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For further information, see CMDT Part 32-05: Other Systemic Viral Diseases

Key Features

Essentials of Diagnosis

  • Onset 1–19 days (average, 4 days) following tick bite

  • Fever, chills, myalgia, headache, prostration

  • Leukopenia, thrombocytopenia

  • Second attack of fever after remission lasting 2–3 days

General Considerations

  • Transmitted by Dermacentorandersoni tick bite

  • Disease is limited to the western United States and Canada and is most prevalent during the tick season (March to November)

  • Virus infects the marrow erythrocyte precursors, leading to viremia lasting the life span of the infected red cells

  • Blood transfusions can be a vehicle of transmission

Clinical Findings

Symptoms and Signs

  • Incubation period is 3–6 days, rarely as long as 19 days

  • Onset is usually abrupt with high fever, sometimes with chills

  • Severe myalgia, headache, photophobia, anorexia, nausea and vomiting, and generalized weakness are prominent

  • Faint rash (occasionally)

  • Acute symptoms resolve within a week

  • Remission is followed in 50% of cases by recurrent fever and a full recrudescence lasting 2–4 days

Differential Diagnosis

  • Influenza

  • Rocky Mountain spotted fever

  • Numerous other viral infections

  • Relapsing fevers


Laboratory Findings

  • Leukopenia with a shift to the left and atypical lymphocytes occurs, reaching a nadir 5–6 days after the onset of illness

  • Thrombocytopenia may occur

  • An RT-PCR assay may be used to detect early viremia

  • Detection of IgM by capture ELISA or plaque reduction neutralization is possible after 2 weeks from symptom onset


  • No specific treatment is available

  • Ribavirin has shown efficacy in an animal model

  • Antipyretics are used

  • Salicylates should be avoided due to potential bleeding



  • Tick avoidance

  • Tick control, particularly from March to November


  • Aseptic meningitis (particularly in children), encephalitis, and hemorrhagic fever occur rarely

  • Malaise may last weeks to months

  • Spontaneous abortion and multiple congenital anomalies associated with Colorado tick fever infection during pregnancy (rarely)


  • Disease is usually self-limited and benign


Kadkhoda  K  et al. Case report: a case of Colorado tick fever acquired in southwestern Saskatchewan. Am J Trop Med Hyg. 2018 Mar;98(3):891–3.
[PubMed: 29363458]
McDonald  E  et al. Notes from the field: investigation of Colorado tick fever virus disease cases—Oregon, 2018. MMWR Morb Mortal Wkly Rep. 2019 Mar 29;68(12):289–90.
[PubMed: 30921304]

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