Colorado Tick Fever 

Updated: Mar 20, 2017
Author: Cassis Thomassin, MD; Chief Editor: Jeter (Jay) Pritchard Taylor, III, MD 



Colorado tick fever is a viral infection transmitted by the bite of the wood 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 whether a serious treatable infection exists. Administration of fluids and antipyretics as needed is also indicated. If the tick is still attached to the patient, it should be removed.

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

Pathophysiology and Etiology

The cause of Colorado tick fever is infection with the causative agent that is transmitted by a tick bite. This agent is a double-stranded RNA virus of the genus Coltivirus in the family Reoviridae. The entire genome has been sequenced. Although the virus has been found in many tick species, the vector and major reservoir is Dermacentor andersoni. A closely related Coltivirus has been implicated in human disease in Europe. Closely related viruses have been isolated from Ixodes ticks in Europe (see the image below).[4]

Two ticks next to common match. On right is Ixodes Two ticks next to common match. On right is Ixodes scapularis, vector for Lyme disease. On left is Dermacentor tick (the larger one and the vector for Colorado tick fever).

Symptoms begin roughly 4-5 days after the tick bite, though incubation periods as long as 20 days are reported. For the first 2 weeks of disease, free virus can be isolated from the blood. This initial period is followed by a period during which the virus circulates inside erythropoietic cells. The virus can live in a red blood cell for the life of the cell, which is roughly 120 days. For this reason, blood donation is prohibited in patients 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.[5]


The virus that causes Colorado tick fever is the second most common arbovirus (after West Nile virus) in the United States, and 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.

In endemic areas, the disease is usually limited to elevations higher than 4000 ft. Most cases occur from May to July, corresponding to the level of activity of the D andersoni tick.[6] It should be noted, however, that reporting is not mandatory, which means that the number of reported cases probably represents only a small fraction of the actual cases. Furthermore, in endemic areas, the disease is common enough that it might not be conscientiously reported. It is likely that many cases of this nonspecific illness remain undiagnosed or unproven.

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 known to exist, 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, not the United States.

Approximately half of patients are between 20 and 47 years, and males predominate. This finding probably reflects their risk of exposure to ticks in various recreational and occupational activities rather than any intrinsic biological age- or sex-based risk.


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, most often in patients whose conditions are diagnosed late.

Severe disseminated intravascular coagulation and thrombocytopenia have been recorded in these fatal cases, as have pathologic changes in the myocardium, brain, and lungs. However, an 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.[7] 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 flulike symptoms.

Most patients have a history of exposure to ticks, but only about half recall tick attachment. Therefore, although a history of a tick bite is an important clue, 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 fever pattern noted in about one half of cases of Colorado tick fever is so-called “saddleback 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 by another 2-3 days of fever.

Common flulike symptoms include the following:

  • Headache

  • Myalgias

  • Arthralgias

  • Fatigue

In addition, a nonspecific evanescent rash may be present (5-15% of cases), sometimes with a palatal enanthema. Stiff neck, retroorbital pain, photophobia[1] , nausea and vomiting, abdominal pain, diarrhea, and sore throat all have been reported in a minority of patients. 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. Findings may include rash and nuchal rigidity.

In 5-15% of patients, a macular, maculopapular, and 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. Some patients have altered sensorium or even coma.

Complications of Colorado tick fever are unusual and rare. Cases with neurologic disease, including meningitis and meningoencephalitis, are reported, especially in children.[6]



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 into recent transfusions in patients with febrile illnesses (not just Colorado tick fever) and also to make sure 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 is 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 (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 frequently are 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.[8]



Approach Considerations

Emergency department (ED) 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; 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. Administration of fluids and antipyretics as needed is indicated. Consultation with an infectious disease specialist may be appropriate in some cases.

If a tick is found to be still attached to the patient, it must be removed. The removal method that is generally recommended is to grasp the tick with a forceps or fine-point tweezers near the point of attachment and then to pull straight outward with steady, even, gentle traction. Twisting and squeezing should be avoided, because they may facilitate the movement of pathogens into the host and may be more likely to leave tick mouthparts embedded in the skin.[9]

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:

  • Wear a long-sleeve shirt, and tuck the shirt into the pants

  • Tuck the pant legs into the socks

  • Wear light-colored clothing

  • Perform daily tick checks

  • Apply insect repellent such as DEET or permethrin[10]



Medication Summary

No specific treatment exists for Colorado tick fever. Regular use of antipyretics provides symptomatic relief. Although ribavirin has 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.