Colorado tick fever

Alternative names
Mountain tick fever; Mountain fever; American mountain fever

Colorado tick fever is an acute viral infection transmitted by the bite of the Dermacentor andersoni tick.

Causes, incidence, and risk factors

This disease is limited to the western US and is most prevalent from March to September, with the highest numbers of infections occurring in May and June.

Symptoms start about 3 to 6 days after the tick bite. Symptoms of fever continue for 3 days, stop, then recur 1 to 3 days later for another few days.

Risk factors are recent outdoor activity and recent tick bite. The incidence is high in Colorado, where up to 15% of regular campers show past exposure (based on antibodies). It is much less common in the rest of the US.


  • abrupt onset of fever  
  • sweating, excessive  
  • severe muscle aches  
  • joint stiffness  
  • headache  
  • photophobia (sensitivity to light)  
  • nausea and vomiting  
  • generalized weakness  
  • occasional faint rash

Signs and tests

  • blood tests that confirm infection (usually several weeks later)       o complement fixation to Colorado tick virus       o immunofluorescence positive for Colorado tick fever  
  • CBC shows low white blood cell count  
  • mild increase in liver function tests  
  • mild increase in creatine phosphokinase

Make sure the tick is fully removed from the skin. Take a pain reliever if necessary (do not give aspirin to children; it is associated with Reye’s syndrome in some viral illnesses). If complications develop, treatment will be aimed at controlling the symptoms.

Expectations (prognosis)
The disease is usually self-limiting and not dangerous.

There is a risk for aseptic meningitis, encephalitis, and hemorrhagic fever, but these complications are extremely rare.

Calling your health care provider
Call your health care provider if you are unable to fully remove a tick embedded in the skin, if you or your child develop symptoms suggestive of this disease, if symptoms worsen or do not improve with treatment, or if new symptoms develop.


When walking or hiking in tick-infested areas, tuck long pants into socks to protect the legs, and wear shoes and long-sleeved shirts. Ticks will show up on white or light colors better than dark colors, making them easier to remove from your clothing.

Check yourself and your pets frequently. If you find ticks, remove them immediately by using a tweezers, pulling carefully and steadily. Insect repellent may be helpful.

Johns Hopkins patient information

Last revised: December 4, 2012
by Harutyun Medina, M.D.

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All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.