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Tick-Borne Diseases, Colorado
Updated: Dec 9, 2008
Introduction
Background
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.
Pathophysiology
The causative agent, an RNA virus formerly classified as an Orbivirus of 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 D andersoni. The newer International Committee on Taxonomy of Viruses has reclassified the agent of Colorado tick fever as a Coltivirus (still in the family of Reoviridae). A closely related Coltivirus has been implicated in human disease in Europe. Closely related viruses have been isolated from Ixodes ticks in Europe.
Two ticks next to a common match. On the right is an Ixodes scapularis, the vector for Lyme disease. On the left is a Dermacentor tick (the larger one and the vector for Colorado tick fever).
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.
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. These cases are contracted in the states of 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 this endemic area, the disease is limited to elevations above 4000 ft. The number of reported cases likely represents a small fraction of actual cases, since 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. A seasonal peak exists from April through August.
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.
- Some fatalities may have occurred; they have been ascribed to Rocky Mountain spotted fever.
- Prolonged weakness has 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.
Clinical
History
Colorado tick fever presents as a nonspecific febrile illness with few historic clues other than the epidemiology to suggest the disease. 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 the following:
- Tick bite
- 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 have a history of participating in activities that put them at risk for a tick bite.
- Fever
- Fever nearly universally is present.
- One characteristic fever pattern noted in about one half of cases of Colorado tick fever is "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 another 2-3 days of fever.
- Flulike symptoms
- Headache
- Myalgias
- Arthralgias
- Fatigue
- Nonspecific evanescent rash (5-15%), sometimes with a palatal enanthem.
- Other
- Stiff neck, 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
Physical examination is not particularly helpful in diagnosis in Colorado tick fever. Findings may include rash and nuchal rigidity.
- Rash
- 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 enanthem is sometimes present.
- Nuchal rigidity is found in 15-20% of cases.
- Splenomegaly may occur.
- Some patients have clouded sensorium or even coma.
Causes
Cause of Colorado tick fever is infection with the causative agent that is transmitted by a tick bite. People who are exposed to the vector D andersoni in the endemic area of the mountainous areas in western North America are at risk.
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References
Emmons RW. Ecology of Colorado tick fever. Annu Rev Microbiol. 1988;42:49-64. [Medline].
Goodpasture HC, Poland JD, Francy DB, et al. Colorado tick fever: clinical, epidemiologic, and laboratory aspects of 228 cases in Colorado in 1973-1974. Ann Intern Med. Mar 1978;88(3):303-10. [Medline].
Klasco R. Colorado tick fever. Med Clin North Am. Mar 2002;86(2):435-40, ix. [Medline].
Leiby DA, Gill JE. Transfusion-transmitted tick-borne infections: a cornucopia of threats. Transfus Med Rev. Oct 2004;18(4):293-306. [Medline].
Romero JR, Simonsen KA. Powassan encephalitis and colorado tick fever. Infect Dis Clin North Am. Sep 2008;22(3):545-59, x. [Medline].
Spruance SL, Bailey A. Colorado Tick Fever. A review of 115 laboratory confirmed cases. Arch Intern Med. Feb 1973;131(2):288-93. [Medline].
Further Reading
Keywords
tick-borne diseases, Colorado tick fever, tick bite, Dermacentor andersoni, D andersoni, Orbivirus, vector-borne disease, wood tick, Coltivirus, RNA virus, tick-borne viral diseases, tick-borne virus, Rocky Mountain spotted fever, tick bite, viral infection
