Rocky Mountain Spotted Fever -- United States, 1981

A provisional total of 1,170 cases of Rocky Mountain spotted fever (RMSF) occurring in the United States in 1981 have been reported to CDC. On the basis of this figure, the incidence rate of RMSF in 1981 for the United States as a whole was 0.51 cases/100,000 population.

The South-Atlantic states accounted for 671 (57%) of the reported cases. The highest rates of RMSF were for North Carolina (301 cases, 5.06/100,000), South Carolina (102 cases, 3.22/100,000), Oklahoma (99 cases, 3.19/100,000), Virginia (105 cases, 1.93/100,000), Tennessee (82 cases, 1.78/100,000), Maryland (66 cases, 1.55/100,000), and Arkansas (35 cases, 1.53/100,000).

States submitted case-report forms on 1,059 (91%) of the reported cases. Of these, 372 (35%) were confirmed by complement-fixation (CF), indirect fluorescent-antibody (IFA), indirect hemagglutination (IHA), latex-agglutination (LA), or microagglutination (MA) tests; isolation of spotted fever group rickettsiae; or fluorescent-antibody staining of biopsy or autopsy specimens. An additional 129 patients (12%), whose specimens reacted positively in the Weil-Felix agglutination test, but were not tested by other methods, were designated as having "probable" cases. The other 558 cases (53%) were reported on the basis of clinical diagnosis alone. Sixty percent of the patients were male, 53% were persons 20 years of age, and 92% were white.

Ninety-six percent of the patients became ill between April 1 and September 30. Symptoms reported included fever (98%), headache (90%), rash on torso (85%), and rash on palms of hands or soles of feet (60%). Rash was significantly more commonly associated with laboratory-confirmed (89%) than with unconfirmed (82%) cases (p0.01); otherwise, the prevalence of symptoms was similar for these 2 groups of patients. Seventy-nine percent of patients were hospitalized during their illness. Sixty-seven percent of the patients for whom exposure information was available reported a tick bite or attachment within 14 days before onset of illness. The case-fatality rate (3.4%) was higher for blacks (6.7%) than whites (3.0%), higher for persons greater than or equal to 30 years of age (4.6%) than for younger individuals (2.8%), higher for persons with unknown or no tick exposure (4.4%) than for persons reporting a tick bite or attachment (2.6%), and higher for persons not reporting treatment with tetracycline or chloramphenicol (8.0%) than for those who received such antibiotic therapy (2.5%).

Twenty-five percent of patients for whom the history was available reported travel outside of the county of residence within 14 days before onset of illness. Reported by participating state and territorial health depts; Consolidated Surveillance and Communications Activity, Epidemiology Program Office, Div of Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Following the rapid rise in the 1970s of RMSF incidence in the United States, infection rates since 1977 have remained about the same (Figure 1). The predominant occurrence of RMSF in the southeastern states and the higher incidence for younger persons, males, and whites have remained unchanged in recent years. The case-fatality rate, which has fluctuated between 3% and 8% since 1970, indicates that RMSF remains a serious illness that requires prompt diagnosis and early treatment with tetracycline or chloramphenicol. Risk factors that have been associated with fatalities include age greater than or equal to 30 years, male sex, black race, absence of skin rash, failure to obtain a history of exposure to ticks, and lack of appropriate antibiotic treatment (1). A history of travel to an area in which infected ticks are endemic may be critical to the diagnosis of RMSF when a patient is seen in an area where the disease does not commonly occur.

An important change in the method of conducting national surveillance of RMSF in 1981 was the adoption of a new case-report form that provides information about symptoms, hospitalization, treatment, tick exposure, and travel, and also defines stricter criteria for laboratory confirmation of cases. A clinically compatible case with diagnostic serologic results determined by CF, IFA, IHA, LA, or MA is considered confirmed (a case with positive titers obtained by the Weil-Felix reaction is only considered a probable case). Patients from whom the causative agent is isolated, or who have positive fluorescent-antibody staining of tissue specimens, are also considered to have confirmed cases. These stricter criteria are responsible for the lower percentage of cases confirmed by laboratory testing in 1981 compared with 1980 (35% versus 62%, respectively). It should be emphasized that confirmation of RMSF is of epidemiologic importance but cannot usually be expected to occur before days 10-14 after onset of illness. Therefore, diagnosis must rely on clinical (fever, headache, rash, myalgia) and epidemiologic (tick exposure) criteria, and treatment must be initiated before laboratory confirmation is available.

Prevention of RMSF entails frequent inspection of persons for ticks when exposure is likely. (Ticks do not usually transmit infection until they have been attached for several hours.) Ticks are best removed by grasping the tick with tweezers as close as possible to the point of attachment and by pulling slowly and steadily. If tweezers are not available, fingers protected with facial tissue may be used. If bare hands touch the tick during removal, the hands should be washed thoroughly with soap and water, because tick secretions can be infective.

A vaccine against RMSF is in the developmental stage, but is not expected to be available in the near future.

Reference

1. Hattwick MA, O'Brien RJ, Hanson BF. Rocky Mountain spotted fever: epidemiology of an increasing problem. Ann Intern Med 1976;84:732-9.

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