Colorado Tick Fever


Colorado tick fever virus

Where Found

The geographic range of Colorado tick fever virus includes the Western United States, primarily Colorado, Utah, Montana, and Wyoming. Although rare, the virus can also be transmitted from person-to-person via blood transfusion.

Colorado Tick Virus bloodsmear

Incubation Period

1–14 days

Signs and Symptoms

  • Fever, chills, headache, myalgias, and lethargy
  • ~50% of patients have a biphasic illness with symptoms remitting after 2 to 4 days, but then recurring 1 to 3 days later.
  • Conjunctival injection, pharyngeal erythema and lymphadenopathy may be present.
  • Maculopapular or petechial rash in <20% of patients
  • Prolonged convalescence characterized by weakness and fatigue is common in adults.
  • Life-threatening complications and death are rare and usually associated with disseminated intravascular coagulation or meningoencephalitis in children.

General Laboratory Findings

  • Leukopenia
  • Moderate thrombocytopenia

Laboratory Diagnosis

Preliminary diagnosis of Colorado tick fever (CTF) is based on signs and symptoms, places and dates of travel, activities, and history of potential tick exposure. Acute samples should be tested by reverse-transcriptase polymerase chain reaction (RT-PCR) to detect viral RNA as antibody production is delayed until 14–21 days after onset of symptoms.

Note: CSF, cerebrospinal fluid; CNS, central nervous system

CTF Testing
Timing of specimen collection Specimens Preferred Test
<14 days after symptom onset Serum (CSF if suspected CNS involvement) RT-PCR for viral RNA
≥14 days after symptom onset Serum (CSF if suspected CNS involvement) Antibody testing*; consider RT-PCR for samples from days 14-21

*If possible, acute and convalescent samples, collected at least 2 weeks apart, with the convalescent sample collected at least 3 weeks after symptom onset, should be obtained to look for seroconversion or a 4-fold rise in antibody titers typically using a plaque reduction neutralization test (PRNT).

Note: CTF testing is available at some commercial and state health department laboratories and at CDC. Contact your state or local health department for assistance with diagnostic testing. CTF cases are reportable to local public health authorities in certain states.


No specific antiviral treatment is available. Patients with suspected CTF should receive supportive care as appropriate. Patients with confirmed CTF should defer blood and bone marrow donation for at least 6 months after recovery.


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