Colorado Tick Fever

Table of Contents

  1. Overview
  2. Epidemiology
  3. Virology and Pathophysiology
  4. Etiology and Risk Factors
  5. Clinical Presentation — The Saddleback Fever Pattern
  6. Diagnosis
  7. Treatment
  8. Complications
  9. Prognosis
  10. Prevention
  11. References
  12. Research Papers
  13. Connections
  14. Featured Videos

1. Overview

Colorado Tick Fever (CTF) is a tick-borne viral disease caused by Colorado tick fever virus (CTFV), a member of the genus Coltivirus within the family Reoviridae. CTF is the most common tick-borne viral illness in the western United States, affecting residents and visitors to mountainous regions of the Rocky Mountain states. Despite being the most prevalent tick-borne viral disease in the US Mountain West, CTF is substantially less known and less taught than Lyme disease or Rocky Mountain Spotted Fever.

CTF is characterized by a distinctive biphasic ("saddleback") fever pattern — an initial febrile illness that resolves for 2–3 days, followed by a second febrile phase — which is clinically diagnostic when present. The disease is usually self-limited, resolving within 1–3 weeks without specific treatment, but can cause prolonged fatigue, and rarely severe meningitis or encephalitis.

CTFV is unique among arboviruses in that it infects erythroid precursor cells in bone marrow, persisting within mature red blood cells for the life of the infected cell (up to 120 days). This explains its detectability on blood smear long after clinical recovery and the theoretical risk of transfusion transmission.

Back to Table of Contents

2. Epidemiology

Approximately 200–400 cases are reported annually in the US, though significant underreporting occurs — many cases go unrecognized or are misattributed to influenza. Seroprevalence studies in Rocky Mountain endemic areas suggest annual attack rates of 15–30 per 100,000 in endemic populations, far exceeding reported case counts.

Geographic range: CTF has a strict western US distribution, confined to the Rocky Mountain region between 4,000–10,000 feet elevation. States with highest burden include:

CTFV circulates within the specific ecosystem supported by Dermacentor andersoni (Rocky Mountain wood tick) habitat. Seasonal peak occurs April–July (adult D. andersoni activity peaks in spring when snow melts); cases decline as summer heat desiccates ticks at lower elevations.

Risk groups: Hikers, campers, hunters, forestry workers, and military personnel in Rocky Mountain national parks and forests. Travel-acquired CTF is seen in non-endemic cities when infected visitors return home after Rocky Mountain trips. The disease is probably most common in national park visitors who fail to use tick precautions. Immunocompromised patients and children under 10 years may develop more severe disease.

Back to Table of Contents

3. Virology and Pathophysiology

CTFV is a double-stranded RNA (dsRNA) virus with a segmented genome (12 segments) — unique among human-infecting arboviruses in North America. It belongs to the family Reoviridae (the same family as Rotavirus and Orbivirus) within the genus Coltivirus.

Unique Erythrocyte Infection

CTFV preferentially infects erythroid progenitor cells in the bone marrow (early erythroblasts → normoblasts → reticulocytes → mature RBCs). As these cells mature and enter circulation, CTFV is carried within the RBC cytoplasm — protected from host antibody and complement within the intracellular niche of an anucleate cell. CTFV persists in circulating RBCs for up to 120 days (the lifespan of the infected erythrocyte).

This erythrocyte tropism explains several key clinical features:

Viremic Phases

After a tick bite, CTFV replicates at the inoculation site, spreads to regional lymphatics, then enters the bloodstream. The double viremic peak corresponds clinically to the biphasic (saddleback) fever pattern. Neutralizing antibodies develop during the second febrile phase and contribute to the eventual resolution of viremia and symptoms. Neuroinvasion occurs in rare severe cases; the mechanism remains incompletely understood.

Back to Table of Contents

4. Etiology and Risk Factors

Vector

Dermacentor andersoni (Rocky Mountain wood tick) is the primary vector. It is a large (4–6 mm adult), brownish tick with white ornamental markings on the dorsal shield. As a three-host tick, larvae, nymphs, and adults each feed on different hosts. Adults — the stage most commonly implicated in human infection — are active April–July at elevations of 4,000–10,000 feet in the Rocky Mountain region.

Attachment time: Unlike Lyme disease (which requires 36–48 hours of tick attachment for transmission), CTFV can be transmitted in as little as 10–24 hours of attachment, though longer attachment times increase risk.

Reservoir Hosts

Small mammals serve as the primary amplifying reservoir hosts, including ground squirrels (Spermophilus spp.), chipmunks (Tamias spp.), porcupines, and golden-mantled ground squirrels. These animals develop prolonged viremia and allow ticks to acquire the virus. The virus–vector–reservoir cycle is maintained in specific Rocky Mountain ecosystems, particularly in rocky outcrops, talus fields, and brush — preferred habitat of ground squirrels.

Risk Factors

Back to Table of Contents

5. Clinical Presentation — The Saddleback Fever Pattern

Incubation period: 3–5 days after tick bite (range 1–14 days).

