Last Update: March 2000
Author: F. A. Leighton
Reviewer: H. Artsob
The CTF virus is classified as a member of the genus Orbivirus in the family Reoviridae; the same genus and family to which the viruses that cause Epizootic Hemorrhagic Disease of deer and Bluetonge belong (both are important diseases of ungulates). It is a double-stranded RNA virus.
In Canada, CTF virus is known to occur only in southern British Columbia and Alberta at elevations of from about 1,200 to 3,000 metres. Its range is known to extend south to northern Mexico, east as far as western South Dakota and west to the Pacific Ocean at these moderately high elevations. Its range appears to be delimited by the zone of overlap of the appropriate altitude and the distribution of its principal arthropod host and vector, the tick Dermacentor andersoni.
About 10% of Snowshoe Hares sampled in the area around Ottawa Ontario in 1962 had antibodies to a CTF-like virus, but this is well outside of the established range of the CTF virus itself.
CTF virus cycles naturally among ticks and wild rodents in western mountains at elevations between about 1,200 and 3,000 metres. It has been found in 8 different species of tick, but the predominant tick vector is Dermacentor andersoni, and the distribution of CTF parallels that of D. andersoni at these elevations. Larval ticks acquire infection by feeding on an infected small mammal host. The tick then remains infected as it changes from larva to nymph and from nymph to adult. The infected nymph can, thus, infect another small mammal when it seeks out and feed on such a host. The virus does not pass from infected adult tick to the tick's eggs. Thus, the cycle of infection is maintained by a transmission cycle involving infected small mammals and the larvae and nymphs of Dermacentor andersoni and perhaps some other tick species as well. Adult ticks may pass infection on to the larger mammals they normally feed upon, including humans, but these larger mammals do not play an important role in the maintenance cycle of the virus.
Many species of rodent have been found infected with CTF virus and have been shown to infect ticks that feed upon them. The species that are probably most important in western Canada are the Golden-mantled Ground Squirrel (Spermophilus lateralis), the Bushy-tailed Woodrat (Neotoma cinerea), the Deer Mouse (Peromyscus maniculatus), and perhaps also the Least Chipmunk (Eutamias minimus) and the porcupine (Erithizon dorsatum). The distribution of the virus in Canada appears more coincident with the preferred habitat of the Golden-mantled Ground Squirrel and the Bushy-tailed Woodrat than with the range of Dermacentor andersoni, which extends eastward from the mountains onto the prairies as far as western Saskatchewan.
CTF virus is not known to cause illness in wild or domestic animal species, although disease has been produced experimentally in laboratory mice, hamsters, guinea pigs and Rhesus monkeys. Infection does not appear to affect survival of the host tick.
While the virus has been found in ticks in British Columbia and Alberta, disease caused by CTF appears to be both rare and under-reported in Canada. Serological surveys have shown that infection occurs far more often than disease is reported. Up to 1990, only one actual case of human disease caused by CTF had been diagnosed in Canada, in a young woman in southern Alberta. In North America generally, CTF has never occurred as an epidemic but it occurs regularly at a low rate among people who share its habitat. Although fatal human cases of CTF infection have occurred, they are rare; only about 0.2% of people with clinical illness due to infection have failed to recover. Most people who become ill after infection with CTF virus experience fever and general illness, sometimes with two periods of illness separated by a few days of reduced illness.
CTF has not been the object of major health surveillance or preventive programs in Canada. In areas where it occurs, human disease can be prevented by preventing contact with ticks. Humans are most likely to be infected by adult ticks which are most active from early spring to the end of snow-melt and the arrival of warm, dry conditions in late June or early July.
Artsob, H. 1990. Arbovirus activity in Canada. Archives of Virology. Supplement 1: 249-258.
Bowen, G.S. 1988. Colorado tick fever. In: Monath T.P.(ed.) The Arboviruses: Epidemiology and Ecology. Boca Raton: CRC Press, Inc. p. 159-176.
Calisher, C.H. 1994. Medically important arboviruses of the United States and Canada. Clinical Microbiology Reviews 7: 89-116.
Emmons, R.W. 1994. Colorado tick fever. In: Beran, G.W.(Editor-in-chief) Handbook of Zoonoses. 2nd Edition. Section B Viral. CRC Press Inc. Boca Raton. p.79- 87.