Laboratory Studies
Laboratory studies are nonspecific and generally not helpful. The white blood cell (WBC) count may be mildly depressed (mean, approximately 3900/µL) in about 66% of patients. Leukopenia may suggest the diagnosis. Rarely, thrombocytopenia occurs. Peripheral smear may show atypical lymphocytes. [1] Occasionally, patients with Colorado tick fever have elevated hepatic transaminase levels (in the mid-hundreds).
Analysis of cerebrospinal fluid (CSF) analysis may demonstrate mild-to-moderate lymphocytic pleocytosis (up to 300 cells/µL) and mildly elevated protein levels.
Because the clinical features of Colorado tick fever are nonspecific, the diagnosis must be established in the proper epidemiologic context. Confirmation is based on serologic test results or virus inoculation in mice. In addition, reverse transcriptase polymerase chain reaction (RT-PCR) techniques are available that may help diagnose the disease in the first 5 days of illness.
Neutralizing antibodies appear in about one third of cases by day 10 and in nearly all patients by 1 month after infection. A 4-fold increase in titers between specimens drawn during the acute phase and those drawn during convalescence is observed in nearly all patients. The assay, performed with complement fixation or immunofluorescent techniques, must be done in a laboratory that has experience with this test.
Antibodies to the Colorado tick virus are often found in perennial campers who frequent endemic areas; thus, a single elevated titer of immunoglobulin G (IgG) does not necessarily indicate acute infection. This finding also suggests asymptomatic seroconversion.
Although viral testing is not routinely available, the virus can be detected in the blood for 2-4 weeks after infection. Laboratory techniques also allow isolation of RNA and DNA from the tick itself to detect bacterial and viral pathogens; coinfection is a possibility. [13]
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Two ticks next to a common match. On right is Ixodes scapularis, vector for Lyme disease. On left is Dermacentor, vector for Colorado tick fever.
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Approximate geographic distribution of Dermacentor andersoni ticks and counties of residence for confirmed and probable Colorado tick fever (CTF) virus disease cases, United States, 2010–2019. All cases were acquired in states where local transmission of CTF virus has been reported previously. Courtesy of the Centers for Disease Control and Prevention (CDC) (https://www.cdc.gov/coloradotickfever/statistics.html).
