Tick-borne viruses

Colorado tick fever

The causative agent of Colorado tick fever (CTF) is an orbivirus of the genus Coltivirus which is related to Eyach virus in Europe. It is the only member of this group found in the USA or Canada apart from Sunday Canyon virus which has been isolated from the tick Argas cooleyi in Texas.

The distribution of CTF is the same as that of its principal tick vector, Dermacentor andersoni. In the USA, the disease is found in California, Colorado, Idaho, Montana, Nevada, Oregon, Utah, Washington, Wyoming, and in Canada in British Columbia and Alberta. Many of the endemic areas are vacation destinations, frequented during the tick season (spring and early summer) by large numbers of tourists. Colorado tick fever virus has been isolated from humans vacationing or travelling through these states and physicians should be aware of the possibility of CTF in febrile patients returning from these regions (Calisher and Craven, 1998). Some 200–400 cases are reported each year. The disease is limited to elevations above 4000 feet. The number of reported cases is probably a small fraction of actual cases because many cases are either not diagnosed or are unreported. Campers, hikers and foresters are commonly infected. Infections occur mainly in April to June, when adult ticks are abundant. In 3–15% of infected children under 10 years of age, CTF virus invades the central nervous system (CNS) and causes encephalitis. Virus has been isolated from the cerebrospinal fluid of some of these patients.