ABSTRACT
A resident of the Rocky Mountain west in the United States and Canada, Colorado tick fever (CTF) virus was possibly responsible for an illness suffered by Capt. William Clark during his expedition to explore the west with Capt. Meriwether Lewis and the Corps of Discovery. Upon his arrival at Three Forks, Montana, in July of 1805, Clark became acutely ill. His written records suggest that his illness was consistent with what we now recognize as the symptoms and circumstances pertaining to CTF [1]. Pioneers that colonized the Rocky Mountain region in the wake of the Lewis and Clark expedition loosely grouped together what were probably several different diseases with the term “Mountain Fever” [2]. Colorado tick fever was rst recognized as a distinct disease when Becker described and named it in 1930 [3]. Strong circumstantial evidence at the time suggested that the etiologic agent responsible for illness was carried by the Rocky Mountain wood tick Dermacentor andersoni [3,4], and concurrent work done by Florio et al. identied the causative agent as a virus [5]. It was isolated in 1948 from its tick vector [6] and thus CTF virus was designated an arthropodborne virus (arbovirus). Due to the similarities between CTF and dengue fever, it was thought that perhaps CTF virus was a tick-borne dengue. Experiments where human volunteers were sequentially infected with these viruses showed that cross-protection did not occur, proving that dengue and CTF viruses were unrelated [7,8]. Colorado tick fever virus remained taxonomically unclassied until 1971 [9] due to the fact that it, along with some other arboviruses, was resistant to lipid solvents and detergents, a feature considered unusual for arboviruses, which often possess lipid-containing envelopes [10]. Following electron microscopic research into the characteristics of the virus [11-13] and physicochemical and serologic studies [9], sufcient evidence was amassed to allow for the rst taxonomic placement of CTF virus.
