Colorado Tick Fever 

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 Canada. A nonspecific febrile illness is the most common manifestation, but the virus occasionally targets other organ systems.^{[1, 2]}

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.

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).^[3]

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.^[4]

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.^[5] 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.

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.