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Colorado tick fever is a viral infection spread by a bite from an infected Dermacentor andersoni wood tick. This species of tick is more commonly referred to as the Rocky Mountain Wood tick.
All ticks are parasites that feed on blood. Ticks are more common at higher elevations. Colorado tick fever is more prevalent in Colorado than in the rest of the United States. The disease is limited to the Western US and Canada. Incidence of Colorado tick fever is highest between February and October, with 90 percent of cases being reported between April and July. You are at increased risk if you spend time outdoors in tick-infested areas.
Symptoms begin within three to six days after a tick bite and include fever, chills, headache, nausea, and vomiting. These 3 to 6 days are referred to as the period of incubation. This means that symptoms immediately after a tick bite are not likely caused by tick fever. In most cases, symptoms resolve on their own within 10 days. Most patients make a full recovery.
Signs and Symptoms of Colorado Tick Fever
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.
Recognizing the Symptoms of Colorado Tick Fever
Symptoms of Colorado tick fever include:
- sudden fever up to 105 degrees Fahrenheit
- headache behind the eyes
- light sensitivity (photophobia)
- muscle aches
- skin pain
- anorexia (loss of the desire to eat)
- nausea and vomiting
- abdominal pain
- general weakness and fatigue
- rash this is faint and occasional
A fever associated with Colorado tick fever generally lasts two to three days, and then subsides for a few days, returning for another 48 hours or so. For most people, symptoms disappear completely in about ten days.
Symptoms of Colorado tick fever are similar to those of other infectious diseases. Consult with your doctor if you develop severe symptoms of Colorado tick fever, your symptoms don’t improve within a week, or if you cannot completely remove a tick embedded in your skin.
How to Diagnosed Colorado Tick Fever
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 for Colorado Tick Fever
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.