The incubation period is three to four days and up to two weeks after the tick bite. A typical flu-like illness ensues with fever, chills, headache, myalgia, malaise, sore throat, vomiting, and lymphadenopathy. 5-16% of afflicted patients have a rash. Colorado tick fever is commonly biphasic (two episodes), especially in adults over the age of 30 years. It remits after two to four days and reoccurs one to three days later with a prolonged convalescence. Death or menigoencephalitis in children has rarely been reported.
General laboratory findings include:
- Leukopenia with relative lymphocytosis (2,000-4,000/mm3)
- Atypical lymphocytes
- Thrombocytopenia (20,000-60,000/mm3 )