Colorado Tick Fever *

Transcription

1 Vol. 37 Colorado Tick Fever * LLOYD FLORIO, M.D., DR.P.H., F.A.P.H.A., AND MABEL 0. STEWART Department of Public Health and Laboratory Diagnosis, University of Colorado School of Medicine, Denver, Colo. DISEASES have occasionally been given geographical names, such as Rocky Mountain spotted fever, because they were originally thought to be limited to sharply circumscribed areas. With the dissemination of information concerning such diseases, it has usually been found that their distribution is much more cosmopolitan than was once supposed. This might also be true for Colorado tick fever, a disease frequently recognized in Colorado and occasionally in other Rocky Mountain states. The larger number of cases diagnosed in Colorado can be explained by the interest created by Becker1 who described it as a distinct disease entity and who gave it the name by which it is now known. EPIDEMIOLOGY Individuals with Colorado tick fever practically invariably give a history of being bitten by the wood tick, Dermacentor andersoni Stiles. In every instance, the patient has been in a tick infested area 4 to 6 days prior to the onset of the illness. The disease, like Rocky Mountain spotted fever, is found only during the spring and early summer when the ticks are active. As soon as it becomes hot and dry, the wood ticks disappear and so does Colorado tick fever. Although experimental * Presented before the Epidemiology Section of the American Public Health Association at the Seventyfourth Annual Meeting in Cleveland, Oh-o, November 14, work on the epidemiology of the disease still continues, the wood tick has been found to be naturally infected.2 Laboratory studies indicate that the infectious agent is transmitted transovarially to at least the next generation of adults.2 CLINICAL The onset is sudden and characterized by chilly sensations and a generalized aching of the whole body. Headache, deep ocular pain, and backache particularly in the lumbar region are the prominent symptoms. Photophobia is relatively frequent. Anorexia, nausea and sometimes vomiting are common. With the onset of symptoms, the temperature begins to rise and within 24 hours is usually between 102 and 1040 F. There is a corresponding elevation of the pulse rate. The attack lasts for approximately 2 days. The temperature returns to normal, the symptoms completely disappear, and the individual feels well enough to resume normal activity. This symptom-free phase of approximately 2 days is followed by a recurrence of the symptoms and fever of about 3 days' duration. Convalescence is characterized by mild weakness and lassitude lasting 3 to 7 days. Single and triple attacks are occasionally observed. While periods of attack, remission and attack usually last approximately 2 or 3 days, each may varv from 1 to 4 days. Either attack may be more severe than the [2931

2 294 AMERICAN JOURNAIL OF PUBLIC HEALTH Mar., 1947 other. During the remission, it is common for the temperature to be subnormal. Conjunctival injection and mild erythema are the usual and only physical findings other than the elevation of temperature and pulse rate. Exanthemata are never observed. Complications are unknown, nor is the disease ever fatal. Treatment is purely symptomatic. Aspirin and codeine suffice in the aileviation of symptoms. The only significant laboratory findings are related to the white blood cells. Beginning with the onset of symptoms, there is a progressive decrease in the number of leucocytes which usually reach the lowest point at the beginning of the second attack. Typically the white blood cell count falls to between 2,000 and 3,000 cells per cubic milli- meter, although one as low as 1,200 has been observed.3 All of the leucocytes are reduced in absolute numbers except the monocytes. There is a definite shift to the left in the polymorphonuclear neutrophils. It is common for the band forms to outnumber the segmenters when the count is lowest. Four to 7 days following clinical recovery, the white blood cell and differential counts have returned to normal. The differential diagnosis is not difficult. There are no other diseases occurring in the Rocky Mountain region with which Colorado tick fever can be confused. The history of exposure to ticks, the saddle-back temperature curve, the symptoms, and the white blood cell picture have made the diagnosis obvious after 4 or 5 days of observation. Figures 1, 2, and 3 depict 105. Deep Ocular Pait v ^ Anorexia White Female, CASE 1 Uadche Age 41I ' Incubation ReY Sdaqfl!99 bv 97_ *Dayi S Leucocy,es in ThQutandi per c.mm. Seymenters % 8f 30 Band Forms %o 17 Ly mphocydes % - 13 SO Monocydes 2 3 Eosinophils % Basoph ils % I *'Schilling hemogram not done FIGuRE 1-A Naturally Acquired Case of Colorado Tick Fever

3 Vol. 37 COLoRADo TicK FEVER 29S Whit e 105- C qa ale - Afge 29 z t Incubatiorx Q. 3 datjs IE9 Muscle and foine Pain Conj.ncef*val l2jection- Deep Ocular dain CIa izly Moaxche IenSazions Backache Days LeucoyeVeg in 'hoiusands per c.mm. Se,menieers Si Band Forms % ; I734 9 Lymphocyfes % t S5 Monocyes Eosinophils I4, Bdsoph ils % I I Si 3 I FIGURE 2-Colorado tick fever in an experimental subject White Male Ale 23 Inecubation 4 datys VS_ CASE 3 Muscle andloint Pain ConyuncLival Inyection Chill Senralions Ieadache Backtche Days leucocycfes in S40 Thousands per cmmr. Seynmenters % Band Forms V% is 9 zymphocyees % fonoc res % Eosinophils % I FGao fpha x 1%'e l FIGURE 3-Colorado tick fever in an experimental subject

