Colorado tick fever is a rare, self-limiting febrile illness caused by an RNA virus of the genus Coltivirus. It is spread through tick bites to people living or traveling in the endemic regions of the western United States and western Canada. It presents clinically with non-specific symptoms, usually in adult males, although central nervous system involvement in children has also been reported.
Colorado tick fever (CTF) is a self-limiting, often benign, febrile illness spread to humans through infected tick bites  . A spurt in CTF cases is usually noticed from April until August as the ticks are active in these months. Ticks get infected with the virus when feeding on the blood of rodents such as mice, squirrels, and chipmunks, and then transmit the virus while feeding on either humans or animals. Rarely, the infection can be transmitted via blood transfusion, but otherwise, person to person transmission has not been reported. The onset of symptoms can occur anytime between 1 to 14 days after the tick bite.
The clinical presentation is non-specific with fever, malaise, myalgias, arthralgias, lassitude, fatigue, sore throat, nausea, or vomiting. A maculopapular rash with palatal spots may be present in some patients. Some patients may have the classic CTF "biphasic" or "saddleback" fever characterized by fever for a few days, followed by improvement in symptoms for the next few days, and then another short phase of fever with constitutional symptoms. A majority of patients have a self-limiting illness and recover without sequelae although the weakness can persist for several weeks. There have been rare reports of central nervous system involvement with meningitis and encephalitis in children     , as well as in some adults . Other uncommon clinical presentations include photophobia , diarrhea, hepatitis, epididymo-orchitis, arthritis, hemorrhagic diathesis, carditis, pneumonia, and even coma    .
Initial clinical diagnosis of CTF is based on the patient's history of travel to endemic areas with probable tick bites followed by fever and fatigue. Physical examination findings are not specific although a maculopapular rash and nuchal rigidity may be present. Routine laboratory tests like complete blood count, erythrocyte sedimentation rate, and liver function tests are ordered. Atypical lymphocytes , relative lymphocytosis, thrombocytopenia, and leukopenia are often noticed in CTF patients. Certain patients may have elevated levels of liver transaminases. The diagnosis of CTF can be confirmed with the detection of viral RNA and specific immunoglobulins in serum. Reverse-transcriptase polymerase chain reaction (RT-PCR) is a more useful test in early phases of the disease since serological tests may not be positive until 2-3 weeks after the patient's initial clinical presentation. Cerebrospinal fluid analysis may be warranted in CTF patients presenting with neck stiffness or features of encephalitis and may reveal mild protein elevation with lymphocytic pleocytosis. Finally, genetic material can be isolated from the tick to confirm the presence of a virus .
Radiological investigations are not helpful in the diagnosis or management of CTF except in cases of central nervous system involvement. Magnetic resonance imaging can be considered in cases presenting with features of encephalitis or other serious neurological diseases.