Colorado Tick Fever

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

Diagnosis

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

Treatment

• Supportive

• Do not use analgesics that modify platelet function

Outcome

Complications

• Meningoencephalitis occurs in 3–7% of patients

• Cardiac and pulmonary complications are rare

Prevention

• Avoid endemic areas

• Use conventional means to avoid tick bite

Reference