Colorado Tick-Borne Diseases 

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 causative agent is a double-stranded RNA virus that is a member 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 Dermatocenter andersoni. A closely related Coltivirus has been implicated in human disease in Europe. Closely related viruses have been isolated from Ixodes ticks in Europe.^[3] See the image below.

Symptoms begin roughly 4-5 days after the tick bite, although incubation periods of as long as 20 days are reported. For the first 2 weeks of disease, free virus can be isolated from the blood. This is followed by a period during which the virus circulates inside of erythropoietic cells. The virus can live in the red blood cells for the life of the cell, which is roughly 120 days. For this reason, blood donation is prohibited in patients for 6 months following infection.

Cases with prominent hepatic or CNS manifestations have been reported. Transfusion-associated cases from viremic patients have occurred.^[4]

Frequency

United States

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 annually. The virus 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 above 4000 ft. Most cases occur from May to July, corresponding to the level of activity of the D Andersoni tick.^[5] However, of note, the number of reported cases likely represents a small fraction of actual cases because reporting is not mandatory. Furthermore, in endemic areas, the disease is sufficiently common that it might not be conscientiously reported. Many cases of this nonspecific illness likely remain undiagnosed or unproven.

International

A closely related virus transmitted by the bite of the European sheep tick Ixodes ricinus has been reported in West Germany. Other tick-borne viral diseases 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.

Mortality/Morbidity

Although prompt recovery is the expected outcome, rare fatalities are 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.

Age

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.