Colorado Tick Fever

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

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

Prevention

• Tick avoidance

• Tick control, particularly from March to November

Complications

• 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)

Prognosis

• Disease is usually self-limited and benign