Colorado Tick Fever
Colorado tick fever is a febrile illness caused by an RNA virus belonging to the genus Coltivirus (short for Colorado Tick virus). The complete genome of the virus was sequenced in the year 2000. The disease is transmitted to humans, apparently exclusively, by the wood tick, Dermacentor andersoni. (Although other tick species have been shown to harbor the virus, none are known to be capable of transmitting it.) As a result, the geographic distribution of Colorado tick fever corresponds roughly to the range of the wood tick, stretching from the western Black Hills through the Rocky Mountains to the West Coast states of North America. The Canadian provinces of Alberta and British Columbia are also endemic for the disease.
Typically, ticks acquire the Colorado tick fever virus from infected small and medium-sized mammals, such as chipmunks, squirrels, mice, rats and porcupines. The principal mammalian hosts vary according to local geography. Once infected, ticks harbor the virus for life, and in turn infect other mammals when taking a new blood meal.
A few hundred human cases of Colorado tick fever are confirmed each year in the United States, but the disease is not nationally reportable and thus its true incidence is assumed to be considerably higher. The vast majority of cases occur at elevations between 4000 to 10,000 feet above sea level. The illness is usually acquired between late March and October, but most cases occur in late spring and early summer. There is little age bias in human infections, but males are more commonly infected than females, presumably because of greater outdoor exposure to ticks during vocational and recreational activities. Transmission by blood transfusion has also been reported.
Signs and Symptoms
Symptoms usually begin between 3-5 days after infection with abrupt onset of fever accompanied by headache, chills, photophobia, myalgias and malaise. Nausea, vomiting, diarrhea and abdominal pain may also be present. Some 5-15% of patients may also develop a non-specific rash. About half of all patients will experience a single recurrence of fever after initial resolution; this pattern has been referred to as a “saddleback” fever.
Generally, signs and symptoms resolve uneventfully, but neurologic complications can occur, particularly in children. Around 5-10% will develop meningitis or encephalitis (or both) within a week of illness onset. Patients of any age may experience sensory alterations, somnolence or even, in rare cases, coma. Also rare but occasionally reported complications of Colorado tick fever are pneumonitis, myocarditis and hepatitis.
Although children are more prone to severe acute disease, their illness tends to resolve more quickly, usually in around a week. In contrast, around 70% of patients 30 years or older tend to have lingering symptoms, primarily fatigue and malaise, for weeks to months.
Death from Colorado tick fever is extremely uncommon, but has been reported in a few pediatric cases.
Clinically, Colorado tick fever is difficult to distinguish from generic viral illnesses; other than the saddleback fever pattern, its signs and symptoms are non-specific. Conventional blood tests are also generally not helpful, although leukopenia is a common finding in many patients. A much smaller percentage of patients may have thrombocytopenia and/or abnormal liver function tests.
In cases of central nervous system involvement, CSF studies may shown a lymphocytic pleocytosis, elevated protein and/or mildly decreased glucose concentration, but these findings too are non-specific.
Serologic diagnosis can be made by indirect immunofluorescence, enzyme immunoassay and Western blot. Polymerase chain reaction (PCR) tests are also available for the virus and can be useful in cases with active infection. However, it is likely that many cases of Colorado tick fever are attributed to other viral infections and never correctly diagnosed.
Treatment consists primarily of supportive care, as there are no specific antiviral therapies available and most cases resolve uneventfully without treatment in any case. Aspirin is generally not recommended as an antipyretic, as it may increase the risk of hemorrhage in patients with thrombocytopenia. There is evidence that the use of ribavirin increases the survival rate in animal models, but no human studies of this treatment modality have been performed.
Infection with the Colorado tick fever virus almost always confers lifelong immunity.
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