Current Trends Rocky Mountain Spotted Fever -- United States, 1983

For 1983, a provisional total of 1,126 cases of Rocky Mountain spotted fever (RMSF) in the United States was reported to CDC, for an incidence rate of 0.5 cases/100,000 population.

Oklahoma reported the most cases and had the highest incidence rate in the country (227 cases, 6.9/100,000 population). Other states with high RMSF rates were: North Carolina (206 cases, 3.4/100,000); South Carolina (80 cases, 2.5/100,000); Arkansas (42 cases, 1.8/100,000); Georgia (68 cases, 1.2/100,000); Virginia (59 cases, 1.1/100,000); and Tennessee (49 cases, 1.1/100,000) (Figure 1).

States submitted case report forms for 957 (85%) of reported cases. Of these, 603 (63%) were confirmed by serologic testing,* by isolation of spotted fever group rickettsiae, or by fluorescent antibody staining of biopsy or autopsy specimens. An additional 60 patients (6%) had "probable" cases, as indicated by a fourfold increase or a single convalescent titer 1:320 or higher in the Weil-Felix (OX-19, OX-2) agglutination tests, or by a single convalescent titer 1:128 or higher by latex agglutination (LA) or indirect hemagglutination (IHA). The other 294 cases (31%) were reported on the basis of clinical diagnoses alone. Fifty-one percent of the patients were under 20 years of age; 61% were male; and 89% were white.

Ninety-four percent of patients became ill between April 1 and September 30. Symptoms reported included fever (97%), headache (87%), rash on torso (85%), and rash on palms of hands or soles of feet (62%). Seventy-seven percent of patients were hospitalized. Sixty-seven percent of patients for whom exposure information was available reported tick bites or attachments within 14 days before onset of illness. The case-fatality rate (4%) was higher for persons 30 years of age or older (9%) than for younger individuals (2%); slightly higher for persons with unknown or no tick exposure (5%) than for persons reporting tick bites or attachments (4%); and higher for persons not reporting treatment with tetracycline or chloramphenicol (7%) than for those who received such antibiotic therapy (4%).

Nineteen percent of 666 patients for whom histories were available reported travel outside their counties of residence within 14 days before onset of illness. Forty-six percent of these patients indicated travel to one of the seven states reporting the highest incidence of RMSF in 1983. Reported by Div of Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: After the rapid rise of RMSF in the United States during the 1970s, infection rates have remained approximately the same since 1977, with the exception of a slight drop in 1982 (Figure 2). In 1983, more cases than usual were reported from Oklahoma, Arkansas, and Texas (Texas reported 100 cases, for an incidence of 0.6/100,000), suggesting increased activity of RMSF in the West South Central states, where the vectors of RMSF are Amblyomma americanum (the Lone-Star tick) and Dermacentor variabilis (the American dog tick). Four hundred seventy-two cases (42% of the total) were reported from the South Atlantic states, where the principal vector is D. variabilis.

The higher incidence of RMSF among younger persons and the case-fatality rate (which has fluctuated between 3% and 8% since 1970) have changed little in recent years. Consistent with previous findings (1), the 1983 data indicate that fatality continues to be associated with persons aged 30 years or older, failure to obtain a history of exposure to ticks, and lack of appropriate antibiotic therapy. Travel history for 19% of patients for whom information was available indicates that travel to highly endemic areas may be critical in diagnosing the disease, especially in areas where RMSF does not commonly occur.

The percentage of laboratory-confirmed (63%) cases in 1983 was higher than that in 1982 (48%) and 1981 (35%), suggesting that the more sensitive and specific laboratory tests to confirm RMSF have achieved wider use. It must be emphasized, however, that RMSF confirmation is of epidemiologic importance and cannot usually be expected to occur before 10-14 days after onset of illness. Therefore, diagnosis must rely on clinical (fever, headache, rash, myalgia) and epidemiologic (tick exposure) criteria, and treatment with tetracycline or chloramphenicol must be initiated before laboratory confirmation is available.

Prevention of RMSF entails frequent inspection of persons when tick exposure is likely. Ticks are best removed by grasping with tweezers as close as possible to the point of attachment and by pulling slowly and steadily. If tweezers are unavailable, 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 ticks' secretions can be infective. Because it is difficult to determine with certainty if a tick is infected with Rickettsia rickettsii, or if transmission has occurred, routine testing of ticks removed from patients is not recommended. Instead, when a tick bite occurs, the patient and the family should be educated about the incubation period of RMSF (3-12 days) and the most common symptoms, and should be instructed to seek medical attention promptly if RMSF symptoms occur. No vaccine against RMSF is currently available.

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. *A fourfold increase in antibody titer between acute- and convalescent-phase serum specimens by complement fixation (CF), indirect fluorescent antibody (IFA), indirect hemagglutination, latex agglutination, or microagglutination; or a single convalescent titer 1:16 or higher (CF) or 1:64 or higher (IFA) in a clinically compatible case.

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