Colorado Tick Fever or Ehrlichiosis

To the Editor.—  I read with interest the recent article by Fishbein et al¹ entitled "Unexplained Febrile Illnesses After Exposure to Ticks: Infection With an Ehrlichia?" Although the serological data are very supportive of disease due to Ehrlichia, I would also note that virological and serological studies were apparently not performed for Colorado tick fever (CTF). This latter infection caused by an orbivirus has many similar clinical features to the illness described by Fishbein et al, with fever, chills, headache, myalgias, and abdominal complaints occurring commonly.² A biphasic course is characteristic, but occurs in only half the cases. In addition the major laboratory abnormality of CTF is leukopenia, and lymphopenia is also frequently present.³ Both thrombocytopenia and liver function abnormalities have been described.⁴Despite its name, CTF is endemic throughout the mountainous regions of the western United States and Canada in the distribution of Dermacentor andersoni