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Tick-Borne Diseases, Rocky Mountain Spotted Fever: Differential Diagnoses & Workup

Author: Allon Amitai, MD, International Emergency Medicine Fellow, Rhode Island Hospital; Consulting Staff, Memorial Hospital of Rhode Island; Doctoring Preceptor, Brown University Medical School
Coauthor(s): Richard Sinert, DO, Associate Professor of Emergency Medicine, Clinical Assistant Professor of Medicine, State University of New York College of Medicine; Consulting Staff, Department of Emergency Medicine, Kings County Hospital Center
Contributor Information and Disclosures

Updated: Jan 15, 2008

Differential Diagnoses

Bronchitis
Pneumonia, Mycoplasma
Gastroenteritis
Pneumonia, Viral
Hepatitis
Syphilis
Idiopathic Thrombocytopenic Purpura
Thrombocytopenic Purpura
Meningitis
Tick-Borne Diseases, Ehrlichiosis
Mononucleosis
Tick-Borne Diseases, Lyme
Pediatrics, Kawasaki Disease
Tick-Borne Diseases, Q Fever
Pediatrics, Measles
Tick-Borne Diseases, Relapsing Fever
Pediatrics, Rubella
Tick-Borne Diseases, Tularemia
Pneumonia, Bacterial
Toxic Shock Syndrome
Pneumonia, Immunocompromised

Other Problems to Be Considered

Influenza
Enterovirus infection
Typhoid fever
Bacterial sepsis
Meningococcemia
Disseminated gonococcal infection
Drug hypersensitivity
Immune complex vasculitis
Staphylococcal sepsis
Murine typhus
Rickettsialpox
Recrudescent typhus
Sylvatic flying squirrel-associated Rickettsia prowazekii infection

Workup

Laboratory Studies

  • Basic laboratory tests should be obtained, including the following: CBC count, electrolytes, renal function tests, liver function tests, and coagulation panel.
  • Hyponatremia, secondary to increased ADH secretion in response to hypovolemia, is observed in 56% of patients.
  • Thrombocytopenia is observed in 50% of patients.
  • Anemia, abnormal liver function test results, or increased BUN level is found in 30% of patients.
  • Blood culture
    • Isolation of R rickettsii from the blood is possible, but few laboratories perform this isolation because of biohazard concerns.
    • This is an insensitive test because most of the Rickettsia are found in the vascular endothelial cells, not in the bloodstream.

Imaging Studies

  • Obtain a chest radiograph in patients who appear significantly ill or have abnormal lung findings on physical examination.
  • Computed tomography or MRI are indicated for altered mental status or neurologic deficits, and may reveal infarction, edema, and meningeal enhancement. 

Other Tests

  • No microbiological or immunological tests are sensitive enough to exclude the diagnosis of RMSF within the time frame of emergency evaluation and management. The decision to administer antibiotics should be based on history, physical examination, and clinical suspicion, not confirmatory tests.
  • Serologic testing is frequently used as a confirmatory test. However, it is only useful after acute infection because serum antibodies become detectable during convalescence.
  • Indirect immunofluorescence has a sensitivity of 94-100% and a specificity of 100%.
    • According to the Centers for Disease Control and Prevention (CDC), titers of 1:64 are considered evidence of current or past infection. A 4-fold increase of titers at a 3-week interval demonstrates new infection.8
    • Indirect immunofluorescence is the most commonly used serologic test.
  • Indirect hemagglutination has a sensitivity of 71-94% and a specificity of 96-99%; 1:128 titers are diagnostic.
  • Latex agglutination can be used; titers of 1:64 are diagnostic.
  • Complement fixation is a much less sensitive test; titers of 1:16 are diagnostic.

Procedures

  • Direct immunofluorescence of cutaneous biopsy specimens is the only timely diagnostic method during the acute phase.
    • Results can be available in 3 hours.
    • It has a sensitivity of 70% and a specificity of 100%.
    • However, with a 30% false-negative rate patients should be treated even if the test is negative and the suspicion is high.
  • Lumbar puncture
    • Lumbar puncture usually is performed as part of the workup for suspected meningitis.
    • Pleocytosis is found in 34-38% of cases. Usually 10-100 cells/µL with a mononuclear cell predominance are found.
    • Increased protein is found in 30-35% of cases.
    • The glucose level usually is normal.

