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Key Features

Essentials of Diagnosis

  • Travel in endemic area; tick bite

  • Fever, chills, headache, retro-orbital pain, myalgia

  • Biphasic fever curve

  • Leukopenia early in the illness

General Considerations

  • Endemic in the high plains and mountains of the central and northern Rocky Mountains and northern Pacific coast of the United States

  • Reservoir of the virus consists of squirrels and chipmunks

  • Many hundreds of cases occur each year in visitors or laborers entering this region, primarily from May through July

Clinical Findings

Symptoms and Signs

  • After a 3- to 4-day incubation period (maximum, 14 days), fever begins suddenly together with chills, lethargy, headache, ocular pain, myalgia, abdominal pain, and nausea and vomiting

  • Conjunctivitis may be present

  • A nondistinctive maculopapular rash occurs in 5–10% of patients

  • Illness lasts 7–10 days

  • Half of patients have a biphasic fever curve with several afebrile days in the midst of the illness

Differential Diagnosis

  • Enterovirus, measles, or rubella infection may be suspected early in course, especially if rash is present

  • Leptospirosis

  • Borreliosis

  • Tularemia

  • Ehrlichiosis

  • Rocky Mountain spotted fever


Laboratory Findings

  • Leukopenia is characteristic early in the illness

  • Platelets are modestly decreased

  • Specific enzyme linked immunosorbent assay (ELISA) testing is available, but 2–3 weeks may elapse before seroconversion

  • Fluorescent antibody staining will detect virus-infected erythrocytes during the illness and for weeks after recovery

  • Reverse transcription polymerase chain reaction (RT-PCR) is positive within the first week of illness


  • Supportive

  • Do not use analgesics that modify platelet function



  • Meningoencephalitis occurs in 3–7% of patients

  • Cardiac and pulmonary complications are rare


  • Avoid endemic areas

  • Use conventional means to avoid tick bite


Meagher  KE, Decker  CF: Other tick-borne illnesses: tularemia, Colorado tick fever, tick paralysis. Dis Mon 2012;58(6):370–376
[PubMed: 22608124]

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