Tick species that transmit Colorado Tick Fever: Rocky Mountain wood tick (Dermacentor andersoni)
What is Colorado Tick Fever?
Colorado tick fever (CTF), also known as Mountain tick fever or American mountain fever, is a viral disease caused by infection with the Colorado tick fever virus (CTFV), a member of the Coltivirus genera. CTF is transmitted to humans most commonly by the bite of an infected adult wood tick, and while there is no evidence of natural person-to-person transmission, rare cases of transmission by blood transfusion have been reported.
The diagnosis of persons with CTF is complicated by non-specific nature of the symptoms. Most persons experience fever, chills, headache, muscular and skeletal pain, and malaise; these signs and symptoms can be confused with other infectious and non-infectious diseases. A petehcial (spotted) rash occurs in 5-12% of CTF cases.
A closely related virus, Eyach virus, transmitted by the European sheep tick, Ixodes ricinus, has been reported in parts of Western Europe.
Where is Colorado Tick Fever Prevalent?
Colorado tick fever occurs primarily in the Rocky Mountain region of the western United States as well as the Canadian provinces of British Columbia and Alberta. More than 90% of all CTF cases in the United States are reported from Colorado, Utah and Montana.
The disease is most prevalent during the summer months between April and August, and is usually limited to mountainous elevations between 1,200 and 3,000 meters. Patients with CTF are most often campers and young men, who have been exposed to tick bites during outdoor recreational activities.
Clinical manifestations of CTF can range from mild to life-threatening depending on the patient’s age and general health.
The first symptoms of CTF usually occur 3-7 days after a tick bite, although the incubation period can be as long as 20 days. The initial symptoms of the disease often include fever, chills, headache, muscular and skeletal pain, and malaise. Other symptoms may include nausea, vomiting, stomach pain, light sensitivity and sore throat. About half of all patients experience a two-staged “saddleback” fever characterized by 2 to 3 days of acute fever followed by a brief remission of the fever, followed by a second acute fever that may be more severe than the first. A petehcial (spotted) rash occurs in 5-12% of CTF cases.
In rare cases, patients experience illnesses of the central nervous system (CNS) ranging from mild to encephalitis with coma and death. CNS illnesses are commonly characterized by severe headache, sensory impairment, neck stiffness and light sensitivity.
An initial diagnosis is based on the patient’s signs and symptoms and confirmation depends on laboratory testing. The appearance of a saddleback fever and the absence of a hemorrhagic rash common to Rocky Mountain spotted fever are clinical indicators of CTF. Leukopenia, a decrease in the number of circulating white blood cells, is the most common laboratory finding in CTF.
The immunoflourescent staining of blood smears may be used to identify CTFV antigens, however a confirmed diagnosis can only be made using PCR (Polymerase Chain Reaction).
No specific treatment for CTF is available. Management of CTF includes treatment of fever and pain with analgesics and acetaminophen, along with standard infection control procedures. Patients infected with CTF should advise blood collection agencies of their illness prior to donation, due to the risk of transmitting CTF through blood transfusion.