Facts about Colorado Tick Fever

Colorado tick fever (CTF) is a tick-borne viral illness, which is found in the Rocky Mountains region of the United States and southwest Canada. In the past, this disease was also known as Mountain tick fever, Mountain fever, or American mountain fever. CTF is caused by the Colorado tick fever virus which is a member of the Coltivirus genera and is an arthropod-borne virus or arbovirus. As an arbovirus this virus is maintained in nature in cycles involving arthropod vectors and susceptible vertebrate hosts. An infected arthropod (in this case a Rocky Mountain wood tick) transmits the virus to a vertebrate host (voles, chipmunks, rabbits, marmots, deer, etc.) in the process of feeding. The virus multiplies in the new host until it reaches a level in the blood that a second tick feeding on this host can pick up the virus. The virus replicates in the tick’s midgut and is disseminated to other tissues including the tick’s saliva gland, where it then replicates some more, and can be passed on to yet another host while this tick is feeding and the cycle continues. CTF virus is transmitted exclusively by female wood ticks in North America. As with many other tick species, female Rocky Mountain wood ticks require certain protein substances in blood in order to produce eggs. Sometimes an infested Rocky Mountain wood tick finds a human to feed on and the Colorado tick fever virus is passed on to the human and the human becomes sick with Colorado tick fever. The human is what is known as a “dead-end host” meaning that the virus does not reach a level in the blood of the host, so that a vector can bite the human and become infected with the virus. This virus has also been associated with exposures to the virus in laboratory settings and one case followed transfusion of blood from a person infected with CTF virus within 4 months of donation.

The distribution of CTF virus roughly approximates that of its vector tick, the Rocky Mountain wood tick. The virus has been isolated from humans and from ticks in California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, South Dakota, Utah, Washington, and Wyoming, and also in Canada’s southern Alberta and British Columbia. Many of these areas are vacation destinations, frequented during tick season (spring and early summer) by large numbers of tourists, and the virus has been isolated from humans vacationing or traveling through these states. This disease continues to be under-recognized and under-reported to state health departments. The primary risk factor for acquiring CTF is exposure to infected Rocky Mountain wood ticks in CFT virus-endemic areas. Other risk factors are male gender, 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. Studies have shown that a tick must usually be attached for several hours to transmit enough virus to cause illness.

Classically, symptoms of CTF appear abruptly, with initial features including high fever, chills, joint and muscle pains, severe headache, ocular pain, conjunctival injection, nausea, and occasional vomiting. Fever, headache, lumbar pain, aching in the extremities, and anorexia may continue for a few days followed by a time of feeling better and then a relapse with symptoms reoccurring again for 2 or 3 days. In the vast majority of cases the illness is mild-to-severe. But can be very severe in cases involving children and have even required hospitalization. Complications with this disease have included aseptic meningitis, encephalitis, and hemorrhagic fever, but these are rare.

The incubation period for CTF ranges from less than 1day to 19 days (average, about 4 days), possibly dependent on the dose of CTF virus the person receives from the infecting tick. In addition to the characteristic early clinical signs and symptoms, a person’s history of tick exposure or knowledge of having been fed on by ticks may provide preliminary evidence of CTF, indeed 90% of patients recall having had an attached tick or having seen a tick crawling on their body or clothing. CTF has been confused with Rocky Mountain spotted fever, tick-borne tularemia, relapsing fever, and acute rheumatic fever. Differential diagnosis of these infections is important because CTF is not treatable with antibiotics.

At present, there is no specific antiviral therapy for CTF: No anti-CTF virus drugs have undergone trials in humans and none are licensed for human use. No vaccines are available for pre-exposure prophylaxis for CTF virus. Therapeutic efforts are directed towards managing symptoms, such as reducing fever and relieving headache. CTF virus is not transmitted person-to-person, but care should be taken in handling tissues and body fluids because they may contain virus and laboratory infections can occur.

Education is the best defense. You should be aware of CTF in your home areas and in the areas you visit, and you should learn to avoid contact with suspected ticks during the transmission season. To avoid ticks:
* It is important to stay out of woods where infected ticks are common and to walk only on recommended paths.
* Wear light colored clothing when walking or hiking. This will allow you to see ticks that are crawling on your clothes.
* Wear long sleeve shirts and tuck your pants legs into your socks.
* Repellants such as permethrin can be sprayed on boots and clothing and will last for days. Repellants containing DEET will only last for a few hours.
* After returning from a tick-infested area, conduct a body search using a mirror.
* Children and pets should be checked for ticks in hidden areas such as hair.

Contracting the CTF virus is thought to provide long lasting immunity against reinfection. However it is always wise to be on the safe side and try to prevent tick bites.