AUTHOR AND EDITOR INFORMATION

Section 1 of 11  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

INTRODUCTION

Section 2 of 11  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

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, is limited to 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.

Attempts to isolate this agent from other species of ticks have failed, although closely related viruses have been isolated from Ixodes ticks in Europe.

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

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

Most patients are young and male; this finding reflects their risk of exposure to ticks in various recreational and occupational activities rather than any intrinsic biological age- or sex-based risk.

CLINICAL

Section 3 of 11  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

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
• • About 50-95% of patients remember a tick biting them or crawling on them.
  • 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%)
• 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
• • It is described as macular, maculopapular, and petechial. 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.
• 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.

DIFFERENTIALS

Section 4 of 11  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

WORKUP

Section 5 of 11  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

Lab Studies

• Laboratory studies are not helpful.
• The WBC count is mildly depressed (mean, about 3900 per mm³) in about 66% of patients. Leukopenia may suggest the diagnosis. Rarely, thrombocytopenia occurs.
• Occasionally, patients with Colorado tick fever have elevated hepatic transaminase levels (in the mid-hundreds range).

Other Tests

• Because the clinical features of Colorado tick fever are nonspecific, establish the diagnosis in the proper epidemiologic context. Confirmation is based on serologic test results or virus inoculation in mice. In addition, newer reverse transcriptase polymerase chain reaction (PCR) techniques exist.
• • 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 from convalescence is observed in nearly all patients.
  • The assay, with complement fixation or immunofluorescent techniques, must be performed in a laboratory with experience in performing this test.
  • Antibodies to the Colorado tick virus frequently are found in perennial campers who frequent endemic areas; thus, single elevated titers of immunoglobulin G do 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.

TREATMENT

Section 6 of 11  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

Emergency Department Care

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. Therefore, history taking and physical examination must be directed toward this issue. Exclusion of treatable infections listed in the differential diagnosis section, as well as any other serious bacterial infection, is the goal of ED care. Administration of fluids and antipyretics as needed is indicated.

Consultations

Consultation with an infectious disease specialist may be appropriate in some cases.

MEDICATION

Section 7 of 11  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

No specific treatment exists. Regular use of antipyretics provides symptomatic relief. Although ribavirin has some activity against the virus in animal experiments, no human data exist.

Drug Category: Antipyretics

Treatment of Colorado tick disease is symptomatic and supportive. Bed rest and mild analgesic-antipyretic therapy often is helpful in relieving associated lethargy, malaise, and fever.

FOLLOW-UP

Section 8 of 11  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

Further Outpatient Care

• Continue antipyretic therapy.
• Instruct the patient to follow up with a primary care physician.

Deterrence/Prevention

• Refer to the Deterrence/Prevention section in Tick-borne Diseases, Introduction.

Complications

• Complications are unusual. Cases with neurologic disease, including meningitis and meningoencephalitis, are reported, especially in children.

Prognosis

• The prognosis is excellent, even in cases complicated by neurologic symptoms.
• Rare fatalities are reported.

Patient Education

• Instruct patients not to donate blood for 6 months following infection.
• For excellent patient education resources, visit eMedicine's Bites and Stings Center. Also, see eMedicine's patient education article Ticks.

MISCELLANEOUS

Section 9 of 11  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

Medical/Legal Pitfalls

• Failure to exclude the treatable tick-borne diseases (eg, Rocky Mountain spotted fever, relapsing fever) and other serious bacterial infections

Special Concerns

• This virus can be transmitted by blood transfusion, so ask about recent transfusions in patients with febrile illnesses (not just for Colorado tick fever) and also make sure that the patient does not donate blood for 6 months.

MULTIMEDIA

Section 10 of 11  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

Media file 1:  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).
	View Full Size Image
Media type:  Photo

REFERENCES

Section 11 of 11

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• 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].
• Spruance SL, Bailey A. Colorado Tick Fever. A review of 115 laboratory confirmed cases. Arch Intern Med. Feb 1973;131(2):288-93. [Medline].

Article Last Updated: Oct 17, 2006