More on Tick-Borne Diseases, Rocky Mountain Spotted Fever

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

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  2. Chapman AS, Murphy SM, Demma LJ, Holman RC, Curns AT, McQuiston JH. Rocky mountain spotted fever in the United States, 1997-2002. Ann N Y Acad Sci. Oct 2006;1078:154-5. [Medline].

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Further Reading

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Keywords

RMSF, Rickettsia rickettsii, American dog tick, Dermacentor variabilis, D variabilis, Rocky Mountain wood tick, Dermacentor andersoni, D andersoni, myalgias, petechial rash, spotted fevers, encephalitis, confusion, lethargy, stupor, delirium, seizures, coma, Rocky Mountain spotless fever, jaundice, ataxia, cranialnerve palsies, hearing loss, meningismus, photophobia, severe vertigo, dysarthria, aphasia, hemiplegia, paraplegia, complete paralysis, nystagmus, hyperreflexia, spasticity, fasciculations, neurogenic bladder, pulmonary edema, heme-positive stools, Rhipicephalus sanguineus, Amblyomma cajennense

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Contributor Information and Disclosures

Author

Allon Amitai, MD, International Emergency Medicine Fellow, Rhode Island Hospital; Consulting Staff, Memorial Hospital of Rhode Island; Doctoring Preceptor, Brown University Medical School
Allon Amitai, MD is a member of the following medical societies: American College of Emergency Physicians
Disclosure: Nothing to disclose.

Coauthor(s)

Richard Sinert, DO, Associate Professor of Emergency Medicine, Clinical Assistant Professor of Medicine, State University of New York College of Medicine; Consulting Staff, Department of Emergency Medicine, Kings County Hospital Center
Richard Sinert, DO is a member of the following medical societies: American College of Physicians and Society for Academic Emergency Medicine
Disclosure: Nothing to disclose.

Medical Editor

Peter MC DeBlieux, MD, Professor of Clinical Medicine and Pediatrics, Section of Pulmonary and Critical Care Medicine, Program Director, Department of Emergency Medicine, Louisiana State University Health Sciences Center
Peter MC DeBlieux, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Emergency Medicine, American College of Emergency Physicians, American Medical Association, Radiological Society of North America, and Society of Critical Care Medicine
Disclosure: Nothing to disclose.

Pharmacy Editor

Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine
Disclosure: Nothing to disclose.

Managing Editor

Jon Mark Hirshon, MD, MPH, Associate Professor, Department of Emergency Medicine, University of Maryland School of Medicine
Jon Mark Hirshon, MD, MPH is a member of the following medical societies: Alpha Omega Alpha, American Academy of Emergency Medicine, American College of Emergency Physicians, American Public Health Association, and Society for Academic Emergency Medicine
Disclosure: Nothing to disclose.

CME Editor

John D Halamka, MD, MS, Associate Professor of Medicine, Harvard Medical School, Beth Israel Deaconess Medical Center; Chief Information Officer, CareGroup Healthcare System and Harvard Medical School; Attending Physician, Division of Emergency Medicine, Beth Israel Deaconess Medical Center
John D Halamka, MD, MS is a member of the following medical societies: American College of Emergency Physicians, American Medical Informatics Association, Phi Beta Kappa, and Society for Academic Emergency Medicine
Disclosure: Nothing to disclose.

Chief Editor

Jonathan Adler, MD, Attending Physician, Department of Emergency Medicine, Massachusetts General Hospital; Division of Emergency Medicine, Harvard Medical School
Jonathan Adler, MD is a member of the following medical societies: American Academy of Emergency Medicine and Society for Academic Emergency Medicine
Disclosure: eMedicine.com, Inc. Consulting fee Consulting

 
 
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