Colorado Tick Fever

Updated: Apr 26, 2022
Author: Nicholas Dulin, MD; Chief Editor: Jeter (Jay) Pritchard Taylor, III, MD 



Colorado tick fever is a viral infection transmitted by the bite of the tick Dermacentor andersoni. The disease occurs almost exclusively in the western United States and southwestern[1] Canada. A nonspecific febrile illness is the most common manifestation, but the virus occasionally targets other organ systems.[2, 3]

The essential management decision is to determine whether a serious treatable infection exists. Administration of fluids and antipyretics may also be necessary for supportive care. A skin examination should be completed, and, if the tick is still attached to the patient, it should be removed.[4]

For patient education resources, see the Bites and Stings Center, as well as Ticks.

Pathophysiology and Etiology

The causative agent for Colorado tick fever is transmitted by tick bite. The agent is a double-stranded RNA virus of the genus Coltivirus in the family Reoviridae, the entire genome of which has been sequenced. Colorado tick fever virus (CTF) is the prototype virus from which the genus coltivirus derives its name (Colorado tick fever).[5]  Although the virus has been found in many tick species, the vector and major reservoir for Colorado tick fever is D andersoni, also known as the Rocky Mountain wood tick. A closely related Coltivirus has been implicated in human disease in Europe, isolated from Ixodes ticks.[6]

Two ticks next to a common match. On right is Ixod 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.

Symptoms of Colorado tick fever typically begin 4-5 days after the tick bite, although incubation periods as long as 20 days have been reported. Free virus can be isolated from the blood for the first 2 weeks of illness. Following the initial period, the virus then circulates inside erythropoietic cells. The virus can live in a red blood cell for the life of the cell, which is approximately 120 days. Thus, blood donation from affected individuals is prohibited for 6 months after infection.

Cases with prominent hepatic or central nervous system (CNS) manifestations have been reported. Transfusion-associated cases from viremic patients have occurred.[7]


The virus that causes Colorado tick fever is the second most common arbovirus after West Nile virus in the United States. Several hundred cases are reported to the Centers for Disease Control and Prevention (CDC) annually. It has been found in California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, South Dakota, Utah, Washington, and Wyoming, as well as the Canadian provinces of British Columbia and Alberta.

Approximate geographic distribution of Dermacentor 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) (

In endemic areas, the disease is usually limited to elevations higher than 4,000 feet. Most cases occur from May to July, corresponding to the level of activity of the D andersoni tick.[8]  Reporting is not mandatory, however, so the number of reported cases could be a fraction of the actual exposures.

A closely related virus transmitted by the bite of the European sheep tick Ixodes ricinus has been reported in West Germany. Of the other tick-borne viral diseases, the most notable is tick-borne encephalitis, which occurs in Scandinavia, central and eastern Europe, and Russia. A different RNA virus belonging to the Flaviviridae family causes this tick-borne encephalitis and is transmitted by the ticks Ixodes persulcatus and Ixodes ricinus. Effective vaccines are available in Europe and Canada, but not the United States.

Approximately half of patients with Colorado tick fever are aged 20-47 years, with a male predominance.


The prognosis for patients with Colorado tick fever is excellent, even in cases complicated by neurologic symptoms. Although prompt recovery is the expected outcome, rare fatalities have been reported. Complications seem to occur more frequently in children than in adults and most often in patients whose conditions are diagnosed late.

Severe disseminated intravascular coagulation and thrombocytopenia have been recorded in these fatal cases, along with pathologic changes in the myocardium, brain, and lungs. However, undiagnosed co-infection with Rocky Mountain spotted fever may be responsible for such complications. Prolonged weakness has also been reported in adults older than 30 years.




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] :

  • Headache
  • Myalgias
  • Arthralgias
  • 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]



Diagnostic Considerations

Failure to exclude the treatable tick-borne diseases (eg, Rocky Mountain spotted fever and relapsing fever) and other serious bacterial infections is a potential diagnostic pitfall.

Because this virus can be transmitted by blood transfusion, it is important to inquire about recent transfusions in patients with febrile illnesses (not just Colorado tick fever) and ensure that the patient does not donate blood for 6 months.

