To the Editor.
—The recent REVIEW ARTICLE "Colorado Tick Fever: A Review of 115 Laboratory-Confirmed Cases" by Spruance and Bailey (131:288, 1973) was timely and of interest. We were disappointed, however, to see no mention of studies from our laboratory on rapid diagnosis of Colorado tick fever by fluorescent antibody staining of blood smears,1-3 and the greater sensitivity of the indirect fluorescent antibody test as compared to that of the complement-fixation test for serologic diagnosis.4The diagnosis can usually be confirmed by fluorescent antibody staining of a peripheral blood smear taken on the first day or two of symptoms or any time thereafter. The viremia, being erythrocyte-associated, persists for weeks to several months thereafter, even though the patient has developed antibody and recovered from all symptoms. This persistent viremia can be demonstrated by fluorescent antibody staining or by isolation of the virus in suckling mice, if erythrocytes washed