of patients.
    • Abnormalities indicative of DIC are present in severely ill patients.
  • Hemoglobin and hematocrit: Anemia is present in 5-24% of patients.
  • Serology
    • Diagnosis is confirmed based on indirect immunofluorescent antibody (IFA) test results, latex agglutination, or enzyme immunoassay.
    • Serology specific for R rickettsii infection develops within 6-8 weeks.
    • Serological test results are negative prior to until convalescence.
  • Other laboratory findings  
    • Mildly elevated aminotransferase levels are present in 36-62% of patients.
    • Hyponatremia is present in 19-56% of cases.
    • Increased bilirubin levels are present in 8-9% of patients.
    • Mild cerebrospinal fluid pleocytosis with monocyte predominance may be present.
    • Azotemia develops in 12-14% of cases.
    • Elevated prothrombin time and activated partial thromboplastin time may be present.

Imaging Studies

  • Chest radiographs that show an early pulmonary infiltrate should prompt consideration of a different diagnosis.

Other Tests

  • Electrocardiography may indicate myocardial or conduction abnormalities.
  • The Weil-Felix test is used to detect cross-reacting antibodies against Proteus vulgaris antigens OX-2 and OX-19.
    • This test lacks sensitivity and specificity, and better tests are now available.
    • If the Proteus titer is greater than or equal to 1:320 or if a 4-fold or greater rise to either Proteus OX-19 or OX-2 antigens is observed, an RMSF case that is clinically compatible is considered probable.

Procedures

  • Skin biopsy
    • Direct immunofluorescent microscopy, if available, may be used for rapid histologic diagnosis of RMSF.
    • Immunofluorescent or immunoperoxidase staining of R rickettsii in a biopsy skin or organ specimen is both sensitive (73%) and specific (100%).5
    • Antibodies to specific rickettsial antigens are detected by indirect immunofluorescence (most specific), latex agglutination, and enzyme immunoassay. The diagnostic titer is 1:64 for indirect immunofluorescence and latex agglutination.
    • Polymerase chain reaction amplification of R rickettsii DNA has not been proven to be a sensitive diagnostic method except for later in the disease course, particularly fatal cases. It has been successful when applied to biopsy skin samples during rickettsioses and to ticks. According to Walker and Raoult in 2000, the primers used amplify genes of the 17-kD protein citrate synthetase and rickettsial OmpA and allow the identification of any rickettsial organism.
  • Lumbar puncture is not routinely indicated but is important to rule out meningitis in some patients with RMSF who have CNS findings.

More on Rocky Mountain Spotted Fever

Overview: Rocky Mountain Spotted Fever
Differential Diagnoses & Workup: Rocky Mountain Spotted Fever
Treatment & Medication: Rocky Mountain Spotted Fever
Follow-up: Rocky Mountain Spotted Fever
References
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References

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Further Reading

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Keywords

Rocky Mountain spotted fever, RMSF, tick fever, spotted fever, tick typhus, New World spotted fever, Rickettsia rickettsii, R rickettsii, Sao Paulo fever, fiebre manchada, fiebre petechial, fiebre maculosa brasiliensis, dog tick, wood tick, the great imitator

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Contributor Information and Disclosures

Author

Burke A Cunha, MD, Professor of Medicine, State University of New York School of Medicine at Stony Brook; Chief, Infectious Disease Division, Winthrop-University Hospital
Burke A Cunha, MD is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, and Infectious Diseases Society of America
Disclosure: Nothing to disclose.

Coauthor(s)

Richard H Snyder, MD, Vice-Chair, Program Director, Department of Medicine, Norfolk General Hospital; Clinical Associate Professor, Department of Internal Medicine, East Virginia Medical School
Richard H Snyder, MD is a member of the following medical societies: American College of Physicians
Disclosure: Nothing to disclose.

Marie Spevak O'Brien, DO, Assistant Clinical Professor of Medicine, Arthritis and Rheumatology, Lehigh Valley Physician Group
Marie Spevak O'Brien, DO is a member of the following medical societies: American College of Physicians, American College of Rheumatology, American Medical Association, American Osteopathic Association, International Society for Clinical Densitometry, and Pennsylvania Medical Society
Disclosure: Nothing to disclose.

Medical Editor

Gary L Gorby, MD, Program Director of Adult Infectious Diseases Fellowship, Associate Professor, Department of Internal Medicine, Division of Infectious Disease, St Joseph Medical Center, Creighton University School of Medicine
Gary L Gorby, MD is a member of the following medical societies: Alpha Omega Alpha, American Medical Association, American Society for Microbiology, Infectious Diseases Society of America, and New York Academy of Sciences
Disclosure: Nothing to disclose.

Pharmacy Editor

Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine
Disclosure: eMedicine Salary Employment

Managing Editor

Aaron Glatt, MD, Professor of Clinical Medicine, New York Medical College; President and CEO, Former Chief Medical Officer, Departments of Medicine and Infectious Diseases, New Island Hospital
Aaron Glatt, MD is a member of the following medical societies: American College of Chest Physicians, American College of Physician Executives, American College of Physicians, American College of Physicians-American Society of Internal Medicine, American Medical Association, American Society for Microbiology, American Thoracic Society, American Venereal Disease Association, Infectious Diseases Society of America, International AIDS Society, and Society for Healthcare Epidemiology of America
Disclosure: Nothing to disclose.

CME Editor

Eleftherios Mylonakis, MD, Clinical and Research Fellow, Department of Internal Medicine, Division of Infectious Diseases, Massachusetts General Hospital
Eleftherios Mylonakis, MD is a member of the following medical societies: American Association for the Advancement of Science, American College of Physicians, American Society for Microbiology, and Infectious Diseases Society of America
Disclosure: Nothing to disclose.

Chief Editor

Michael Stuart Bronze, MD, Professor, Stewart G Wolf Chair in Internal Medicine, Department of Medicine, University of Oklahoma Health Science Center
Michael Stuart Bronze, MD is a member of the following medical societies: Alpha Omega Alpha, American College of Physician Executives, American College of Physicians, American College of Physicians-American Society of Internal Medicine, American Federation for Clinical Research, American Medical Association, American Society for Microbiology, Association of Professors of Medicine, Association of Program Directors in Internal Medicine, Infectious Diseases Society of America, Oklahoma State Medical Association, and Southern Society for Clinical Investigation
Disclosure: Nothing to disclose.

 
 
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