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Current Trends Rocky Mountain Spotted Fever -- United States, 1985

For 1985, a provisional total of 700 cases of Rocky Mountain spotted fever (RMSF) was reported to MMWR, an incidence rate of 0.29 cases per 100,000 population. Oklahoma had the highest incidence rate (94 cases, 2.8/100,000). North Carolina reported the largest number of cases (143 cases, 2.3/100,000). Two other states had incidence rates of 1/100,000 or higher--South Carolina (73 cases, 2.2/100,000) and Kansas (27 cases, 1.1/100,000) (Figure 1).

States submitted case report forms for 587 (84%) of the 700 reported cases. Of these 587 cases, 335 (57%) were laboratory-confirmed by either serologic testing, isolation of spotted fever group rickettsia, or fluorescent antibody staining of biopsy or autopsy material. A case is considered serologically confirmed if testing reveals an indirect fluorescent antibody titer (IFA) of 1:64 or greater, a complement fixation (CF) titer of 1:16 or greater, or a fourfold rise in titer by the CF, IFA, microagglutination (MA), latex agglutination (LA), or indirect hemagglutination (IHA) assay. An additional 34 (6%) cases were classified as probable cases, as indicated by a fourfold rise in titer or a single titer 1:320 or higher in the Weil-Felix assay or an LA, MA, or IHA single titer of 1:128 or higher. The other 218 (37%) cases were supported by clinical diagnoses alone.

The 1985 surveillance revealed case characteristics similar to those previously reported. Ninety-five percent of patients reported onset of illness between April 1 and September 30, with 66% becoming ill in May, June, or July. Sixty percent of patients were male; 41% were under 20 years of age; and 90% were white. Symptoms reported included fever (94%), headache (88%), and myalgia (85%). A rash was reported by 83% and, of these, 73% reported that the rash was noted on the palms or soles. Seventy-three percent of the patients were hospitalized. The overall case-fatality rate was 4%. The case-fatality rate was higher for blacks (16%) than whites (3%) and was higher for individuals 40 years of age or older (9%) than for individuals under age 40 years (2%). Of the patients for whom exposure histories were available, 68% reported a tick bite or attachment, and an additional 24% reported being in a tick-infested area within 14 days (but no tick bite or attachment). Eight percent did not have a known exposure of either type. Reported by Viral and Rickettsial Zoonoses Br, Div of Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The number of reported RMSF cases has waned considerably from the peak of 1,192 cases (0.51/100,000) reported in 1981 (1). The increase in the early 1970s appeared simultaneously in many regions of the United States and was stimulated by the 1970 initiation of a CDC surveillance program (2). The rate of RMSF reported in the South Atlantic states, which increased steadily from 0.76/100,000 in 1970 to a peak of 1.91/100,000 in 1980, has now fallen to 0.82/100,000 (Figure 2). Excluding the South Atlantic states, the rate of RMSF in the other states rose through 1977 and remained fairly constant between 1978 and 1985 (Figure 2). For the third consecutive year, Oklahoma reported the highest incidence of any state.

Although the number of reported deaths has decreased with the decrease in the number of reported cases, the case-fatality rate has changed little over the last 5 years. Morbidity and mortality may be decreased by knowledge of the epidemiology and early clinical signs of RMSF (3). RMSF should be suspected, and treatment with chloramphenicol or tetracycline strongly considered, among residents of, or visitors to, RMSF-endemic areas who report fever, headache, and myalgias, even without a rash, particularly during April through October. Symptoms and signs referable to the pulmonary system (such as cough or rales), the gastrointestinal system (such as nausea, vomiting, or abdominal pain), or the central nervous system (such as stupor, meningismus, or ataxia), are seen with RMSF and should not delay diagnosis or treatment (3). Special attention is warranted for individuals 40 years of age and older, who have a greater likelihood of a fatal outcome, and dark-skinned individuals, in whom a rash may be more difficult to diagnose.

In a recent study of RMSF in a hyperendemic area, a tick bite or exposure was reported for 85% of serologically confirmed cases, compared with 54% of matched controls (4). Rash was reported in 84% of the serologically confirmed cases. However, in 16% of patients, a rash was never noted, and in an additional 10%, a rash did not develop until later than the fifth day after onset of the illness.

Prevention of RMSF is best accomplished by careful inspection of persons who may have been exposed to ticks. Ticks should be removed by grasping them with tweezers as closely as possible to the point of attachment and pulling slowly and steadily (5). If a portion of the mouth part remains, it should be treated like any other small foreign body; it may cause irritation, but it will not increase the risk of contracting RMSF. The fingers, protected with tissue paper, may be used to remove a tick from a person if tweezers are not available, but should always be washed after the removal of a tick. The fingers should not be used to detick dogs. Persons living and working in tick-infested areas should be educated about the prevention, symptoms, and signs of the disease. No vaccine against RMSF is currently available. RMSF cases should be reported to appropriate local and state health departments.

References

  1. CDC. Rocky Mountain spotted fever--United States, 1981. MMWR 1982;31:261-3.

  2. Hattwick MAW, Peters AH, Gregg MB, et al. Surveillance of Rocky Mountain spotted fever. JAMA 1973; 225:1338-43.

  3. Woodward TE. Rocky Mountain spotted fever: epidemiological and early clinical signs are keys to treatment and reduced mortality. J Infect Dis 1984;150:465-8.

  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. Needham GR. Evaluation of five popular methods for tick removal. Pediatrics 1985;75:997-1002.



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