Colorado Tick Fever

Colorado Tick Fever 

Colorado tick fever is an acute, self-limited febrile illness caused by infection with a Coltivirus.

Epidemiology & Demographics

  • •Incidence: Approximately 330 cases reported per yr in the U.S.
  • •Demographics: Children and adults of both genders.
  • •Geography: Rocky Mountains at elevations of 4000 to 10,000 feet. Sporadic cases have been reported from areas of California outside the range of Dermacentor andersoni.

Physical Findings & Clinical Presentation

  • •Incubation: 3 to 4 days is usual but can be up to 14 days.
  • •First symptoms: Fever, chills, severe headache, severe myalgias, and hyperesthetic skin.
  • •Initial signs and symptoms:
    • 1.Tick bite (reported by more than 90% of parents)
    • 2.Fever and chills
    • 3.Headache
    • 4.Myalgias
    • 5.Weakness
    • 6.Prostration and indifference
    • 7.Injected conjunctivae
    • 8.Erythematous pharyngitis
    • 9.Lymphadenopathy and hepatosplenomegaly may be found in some patients
    • 10.Maculopapular or petechial rash 5.6% to 12% of cases
  • •These first symptoms last for 1 wk or less, but 50% of patients have a febrile relapse 2 to 3 days after an initial remission. Weakness and fatigue may persist for several months after the acute phase(s). This chronic phase is more likely in older patients.
  • •In children, 5% to 10% of cases are complicated by aseptic meningitis. In adults, rare complications include pneumonia, hepatitis, myocarditis, and epididymo-orchitis. Vertically transmitted fetal infection is possible.


  • •Infectious agent: Coltiviruses; seven species, including three in the U.S.
  • •Vector: Wood tick, D. andersoni 
  • •Pathogenesis: Human transmission occurs by tick bite. Tick season spans from March to September. The virus infects marrow erythrocytic precursors, explaining the protracted disease course because viremia lasts for the life span of the infected red blood cell. 


People become infected with Colorado tick fever (CTF) virus from the bite of an infected Rocky Mountain wood tick (Dermacentor andersoni).

Rocky mountain wood ticks are found in the western United States and western Canada at 4,000–10,000 feet above sea level.

Ticks become infected with CTF virus when they feed on the blood of a reservoir animal (an animal that has the virus circulating in its bloodstream).

The most important reservoirs for CTF virus are small rodents such as squirrels, chipmunks, and mice.

After the tick becomes infected, it passes the virus to other hosts (animal or human) while it feeds.

The virus is not transmitted from person to person, except in rare instances by blood transfusion.

Since CTF virus can remain in red blood cells for several months, blood and bone marrow should not be donated for 6 months following infection.

Differential Diagnosis

  • •Rocky Mountain spotted fever
  • •Influenza
  • •Leptospirosis
  • •Infectious mononucleosis
  • •CMV infection
  • •Pneumonia
  • •Hepatitis
  • •Meningitis
  • •Endocarditis
  • •Scarlet fever
  • •Measles
  • •Rubella
  • •Typhus
  • •Lyme disease
  • •Immune thrombocytopenic purpura (ITP)
  • •Thrombotic thrombocytopenic purpura (TTP)
  • •Kawasaki disease
  • •Toxic shock syndrome
  • •Vasculitis


Consider Colorado tick fever in the presence of the above symptoms associated with travel to an endemic area coupled with a history of tick exposure.

Laboratory Tests

  • •Complete blood count:
    • 1.Leukopenia
    • 2.Atypical lymphocytes
    • 3.Moderate thrombocytopenia
  • •Virus identification in red blood cells by indirect immunofluorescence
  • •Serology with enzyme-linked immunosorbent assay, neutralization, or complement fixation


  • •No specific therapy, although Coltiviruses are sensitive to ribavirin.
  • •Bed rest, fluids, acetaminophen
  • •Avoid aspirin because of thrombocytopenia
  • •Prevention: Tick avoidance measures


No CTF virus vaccines are currently available for use in people. In the absence of a vaccine, prevention of CTF depends on personal protective measures to decrease exposure to infected ticks.

This includes avoiding wooded and bushy areas with high grass, using repellents to discourage tick attachment, and finding and removing ticks before they have a chance to attach.

CTF virus can be transmitted through blood transfusions and bone marrow transplants. People with confirmed CTF virus infections should not donate blood or bone marrow for 6 months after their illness.

CTF virus infections temporally associated with blood transfusion or bone marrow transplant should be reported promptly to the appropriate state health department.

Seek Additional Information

More information about reducing exposure to ticks is available on the CDC Ticks website.


Sign up to receive the trending updates and tons of Health Tips

Join SeekhealthZ and never miss the latest health information

Scroll to Top