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Rocky Mountain Spotted Fever -- United States, 1984

For 1984, a provisional total of 847 cases of Rocky Mountain spotted fever (RMSF) in the United States was reported to the MMWR, for an incidence rate of 0.36 cases per 100,000 population. Oklahoma had the highest incidence rate (119 cases; 3.6/100,000). Other states with high RMSF rates were North Carolina (178 cases; 2.9/100,000), South Carolina (80 cases; 2.4/100,000), Arkansas (28 cases; 1.2/100,000), Tennessee (49 cases; 1.0/100,000), Montana (8 cases; 1.0/100,000), Virginia (48 cases; 0.9/100,000), and Georgia (48 cases; 0.8/100,000) (Figure 1).

States submitted case report forms for 717 (85%) of the cases reported to the MMWR. Of the 717 cases, 399 (56%) were confirmed either by serologic testing, isolation of spotted fever group rickettsia, or fluorescent antibody staining of biopsy or autopsy specimens. Serologic confirmation requires a single complement fixation (CF) titer 1:16 or higher or single indirect fluorescent antibody (IFA) titer 1:64 or higher or fourfold rise in the CF, IFA, microagglutination (MA), latex agglutination (LA), or indirect hemagglutination (IHA) assays. An additional 66 patients (9%) were classified as "probable" cases as indicated by a fourfold rise in titer or single titer 1:320 or higher in the Weil-Felix assay (Proteus 0X-19 or 0X-2) or a LA, MA, or IHA titer 1:128 or higher. The other 252 diagnoses (35%) were supported by clinical findings alone. Ninety-six percent of the patients became ill between April 1 and September 30.

Like that of previous years (1,2), 1984 surveillance revealed that 51% of the patients were under 20 years of age; 61% were male; and 91% were white. Symptoms reported included fever (96%), headache (90%), myalgias (86%), rash (84%), and rash on the palms of the hands or on the soles of the feet (61%). Seventy-five percent of the patients were hospitalized. Sixty-six percent of patients for whom exposure information was available reported a tick bite within 14 days of onset of illness. The case-fatality rate (3.6%) was higher for older individuals and for persons not receiving treatment with either tetracycline or chloramphenicol. Of the 613 patients from whom information about treatment and clinical outcome was available, only 13 (2%) received neither chloramphenicol nor tetracycline. Of these 13 patients, three (23%) died, compared with 16 deaths (3%) among the 600 patients who received treatment with chloramphenicol or tetracycline. For persons 30 years of age or older, the case-fatality rate was 6.5%, compared with 2.0% for individuals under 30. Reported by Div of Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial note: RMSF, the most commonly reported rickettsial infection in the United States, is transmitted to humans by ticks. The incidence of infection begins to increase in April and is highest in May and June.

After the rapid increase in RMSF noted in the United States during the 1970s, infection rates remained approximately the same from 1977 through 1981, when a decrease in the number of cases began. In 1984, 25% fewer cases were reported than in 1983, and all states reporting over 10 cases in 1984 reported either a decrease or no change in number of cases from 1983. This decrease occurred in both of the major foci of RMSF in the United States, the West South Central and South Atlantic states. The West South Central states reported 45% fewer cases, and the South Atlantic states, 18% fewer cases. The reason for the decrease in RMSF is not known but does not seem attributable to reporting artifact. The decrease was widespread geographically, occurred in both the cases reported to the MMWR and in cases reported by case report forms, was distributed uniformly over the April 1-September 30 period, and occurred in the absence of any changes in the reporting system. The decrease may be part of a cyclic pattern of RMSF incidence that appears to be occurring for the second time since reporting began in 1920 (3) (Figure 2).

Laboratory confirmation of a clinical diagnosis of RMSF by serologic or other methods remains important in distinguishing RMSF from other diseases with similar clinical presentations, even though treatment frequently precedes confirmation. Laboratory confirmation is also important for improving the specificity of national RMSF surveillance. The importance of obtaining serologic confirmation of clinically diagnosed cases has been reinforced by a recent study that showed at least 36% of clinically diagnosed cases in an endemic area were found not to be RMSF when serologic testing was performed (4).

No vaccine against RMSF is currently available; RMSF is best prevented by inspecting persons who may have been exposed to ticks. If discovered, ticks should be removed by grasping them with tweezers as close as possible to the point of attachment and pulling slowly and steadily. Fingers, protected with facial tissue, may be used when tweezers are not available. Because ticks' secretions can be infective, hands should always be washed after removal of ticks. Particularly during the spring and summer months in RMSF-endemic areas and during the 3-12 day period after bites or exposures to ticks, RMSF should be considered and medical treatment sought by any individual who develops fever, myalgia, or headache, even in the absence of rash (5). Failure to treat cases with tetracycline or chloramphenicol, particularly early in disease, remains a risk factor for deaths from RMSF (5,6).


  1. Bernard KW, Helmick CG, Kaplan JE, Winkler WG. Surveillance of Rocky Mountain spotted fever in the United States, 1978-1980. J Infect Dis 1982;146:297-9.

  2. Fishbein DB, Kaplan JE, Bernard KW, Winkler WG. Surveillance of Rocky Mountain spotted fever in the United States, 1981-1983. J Infect Dis 1984;150:609-11.

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

  4. Wilfert CM, MacCormack JN, Kleeman K, et al. Epidemiology of Rocky Mountain spotted fever as determined by active surveillance. J Infect Dis 1984;150:469-79.

  5. Helmick CG, Bernard KW, D'Angelo LJ. Rocky Mountain spotted fever: clinical, laboratory, and epidemiological features of 262 cases. J Infect Dis 1984;150:480-8.

  6. Hattwick MA, Retailliau H, O'Brien RJ, Slutzker M, Fontaine RE, Hanson B. Fatal Rocky Mountain spotted fever. JAMA 1978;240:1499-503.

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