Colorado Tick Fever

Synonyms: Mountain tick fever; Mountain fever; American mountain fever

Colorado tick fever is an acute viral infection transmitted by the bite of the wood tick, Dermacentor andersoni.¹ The disease occurs almost exclusively in the western United States and Canada and is most prevalent from March to September.² The causative organism, Coltivirus, is a member of the Reovirus family, distinguished by their genome of double-stranded RNA.

• Several hundred cases are reported annually in the US.
• The disease is limited to elevations above 4000 feet.
• The number of reported cases is probably a small fraction of actual cases because many cases are either not diagnosed or are unreported.
• Most patients are aged 15-45 years, because they are the most likely to have participated in recent outdoor activity in endemic areas.

• Symptoms usually begin about 4-5 days after the tick bite.
• Fever starts abruptly and then continues for 3 days, resolves and then recurs 1 to 3 days later for another few days.
• Flu-like symptoms of headache, myalgia, arthralgia, conjunctivitis and fatigue also develop.³
• A non-specific evanescent rash develops in about 10% of infections.
• Examination is not very helpful in diagnosis. Findings may include a maculopapular and petechial rash on the trunk. The rash tends to be short lived.
• The disease usually lasts 7 to 10 days.

Similar presentations may occur with a variety of infections, including Q fever, Lyme disease, Ehrlichiosis, Tularaemia, Rocky Mountain Spotted Fever, Relapsing Fever.

• Full blood count may show leucopenia and thrombocytopenia.
• Diagnosis usually is made with blood smears, which are stained for the virus with immunofluorescence.¹
• Antibodies to the Colorado tick virus are often found in campers who regularly visit endemic areas and so single elevated titres of IgG do not necessarily indicate acute infection.
• The virus can be detected in the blood for 2-4 weeks after infection but viral testing is not routinely available.

• Ensure the tick is fully removed from the skin.
• No specific treatment exists and management is supportive and focused on controlling temperature and increasing fluid intake.
• At the onset of symptoms, empirical treatment with tetracycline, doxycycline or chloramphenicol is usually started to cover for other possible tick-borne diseases until the diagnosis is confirmed.¹

• There is a risk for aseptic meningitis, encephalitis, thrombocytopenia and haemorrhagic fever, but these complications are extremely rare.
• However patients who are immunocompromised or who have undergone a splenectomy are at increased risk for severe complications.¹

• The disease is usually self-limiting and the prognosis is excellent, even in cases complicated by neurological symptoms.
• Rare fatalities have been reported and these cases have shown evidence of severe disseminated intravascular coagulation and thrombocytopenia.

• Protection against tick bites by tucking long trousers into socks, wearing long-sleeved shirts.
• Remove ticks immediately with tweezers, avoiding squeezing the body.
• Insect repellent may be helpful.
• Routine prophylaxis with antibiotics following uncomplicated tick bites is not indicated.
• The virus can live in red blood cells for the life of the red cell and so blood donation is prohibited in patients for 6 months following infection.