4 296 AMERICAN JOURNAL OF PUBLIC HEALTH Mar., 1947 the history, physical findings, temperature curve, and white blood cell findings in three instances of the disease. Reinfection with Colorado tick fever has never been reported. Three volunteers were inoculated with a different strain of the infectious agent 9 to 12 months after the original infection.3 They did not develop the disease. ETIOLOGY Previous attempts to visualize the infectious agent of Colorado tick fever microscopically or by culture had been unsuccessful as had all previous attempts to induce infection in experimental animals.4 We were able to infect the golden hamster (Cricetus auratus).3 5 Filtrates from gradacol membranes of known porosity were tested on these animals and it was established that the infectious agent is an extremely small virus of a particle size approximating that of yellow fever and poliomyelitis.0 Kaprowski and Cox 7 have recently reported the adaptation of the virus to mice and to developing chick embryo. RELATIONSHIP OF COLORADO TICK FEVER TO ROCKY MOUNTAIN SPOTTED FEVER AND DENGUE Because Colorado tick fever is transmitted by the same vector as Rocky Mountain spotted fever, there has arisen a belief that this disease is a mild manifestation of Rocky Mountain spotted fever. Immunization against Rocky Mountain spotted fever does not protect against experimental3 or naturally acquired8 Colorado tick fever. Two cases of individuals who had had both diseases have been brought to our attention.9 Another is reported by Shaffer.10 Finally the fact that Rocky Mountain spotted fever is due to a Rickettsia while Colorado tick fever is caused by a virus, establishes these diseases as separate clinical entities. The striking clinical and hematological resemblance to dengue, however, has caused various investigators to wonder whether Colorado tick fever might not be tick-borne dengue. The lack of a rash in Colorado tick fever, and the absence of a prolonged convalescence, are the important clinical differences. Since both diseases confer at least a short immunity to themselves, each should confer immunity against the other if they are identical. The problem was resolved by inoculating human subjects first with the virus of one disease and, after a period long enough to develop an immunity, with the virus of the other. They developed both diseases, indicating that the two conditions are probably distinct entities." Using essentially the same technic, Pollard and his coworkers confirmed these results.12 SUMMARY Colorado tick fever is a tick-borne viral disease, confined as far as is now known to the Rocky Mountain region. It is characterized by generalized aching, a saddle-back temperature with complete disappearance of symptoms during the remission and a markedly reduced white blood cell count with a shift to the left. The disease confers a definite immunity. Neither death nor complications occur. Treatment is symptomatic. While clinically and hematologically similar to dengue and often assumed to be a mild form of Rocky Mountain spotted fever, Colorado tick fever is a distinct disease entity. REFERENCES 1. Becker, F. E. Tick-Borne Infections in Colorado. Colorado Med., 27:36, 87 (Feb.), Unpublished data. 3. Florio, L., Stewart, M. O., and Mugrage, E. R. The Experimental Trangmission of Colorado Tick Fever. J. Exper. Med., 80:165 (Sept.), Topping, 'N. H., Cullyford, J. S., and Davis, G. E. Colorado Tick Fever. Pub. Health Rep., U.S. P.H.S., 55:2224 (Nov.), Stewart, M. 0., Florio, L., and Mugrage, E. R.

5 Vol.37 COLORADO TICK FEVER 297 Hematological Findings in the Golden Hamster. J. Exper. Med., 80:189 (Sept.), Florio, L., Stewart, M. O., and Mugrage, E. R. The Etiology of Colorado Tick Fever. J. Exper. Med., 83:1 (Jan.), Kaprowski, H., and Cox, H. Adaptation of Colorado Tick, Fever Virus to Mouse and Developing Chick Embryo. Proc. Soc. Exper. Biol. & Med., 62:320 (June), Florio, L., Mugrage, E. R., and Stewart, M. 0. Colorado Tick Fever. Ann. Int. Med., 25:466 (Sept.), Deem, A. W., and Huddleston, 0. L. Personal communications. 10. Shaffer, F. C. Personal Experiences with Colorado T.ck Fever. Colcrado Med., 32:226 (Mar.), Florio, L., Hammon, W. McD., Laurent, A., and Stewart, M. 0. Colorado Tick Fever and Dengue. J. Exper. Mled., 83:295 (Apr.), Pollard, M., Livesay, H. R., Wilson, D. J., and Woodland, J. C. Immunological Studies of Dengue Fever ind Colorado Tick Fever. Proc. Soc. Exper. Biol. & Med., 61:396 (Apr.), Best Sellers in thlc Book Service for February The Control of Communicable Diseases. American Public Health Association. 6th ed $.35 Diagnostic Procedures and Reagents. American Public Health Association. 2nd ed Industrial Toxicology. Hamilton and Johnstone Infant and Child in the Culture of Today. Gesell and Ilg Local Health Units for the Nation. Emerson and Luginbuhl Municipal and Rural Sanitation. Ehlers & Steel. 3rd ed Preventive Medicine and Public Health. Wilson G. Smillie Public Health-A Career with a Future. American Public Health Association Recommended Practice for Design, Equipment and Operation of Swimming Pools and Other Public Bathing Places. American Public Health Association Standard Methods for the Examination of Water and Sewage. American Public Health Association. 9th ed Order from the Book Service AMERICAN PUBLIC HEALTH ASSOCIATION