Colorado tick fever (CTF) is an underreported, usually mild febrile disease occurring in the US and Canada. It is caused by a virus belonging to the genus Coltivirus (named from Colorado tick fever virus). CTF manifests with symptoms including abrupt onset and initial features of high fever, chills, joint and muscle pains, severe headache, ocular pain, conjunctival injection, nausea, and occasional vomiting. Typically there are two periods of febrile illness occurring in the spring or summer, interrupted by a brief, symptom-free interval. Infection may remain even when symptoms have passed. CTF is not usually a life-threatening illness but can cause severe discomfort, and symptoms may last for some weeks. Children can exhibit a more severe form of the disease, with the involvement of internal organs and in extreme form affecting the central nervous system, producing aseptic meningitis and encephalitis. Treatment is symptomatic.
CTF virus is transmitted exclusively by female wood ticks Dermacentor andersoni in North America. As with many other tick species, female D. andersoni ticks are blood feeders, requiring certain protein substances in blood in order to produce eggs. The virus over-winters in nymphal ticks and emerges in warmer months. Infection of ticks by CTF virus is considered lifelong, with no observable pathologic changes in, or untoward effects on, the tick.
In the 19th Century, settlers and visitors to the Rocky Mountain region of North America mentioned "mountain fever" as a cause of morbidity and occasional mortality. "Mountain fever" likely was any of a number of febrile illnesses, including typhoid fever, Rocky Mountain spotted fever (RMSF) and Colorado tick fever.
The disease is most frequent in males age 10 to 50 years, having an outdoor occupation, engaging in frequent or lengthy outdoor recreational activities, and being exposed to ticks in endemic mountain areas in the period April through June. An analysis of 606 CTF cases by age and sex revealed that males in general and people aged 20 to 29 had the highest risk of acquiring the disease.
Landscapes where viral incidence is high occur in western USA and Canada; they typically have have open stands of ponderosa pine, and shrubs on dry, rocky surfaces. These specific ecologic characteristics comprised specific habitats for the small mammals on which the ticks depend for both blood meals and shelter, which include the golden mantled ground squirrel Spermophilus lateralis, porcupine Erethizon dorsatum, least chipmunk Tamias minimus, deer mouse Peromyscus maniculatus, and bushy-tailed woodrat Neotoma cinerea.
The geographic distribution, principal arthropod vector, and vertebrate hosts of CTF virus have been well established for many years, yet this disease continues to be under-recognized and underreported to health departments.