Colorado Tick Fever

• Fred F. Ferri, M.D.

 Basic Information

Definition

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

ICD-10CM CODE
A93.2	Colorado tick fever

Epidemiology & Demographics

1. •

   Incidence: approximately 330 cases reported per year in the U.S.

2. •

   Demographics: children and adults of both genders.

3. •

   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.

4. •

   Colorado has the highest incidence (Fig. E1).

Physical Findings & Clinical Presentation

1. •

   Incubation: 3 to 4 days is usual but can be up to 14 days

2. •

   First symptoms: fever, chills, severe headache, severe myalgias, and hyperesthetic skin

3. •

   Initial signs and symptoms:

   1. 1.

      Tick bite

   2. 2.

      Fever and chills

   3. 3.

      Headache

   4. 4.

      Myalgias

   5. 5.

      Weakness

   6. 6.

      Prostration and indifference

   7. 7.

      Injected conjunctivae

   8. 8.

      Erythematous pharyngitis

   9. 9.

      Lymphadenopathy

   10. 10.

       Maculopapular or petechial rash

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.

Etiology

1. •

   Infectious agent: Coltiviruses; seven species, including three in the U.S.

2. •

   Vector: wood tick, D. andersoni.

3. •

   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.

Diagnosis

Differential Diagnosis

1. •

   Rocky Mountain spotted fever

2. •

   Influenza

3. •

   Leptospirosis

4. •

   Infectious mononucleosis

5. •

   CMV infection

6. •

   Pneumonia

7. •

   Hepatitis

8. •

   Meningitis

9. •

   Endocarditis

10. •

    Scarlet fever

11. •

    Measles

12. •

    Rubella

13. •

    Typhus

14. •

    Lyme disease

15. •

    Immune thrombocytopenic purpura (ITP)

16. •

    Thrombotic thrombocytopenic purpura (TTP)

17. •

    Kawasaki disease

18. •

    Toxic shock syndrome

19. •

    Vasculitis

Workup

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

1. •

   Complete blood count

   1. 1.

      Leukopenia

   2. 2.

      Atypical lymphocytes

   3. 3.

      Moderate thrombocytopenia

2. •

   Virus identification in red blood cells by indirect immunofluorescence

3. •

   Serology with enzyme-linked immunosorbent assay, neutralization, or complement fixation

Treatment

1. •

   No specific therapy, although Coltiviruses are sensitive to ribavirin.

2. •

   Bed rest, fluids, acetaminophen.

3. •

   Avoid aspirin because of thrombocytopenia.

4. •

   Prevention: tick avoidance measures.