Rocky Mountain Spotted Fever (RMSF) Differential Diagnoses

Updated: Mar 23, 2021
  • Author: Sunny Patel, DO; Chief Editor: Michael Stuart Bronze, MD  more...
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Diagnostic Considerations

The early phase of Rocky Mountain spotted fever (RMSF) is characterized by nonspecific symptoms that may mimic any number of other viral or bacterial infections. This delays proper therapy and contributes to higher mortality. [17]

Human ehrlichiosis caused by Ehrlichia canis mimics RMSF in its clinical manifestations and geographic and seasonal distribution. Rash is not present with ehrlichiosis, so spotless RMSF is usually ehrlichiosis. Doxycycline is the preferred drug for human ehrlichiosis.

If RMSF is treated with antibiotics aimed at another pathogen, the characteristic rash in RMSF may be mistaken for a drug eruption.

Conditions to consider in the differential diagnosis of RMSF include the following:

  • Meningitis

  • Meningococcemia

  • Q Fever

  • Rickettsialpox

  • Streptococcus group A infections

  • Streptococcus group B infections

  • Streptococcus group D infections

  • Syphilis

  • Thrombotic thrombocytopenic purpura

  • Toxic shock syndrome

  • Tularemia

  • Typhus

  • Bronchitis

  • Gastroenteritis

  • Hepatitis

  • Idiopathic thrombocytopenic purpura

  • Meningitis

  • Mononucleosis

  • Henoch-Schönlein purpura

  • Kawasaki disease

  • Measles

  • Rubella

  • Bacterial pneumonia

  • Immunocompromised pneumonia

  • Mycoplasma pneumonia

  • Viral pneumonia

  • Syphilis

  • Thrombocytopenic purpura

  • Ehrlichiosis

  • Lyme disease

  • Q fever

  • Relapsing fever

  • Tularemia

  • Toxic shock syndrome

  • Branch retinal artery occlusion

  • Branch retinal vein occlusion

  • Central retinal artery occlusion

  • Central retinal vein occlusion

  • Acute hemorrhagic conjunctivitis

  • Viral conjunctivitis

  • Eales disease

  • Diffuse unilateral subacute neuroretinitis

  • Ocular ischemic syndrome

  • Optic neuritis

  • Papilledema

  • Red eye evaluation

  • Anterior uveitis

  • 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

  • Influenza

  • Typhoid fever

  • Disseminated gonococcal infection

  • Drug hypersensitivity

  • Immune complex vasculitis

  • Staphylococcal sepsis

  • Recrudescent typhus

  • Luetic exanthem

Differential Diagnoses