Differential Diagnoses



Laboratory Studies

Laboratory studies are nonspecific and generally not helpful. The white blood cell (WBC) count may be mildly depressed (mean, approximately 3900/µL) in about 66% of patients. Leukopenia may suggest the diagnosis. Rarely, thrombocytopenia occurs. Peripheral smear may show atypical lymphocytes.[1] Occasionally, patients with Colorado tick fever have elevated hepatic transaminase levels (in the mid-hundreds).

Analysis of cerebrospinal fluid (CSF) analysis may demonstrate mild-to-moderate lymphocytic pleocytosis (up to 300 cells/µL) and mildly elevated protein levels.

Because the clinical features of Colorado tick fever are nonspecific, the diagnosis must be established in the proper epidemiologic context. Confirmation is based on serologic test results or virus inoculation in mice. In addition, reverse transcriptase polymerase chain reaction (RT-PCR) techniques are available that may help diagnose the disease in the first 5 days of illness.

Neutralizing antibodies appear in about one third of cases by day 10 and in nearly all patients by 1 month after infection. A 4-fold increase in titers between specimens drawn during the acute phase and those drawn during convalescence is observed in nearly all patients. The assay, performed with complement fixation or immunofluorescent techniques, must be done in a laboratory that has experience with this test.

Antibodies to the Colorado tick virus are often found in perennial campers who frequent endemic areas; thus, a single elevated titer of immunoglobulin G (IgG) does not necessarily indicate acute infection. This finding also suggests asymptomatic seroconversion.

Although viral testing is not routinely available, the virus can be detected in the blood for 2-4 weeks after infection. Laboratory techniques also allow isolation of RNA and DNA from the tick itself to detect bacterial and viral pathogens; coinfection is a possibility.[13]



Approach Considerations

Emergency department care of patients with Colorado tick fever is the same as that for any patient with a febrile illness. The essential decision is whether a serious treatable infection exists. Thus, history taking and physical examination must be directed toward this issue. Exclusion of the treatable infections listed in the differential diagnosis, as well as any other serious bacterial infection, is the goal of care. Fluids and antipyretics should be administered in a supportive manner as needed. Consultation with an infectious disease specialist may be appropriate in some cases.

If a tick is found attached to the patient, it must be removed.[4] The recommended removal method is to grasp the tick with forceps or fine-point tweezers near the point of attachment and to pull straight outward with steady, even, and gentle traction. Twisting and squeezing should be avoided, as this may facilitate the movement of pathogens into the host and may be more likely to leave tick mouthparts embedded in the skin.[14]

Patients diagnosed with Colorado tick fever should continue antipyretic therapy. They should be instructed to follow up with a primary care physician and to refrain from donating blood or bone marrow for at least 6 months after infection. The emergency physician should be aware that weakness and fatigue caused by this illness may last for several weeks.[1]


For individuals who are planning to spend extended periods of time outside in endemic areas, the following precautions are appropriate:

  • Wearing long-sleeve shirts and tucking the shirt into the pants
  • Tucking the pant legs into the socks
  • Wearing light-colored clothing
  • Performing daily tick checks
  • Applying insect repellent such as DEET or permethrin [15]


Medication Summary

No specific treatment exists for Colorado tick fever. Regular use of antipyretics provides symptomatic relief. Although ribavirin has shown some activity against the causative viral pathogen in animal experiments, there are no human data to support its use in this setting.

Analgesics, Other

Class Summary

Treatment of Colorado tick disease is symptomatic and supportive. Bed rest and mild analgesic-antipyretic therapy often helps relieve the associated lethargy, malaise, and fever.

Aspirin (Bayer Aspirin, Ecotrin, Aspercin, Ascriptin, Bufferin)

Aspirin lowers elevated body temperature by dilating peripheral vessels, enhancing the dissipation of excess heat. It also acts on the heat-regulating center of the hypothalamus to reduce fever.

Ibuprofen (Motrin, Advil, NeoProfen, Caldolor, Ultraprin)

Ibuprofen is one of the few nonsteroidal anti-inflammatory drugs (NSAIDs) indicated for reduction of fever.

Acetaminophen (Tylenol, APAP 500, Mapap, FeverAll)

Acetaminophen reduces fever by acting directly on hypothalamic heat-regulating centers, thereby bringing about increased dissipation of body heat with vasodilation and sweating.


Questions & Answers