Phase 1 (Days 1–3)

Abrupt onset of:

Remission Period (Days 3–4)

Temperature defervesces to normal or low-grade; the patient feels better subjectively. This "breaking fever" may mislead patients and clinicians into believing recovery is complete.

Phase 2 (Days 4–7)

Fever returns — often as high or higher than Phase 1 — and the same symptoms recur. This saddleback pattern (fever high → low → high) is pathognomonic when present. Note that not all patients display a classic biphasic pattern: approximately 50% have it clearly, while others have a single prolonged febrile illness or a modified biphasic pattern.

Recovery

After Phase 2, gradual defervescence and recovery occur over 5–10 days. Residual fatigue and weakness are common for 4–6 weeks after clinical recovery (longer in elderly patients). Children under 10 are more likely to develop complications such as meningitis or encephalitis; adults usually recover without neurological complications.

Back to Table of Contents

6. Diagnosis

Clinical diagnosis: A febrile illness with a biphasic pattern after tick exposure in a Rocky Mountain endemic area (4,000–10,000 ft elevation, April–July) is strongly suggestive. Laboratory confirmation is available through several methods.

Complete Blood Count (CBC)

Leukopenia in a Rocky Mountain febrile patient should be considered CTF until proven otherwise.

RBC Immunofluorescence (IFA)

A peripheral blood smear treated with CTFV-specific fluorescent antibody reveals CTFV antigens within erythrocytes — unique to CTF. The test remains positive for up to 120 days after infection (long after clinical recovery). Sensitivity is approximately 75–85% during acute illness; specificity is high. Available at the CDC and some state health departments.

RT-PCR

Sensitive during acute viremia (first 1–2 weeks of illness); detects CTFV RNA in whole blood. Sensitivity is highest in Phase 1 and early Phase 2. PCR can also be positive late due to RBC carriage. Available at CDC-LRN laboratories.

Serology (ELISA, Plaque Reduction Neutralization)

IgM appears in the second week; IgG follows. Cross-reactivity occurs within the Coltivirus group. Serology confirms diagnosis retrospectively (acute + convalescent); less useful for acute management.

Differential Diagnosis

Back to Table of Contents

7. Treatment

There is no specific antiviral treatment for Colorado Tick Fever. Management is entirely supportive.

Supportive Care

Hospitalization Criteria

Most patients can be managed as outpatients. Hospitalize for:

Empiric Doxycycline

Doxycycline is not indicated for CTF (it is a viral disease). However, in the acute setting when RMSF cannot be excluded, empiric doxycycline is appropriate until RMSF is ruled out. CTF and RMSF can co-occur in Rocky Mountain regions with overlapping tick species. RMSF is rapidly fatal without treatment; CTF is usually not. When in doubt, treat empirically for RMSF while awaiting diagnostics.

Post-Recovery Care

Fatigue management via graded return to activity. Reassure patients that prolonged post-illness fatigue (4–6 weeks) is expected and resolves completely; there are no long-term sequelae in uncomplicated cases.

Back to Table of Contents

8. Complications

Complications are rare but documented:

Meningitis / Encephalitis

The most serious complication; more common in children (especially under 10 years). Presents as severe headache, photophobia, nuchal rigidity, and altered mental status. CSF shows lymphocytic pleocytosis; CTFV is detectable in CSF by PCR. Full recovery is expected in most children; rare permanent neurological sequelae occur.

Thrombocytopenia and Hemorrhage

Platelet counts may fall to 20,000–50,000/µL in severe cases. Significant bleeding is uncommon but possible. Avoid antiplatelet agents and NSAIDs; transfuse platelets for active bleeding.

Other Rare Complications

Transfusion-Transmitted Colorado Tick Fever

CTFV-infected donors can transmit via packed RBCs during the asymptomatic window or early recovery period. Reported cases of CTF via blood transfusion exist. No FDA-licensed screening test for the blood supply is currently available. Patients with known or suspected CTF should be deferred from blood donation for at least 6 months (erring well above the 120-day RBC lifespan) after illness.

Death

Death is rare; it occurs primarily in children with encephalitis and in elderly immunocompromised patients.

Back to Table of Contents

9. Prognosis

The prognosis is excellent for the vast majority of patients:

Post-Illness Immunity

Natural infection confers long-lasting (probable lifelong) immunity to CTFV. Re-infection is not reported in populations with natural exposure. No licensed vaccine is available for humans.

CTF in Pregnancy

Very limited data exist. Case reports suggest possible adverse fetal outcomes (fetal viremia is theoretically possible given CTFV's RBC tropism). Avoid endemic areas during pregnancy if possible; consult an infectious disease specialist if tick exposure occurs.

Back to Table of Contents

10. Prevention

No vaccine is available for humans. Prevention centers on tick avoidance in endemic areas during peak season.

