Petechiae are associated with rare cases, complicated by thrombocytopenia. People who are exposed to the vector, D andersoni , in the endemic area of the mountainous areas in western North America are at risk. Laboratory studies generally are not helpful. Leukopenia may suggest the diagnosis. Given that the clinical features of Colorado tick fever are nonspecific, establish the diagnosis in the proper epidemiologic context.
Confirmation is based on serologic testing.
Neutralizing antibodies appear in about one third of cases by day 10 and in nearly all patients by 1 month from infection. A 4-fold rise in titers between specimens drawn during the acute phase and convalescence is observed in nearly all patients. The assay, by complement fixation or immunofluorescent techniques, must be done in a laboratory with experience in performing this test. This finding also suggests asymptomatic seroconversion. Although not a routinely available test, the virus can be detected in the blood for 2-4 weeks.