From Big Medical Encyclopedia

COLORADO FEVER (synonym: Colorado tick-borne fever, American mountain tick-borne fever) — an acute feverish disease of the person, most often a two-phase current, caused by the virus transmitted by ixodic mites.

The disease is described in 1930 by Becker (F. E. Becker, 1930) under the name «Colorado tick-borne fever».

The natural centers To. l. are registered in mountain and forest districts zap. states of the USA.


Activator K. l. belongs to to an arbovirus (see) the sorts Orbivirus, the Reoviridae families. Virus K. l. also Magreydzhem (L. Florio, M. O. of Stewart, E. R. Mugrage, 1944) from blood of sick people and from ixodic mites was for the first time allocated by Florio, Stewart. The size of a virus 35 — 50 nanometers. It is steady against freezing and drying, but is inactivated within 30 min. at t ° 60 °.


Carrier of a virus K. l. — forest tick of Derma centor andersoni. The virus remains in mites at all stages a metamorphosis. Larvae of mites catch, eating blood of the infected rodents. In winter time the virus remains in an organism of nymphs who attack rodents in the spring and infect them, causing rather long (15 — 20 days) virusemia (see).

The disease of people is connected with a sting of a tick of D. andersoni in the natural centers (see. Natural ochagovost ) also occurs among the population having contact with this species of mites — foresters, hunters, etc., and also tourists because of whom separate cases of a disease are registered far from the natural centers. More often men catch. The highest incidence is registered in May — June and matches the period of activity of a tick of D. andersoni.

Pathogeny it is studied insufficiently.

Pathological anatomy

Pathological anatomy at To. l. at the person it is not studied since the disease proceeds it is good-quality and does not give deaths. At an experimental infection of hamsters note reduction of number of lymphocytes in the centers of follicles of an adenoid tissue of a spleen and emergence of large one-nuclear cells; coloring across Romanovsky (see. Romanovsky — Gimza a method ) reveals in them eosinophilic and basphilic cytoplasmic inclusions.

Immunity after the postponed disease, apparently, lifelong since cases of recurrent diseases are unknown.

Clinical picture

Incubation interval of 3 — 6 days. The disease develops quickly, begins with a fever y all body pains. The headache, retroorbital pains, a photophobia, nausea, a skin hyperesthesia are characteristic. Temperature quickly increases to 39 — 40 °. In 3 — 5 days critical recession of temperature and the recovery which is followed in the first days by weakness follows. The disease often proceeds dvukhfazno: the first ostrolikhoradochny attack is followed by remission (2 — 3 days) then the attack of fever repeats. Sometimes observe 3 — 4 feverish waves. Rash does not belong to usual symptoms, develops in some cases in a look makulopapul on a body, extremities and a face or in the form of petechias on extremities.

Complications are observed rather seldom, preferential at children aged up to 10 years, in the form of encephalitis or heavy bleedings (nasal, oral, gastrointestinal, vaginal) which are followed by usually skin rashes like purpura or petechias.

The diagnosis

the Diagnosis is made on the basis of the described a wedge, pictures, characteristic two-phase fever, a leukopenia (to 2000 — 3000 cells in 1 mm3) appearing for the 3rd day and proceeding to 5 — the 6th day of a disease. Help diagnosis of the data on stay of the patient in the local center.

As confirmation of the diagnosis serves allocation of a virus from blood or the cerebrospinal liquid taken in time of the first feverish wave. A research of blood serum had in RSK with antigen from a virus K. l. yields positive takes in 4 — 6 weeks from an onset of the illness. The neutralization test of a virus becomes positive already on 2 — 4th week.

Differentiate To. l. from spotty fevers of the Rocky Mountains (see), having similar geographical distribution, and flu. Spotty fever of the Rocky Mountains usually proceeds odnofazno and more often is followed by a leukocytosis, than a leukopenia. Flu differs from To. l. the respiratory symptoms which are absent at To. l. Virusol. and serol, researches allow to differentiate accurately To. l. from the specified diseases.

Treatment and Forecast

symptomatic treatment.

The forecast favorable even in the complicated cases.


Prevention is based on measures of protection of people from attack ixodic mites (see). For this purpose use special suits, protecting screens (see) and repellents (see). At stay in the center periodic surveys for removal of the mites which got on clothes and got under it are effective.

Bibliography: Viral and rickettsial infections of the person, under the editorship of T. Rivers, the lane with English, page 587, M., 1955; Laboratory diagnosis of viral and rickettsial diseases, under the editorship of E. Lennet and N. Schmidt, the lane with English, page 185, M., 1974; Control of communicable diseases in man, ed. by A. S. Benenson, p. 24, N. Y., 1970; Silver H. K., Meiklejohn G. K e m p e of Page H. Colorado tick fever, Amer. J. Dis. Child., v.-101, p. 30, 1961; Spr nance S.L. Bailey A. Colorado tick fever, Arch, intern. Med., v. 131, p. 288, 1973; Viral and rickettsial infections of man, ed. by F. L. Horsfall a. I. Tamm, Philadelphia — Toronto, 1965.

S.G. Drozdov.