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 an RNA virus and a member of the Reovirus family.

Epidemiology^[2]

• Several hundred cases are reported annually in the US.
• The disease is limited to altitudes above 4000 feet. The tick favours grassy areas, and the plant "big sagebrush" (Artemisia tridentata) can be an indicator of such areas, where risk of this tick bite is increased.^[3] Small mammals eg chipmunks and squirrels are the ticks' host.
• Transmission by blood transfusion is also possible.
• The number of actual cases may be higher than those reported, because many may be unrecognised.

Presentation^{[2, 4, 5]}

• Patients may not have noticed the tick bite.
• Symptoms usually begin about 4-5 days after the tick bite, but the incubation period can range from 1-19 days.
• Typical symptoms are fever, severe myalgia and headache. The fever is typically a saddleback pattern, which starts abruptly, continues for 3 days, resolves and then recurs 1-3 days later for another few days.
• Other symptoms include orbital pain, conjunctivitis, arthralgia, nausea/vomiting and possibly sore throat.
• 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 7-10 days.

Differential diagnosis^[1]

• Note that more than one tick-borne infection can occur from a single tick bite.
• Similar presentations may occur with a variety of tick-borne infections, including Q fever, Lyme disease, Ehrlichiosis, Tularaemia, Rocky Mountain Spotted Fever, Relapsing Fever.

Investigations^[1]

• Full blood count may show leucopenia and thrombocytopenia.
• Laboratory testing for the virus will depend on local availability, but the following techniques may be used:
  • Blood smears stained for the virus with immunofluorescence.^[1]
  • PCR assay.^[6]
  • Antibodies to the Colorado tick virus appear from about day 10 of the illness. However, antibodies can also be found in campers who regularly visit endemic areas, so single elevated titres of IgG do not necessarily indicate acute infection. A rise in titres during the acute illness helps confirm the diagnosis. An ELISA assay for antibodies has been developed.^[7]
• The virus can be detected in the blood for 2-4 weeks after infection.

Management^[1]

• Ensure the tick is fully removed from the skin (see prevention, below).
• Management is supportive.
• At the onset of symptoms, empirical treatment such as doxycycline is usually started, to cover for other possible tick-borne diseases until the diagnosis known.
• No specific treatment exists, though ribavirin may have a role in some cases.^[8]

Complications

Complications are rare. The following have been reported:

• Aseptic meningitis, encephalitis, thrombocytopenia, haemorrhagic fever, atypical pneumonia, hepatitis, pericarditis and orchitis. Children occasionally have haemorrhagic complications such as severe rash, GI bleeding or disseminated intravascular coagulation.^[4]
• However patients who are immunocompromised or who have undergone a splenectomy are at increased risk for severe complications.^[1]

Prognosis^{[1, 4]}

• 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.

Prevention^[1]

• Protection against tick bites by tucking long trousers into socks, wearing long-sleeved shirts and using bed nets.
• Remove ticks as soon as possible. Prompt removal helps prevent virus transmission, because transmission of infection requires 24-48 hours of tick attachment to host.
• To remove a tick, use a commercial device for tick removal, or blunt, angled forceps. Grasp the body of the tick gently and use vertical traction to dislodge it.
• Tick repellants include those containing DEET; permethrin may help on clothing.
• 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.