History
Colorado tick fever presents as a nonspecific febrile illness with few historical clues (other than the epidemiology) to suggest the disease. [9] Consider the diagnosis in any patient with a febrile illness who lives in or who recently visited an endemic area. Most patients are males aged 15-45 years who present between April and August. Findings may include a history of tick bite, fever, and flu-like symptoms. [4]
Most patients have a history of tick exposure, but roughly half actually recall tick attachment. Therefore, a history of a tick bite may be a clue, but its absence does not exclude the diagnosis. The patient may also have a history of participation in activities that put him or her at risk for a tick bite.
Fever is present in nearly all cases. A characteristic "saddleback" or biphasic fever pattern has been noted in about half of cases of Colorado tick fever, which strongly suggests the diagnosis. Patients with this pattern have a fever for 2-3 days, followed by an afebrile period of similar duration and then another 2-3 days of fever. [10, 11, 12]
Common flu-like symptoms include the following [4] :
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Headache
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Myalgias
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Arthralgias
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Fatigue
In addition, a nonspecific evanescent rash may be present in 5-15% of cases, sometimes with palatal enanthema. Stiff neck, retro-orbital pain, photophobia, [1] nausea, vomiting, abdominal pain, diarrhea, and sore throat have all been reported in a minority of patients. [4] In one series, patients with suspected Colorado tick fever and symptoms of abdominal pain, rash, or sore throat were less likely to have Colorado tick fever on the basis of serologic diagnoses.
Physical Examination
Physical examination is not particularly helpful for diagnosing Colorado tick fever.
In 5-15% of patients, a macular, maculopapular or petechial rash is present. Occasionally, a small, red, painless papule (presumably at the bite site) is present. The distribution is often truncal, in contrast to the more acral rash in Rocky Mountain spotted fever. The rash tends to be short lived, which is another difference compared with Rocky Mountain spotted fever. Petechiae occur in rare cases and may be complicated by thrombocytopenia. A palatal enanthema is sometimes present.
Nuchal rigidity is found in 15-20% of cases. Splenomegaly may occur. In severe cases, patients can present with altered sensorium or even coma.
Complications of Colorado tick fever are uncommon. However, cases with neurologic sequelae, including meningitis and meningoencephalitis, are reported, especially in children. [8]
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Two ticks next to a common match. On right is Ixodes scapularis, vector for Lyme disease. On left is Dermacentor, vector for Colorado tick fever.
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Approximate geographic distribution of Dermacentor andersoni ticks and counties of residence for confirmed and probable Colorado tick fever (CTF) virus disease cases, United States, 2010–2019. All cases were acquired in states where local transmission of CTF virus has been reported previously. Courtesy of the Centers for Disease Control and Prevention (CDC) (https://www.cdc.gov/coloradotickfever/statistics.html).
