What Is Colorado Tick Fever:Symptoms,Treatment,Diagnosis

Colorado tick fever is an acute, tick-transmitted viral disease that occurs through­out the Rocky Mountain area, and is character­ized by a biphasic febrile course and profound leukopenia.


Colorado tick fever virus is a member of the arbovirus group. The virus contains RNA. is small in size, and has not yet been shown be related to other members of the arbc virus gr :c: The virus is readily isolated from  suckling mice or tissue culture. The nurse has been isolated from a wide range of ticks and number of species of small mammals in the Body  Mountain area. It appears, in the wood tick, Dermacentor andersonI is impor­tant in human transmission, because human  are limited to the geographic distribution of this vector. All the states in the Rocky Mountain area have reported cases, the largest number occurring in Colorado. Cases also occur along the eastern border of California and in the western canada the provinces


The disease occurs in the spring and summer, when tick exposure is common. At lower elevations, cases are observed from March onward; at higher altitudes, they occur late into the summer, reflecting the slower emergence of ticks at the higher altitudes. The reservoir of the disease is probably in numerous small mammalian hosts, particularly ground squirrels and chip­munks, which have a prolonged viremia and infect larvae or nymphs. The latter maintain the virus over the winter months, and the adult tick then transmits the virus to another small mammalian host or to man.

In the majority of patients, tick attachment has been observed, and the tick may be present at the time of examination. Other patients, however, state that they have seen ticks on themselves or in their clothes, but that the ticks have not become attached. In a few instances, the ticks have been transported in bedding to distant locations and have caused disease outside the endemic area. Patients usually develop symptoms of illness three to seven days after tick exposure.


Viremia can be detected at the time of onset of fever, and persists throughout the febrile course. The virus may persist in or in association with the red blood cells long after it has disappeared from the serum. This has been shown, in both experimentally infected rodents and man. Red cells from human patients, pre­sumably washed free of antibody, were found to contain virus as long as 39 days after the onset of illness.

The disease is characterized by a profound leukopenia, which is usually greatest during the second febrile episode. The most striking decrease is found in the granulocytes, but there is also frequently a marked thrombocytopenia and some anemia. Pathologic data on the disease in man are meager, because there has been only one reported fatal case. In this instance, the pathologic changes were those of an extensive hemorrhagic diathesis and encephalitis.

Symptoms of Colorado Tick Fever and Clinical Course.

The onset is usu­ally sudden with chilly sensations, malaise, and fever. Muscle achirig and pain about the joints are common manifestations, along with headache and backache. In children, anorexia, nausea, and vomiting are common. The disease is character­istically biphasic. The first episode of fever usu­ally lasts two or three days, following which the temperature returns to normal. After a day of nor­mal temperature, the fever again rises, and the second cycle usually approximates the first in length. Occasionally, there may be only a single febrile bout, or there may be three febrile episodes.

The height of fever is extremely variable, ranging from 99.6 up to 105° F. Rashes are uncommon, but a small proportion of patients have petechial or macular rashes. Involvement of the central nervous system, especially in children, is an un­common but more serious manifestation. In such patients,’ headache is frequently more severe. Signs of meningeal irritation are present, and the cerebrospinal fluid may show a pleocytosis. Pa­tients who develop encephalitis have findings similar to those infected with other arboviruses such as disorientation, coma, and convulsions. Hemorrhagic manifestations, including epistaxis and gastrointestinal bleeding, occur very infre­quently and are most commonly seen in patients with encephalitis.


The diagnosis is based on a history of tick exposure during the preceding three to seven days, followed by a biphasic course with leukopenia. The initial febrile episode cannot be readily differentiated from many other acute febrile illnesses. However, by the time the second cycle has started, after an afebrile day during which the patient has felt moderately well, it becomes apparent that the illness is different. At this time, the peripheral blood count shows a profound leukopenia. The physical findings are not specific. A rash may be present that suggests an enterovirus infection. Moderate lymphadenopathy may be observed, and splenomegaly of mild degree is often present.

The diagnosis is confirmed by isolation of the virus from either serum or whole blood by the inoculation of suckling mice. These are still more sensitive indicators of the presence of virus than available tissue culture systems. The examination of acute and convalescent sera will also usually reveal the development of both neutralizing and complement-fixing antibodies.

The most difficult point in differential diagnosis arises from the fact that both Colorado tick fever and Rocky Mountain spotted fever are transmitted by Dermacentor andersoni. Rocky Mountain spotted fever may resemble Colorado tick fever during the first day or two before a rash becomes obvious. This differentiation is extremely impor­tant, because the early treatment of Rocky Mountain spotted fever with tetracycline will abort the illness and cure the patient.


Protection against ticks by appro­priate clothing or repellents is possible in tick- infested areas, but is rarely carried out. Frequent inspection for ticks and their removal may be helpful, but it has been noted earlier that pro­longed tick attachment does not appear to be necessary for transmission of the disease.Vaccines have been prepared from purified, infected suckling mouse brains. Repeated injec­tions of these vaccines produce neutralizing anti­body titer in recipients at levels that would prob­ably provide a high degree of protection. These vaccines, however, have not been tested under field conditions.


The disease almost invariably runs a benign course, and the patient recovers in a relatively short period after defervescence. The rare exception is the patient who develops either an encephalitic or hemorrhagic tendency. Im­munity is long. lasting, and second infections in later life have not been reported.

Treatment of Colorado Tick Fever

Treatment is symptomatic. Anal­gesic drugs such as salicylates are usually ade­quate to control the myalgia and headache. When the patient appears seriously ill and the diagnosis of Rocky Mountain spotted fever is seriously entertained, it is advisable to give tetracycline in doses of 2.0 grams per day. If the patient has Rocky Mountain spotted fever, the temperature usually falls to normal within 48 hours, whereas the course cf Colorado tick fever is not altered by this.regimen.

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