Colorado Tick Fever

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