Colorado Tick Fever


With fewer than ten cases per year in the US, Colorado tick fever is exclusively transmitted by the Rocky Mountain wood tick, Dermacentor andersnoi. The tick itself acquires the virus from small and medium sized mammals. Apart from the Rocky Mountains, it can be found in the Canadian provinces of Alberta and British Columbia.

Signs and Symptoms

Following infection, symptoms that may occur include fever, headache, chills, light sensitivity, muscle aches, malaise, nausea, vomiting, diarrhea, abdominal pain, and rarely a rash. Symptoms tend to appear abruptly, three to five days following infection. "Saddleback fever" occurs in about half of patients, which means that following initial resolution of symptoms, a single recurrence of fever comes about.


Other than the above mentioned "saddleback fever", the progression of Colorado tick fever is nonspecific making it difficult to diagnose. It is assumed that many cases are not diagnosed and rather attributed to another infection. Blood tests may show low white blood cell count, low platelets, or abnormal liver function. Cerebrospinal fluid may show a lymphocyctic pleocytosis, elevated protein, or mildly decreased glucose concentration. Also of these findings are nonspedcific.