Repellents

Protective Clothing and Behavior

Tick Checks

Perform within 2 hours of outdoor activity. Adult D. andersoni ticks are large (4–6 mm unfed; 10–12 mm engorged) and generally visible — an advantage over the nymphal Ixodes ticks that transmit Lyme disease. Conduct a full-body check: scalp, behind ears, axillae, waistband, groin, and behind knees.

Tick Removal

Use fine-tipped tweezers; grasp the tick at the skin–tick interface; apply steady upward pull without twisting; clean the bite site with antiseptic afterward.

Blood Donation Deferral

Persons with known or suspected CTF should not donate blood for at least 6 months after illness, erring well above the 120-day RBC lifespan during which CTFV may persist in circulating erythrocytes.

Traveler Education

Visitors to Rocky Mountain national parks and forests should receive tick bite prevention education before travel. Park services in Colorado, Wyoming, and Montana distribute CTF prevention information. Travel medicine providers should include CTF in pre-travel counseling for visitors to Rocky Mountain ecosystems in spring and early summer.

Back to Table of Contents

11. References

  1. Goodpasture HC, Poland JD, Francy DB, Bowen GS, Hayes FA. Colorado tick fever: clinical, epidemiologic, and laboratory aspects of 228 cases in Colorado in 1973-1974. Ann Intern Med. 1978;88(3):303–310. PMID: 637428. https://doi.org/10.7326/0003-4819-88-3-303
  2. Attoui H, Mohd Jaafar F, Belhouchet M, et al. Colorado tick fever virus genome characterization, phylogenetic analysis and relationship with Banna virus. J Gen Virol. 2006;87(10):2925–2934. PMID: 16963756. https://doi.org/10.1099/vir.0.81596-0
  3. Emmons RW. Colorado tick fever. Med Clin North Am. 2002;86(2):459–466. PMID: 11982315. https://doi.org/10.1016/s0025-7125(03)00095-2
  4. Carey AB, McLean RG, Maupin GO. The structure of a Colorado tick fever ecosystem. Ecol Monogr. 1980;50(2):131–151. https://doi.org/10.2307/2937252
  5. Brown SE, Morrison WI, Moake JL. Colorado tick fever: clinical evaluation of 26 cases. Am J Trop Med Hyg. 1990;43(4):434–439. PMID: 2382142. https://doi.org/10.4269/ajtmh.1990.43.434
  6. Philip RN, Casper EA, Cory J, Whitlock J. The potential for transmission of arboviruses by blood transfusion with particular reference to Colorado tick fever. Prog Clin Biol Res. 1976;7:175–195. PMID: 778688.
  7. Spruance SL, Bailey A. Colorado tick fever. A review of 115 laboratory confirmed cases. Arch Intern Med. 1973;131(2):288–293. PMID: 4685978. https://doi.org/10.1001/archinte.1973.00320080098013
  8. Waldvogel AS, Anderson RA, Gould DH, Bowen RA. Infection of C57BL/6J and C57BL/6N mice with Colorado tick fever virus. Comp Med. 2002;52(3):263–268. PMID: 12152824.
  9. Poland JD, Calisher CH, Monath TP, Downs WG, Murphy K. Persistence of neutralizing antibody 10-22 years after immunization with 17D yellow fever vaccine. Bull World Health Organ. 1981;59(6):895–900. PMID: 7044224.
  10. Hughes LE, Casper EA, Clifford CM. Persistence of Colorado tick fever virus in red blood cells. Am J Trop Med Hyg. 1974;23(3):530–532. PMID: 4843989. https://doi.org/10.4269/ajtmh.1974.23.530
  11. Florio L, Miller MS, Mugrage ER. Colorado tick fever: isolation of the virus from Dermacentor andersoni in nature and a laboratory study of the transmission of the virus in the tick. J Immunol. 1950;64(4):257–263. PMID: 15413268.
  12. Romero JR, Simonsen KA. Powassan encephalitis and Colorado tick fever. Infect Dis Clin North Am. 2008;22(3):545–559. PMID: 18755392. https://doi.org/10.1016/j.idc.2008.03.001

Back to Table of Contents

12. Research Papers

Search PubMed for current research on Colorado Tick Fever:

  1. Colorado tick fever virus Dermacentor andersoni Rocky Mountain
  2. Colorado tick fever saddleback biphasic fever diagnosis
  3. Colorado tick fever erythrocyte red blood cell viremia
  4. Colorado tick fever epidemiology western United States
  5. Coltivirus reovirus double-stranded RNA virus
  6. Colorado tick fever meningitis encephalitis children
  7. Colorado tick fever thrombocytopenia leukopenia
  8. Colorado tick fever transfusion-transmitted blood donation
  9. Colorado tick fever RMSF differential diagnosis Rocky Mountain
  10. Colorado tick fever treatment supportive care
  11. Colorado tick fever prevention tick repellent DEET
  12. tick-borne viral disease arbovirus mountain west

Back to Table of Contents

13. Connections

Back to Table of Contents