No abstract

A study of Rickettsia rickettsii was conducted in Rowan, Cabarrus, and Granville counties, North Carolina in an attempt to define the prevalence of endemic infection in this area. Serum samples were obtained from 1,976 healthy persons and tested by indirect hemagglutination for detectable antibodies to R. rickettsii. Of this group, 568 (28.7%) had detectable antibody (greater than or equal to 1:8), 80 (4%) had titers greater than or equal to 1:64, and 1,408 (70%) had no detectable antibody (less than or equal 1:8). Indirect immunofluorescence testing for antibody was also performed for 315 (15%) of the serum samples, of which 301 (95%) had undetectable titers and 14 (5%) had detectable titers ranging from 1:8 to greater than or equal to 1:64. Serological reactivity by indirect hemagglutination was detected in persons in the absence of known Rocky Mountain spotted fever. The study failed to show a good correlation of either the height of the geometric mean titer or percentage of seropositive persons with the previously determined age-related rates of acquisition of the disease. These data suggest that the antibodies measured may not be specific for R. rickettsii or that the antibody levels wane with time or both. It is probable that unrecognized infection occurs, but the true incidence or prevalence cannot be determined by available serological tests.

Prospective, active surveillance for Rocky Mountain Spotted fever (RMSF) was undertaken in two North Carolina counties for the years 1979-1981. Appropriate age, sex, and geographically matched controls were compared with the serologically confirmed cases of disease. Patients who were suspected of having infection, but were shown not to have RMSF, were also compared with the serologically proven cases. Ninety-eight cases were identified, and three of these patients died. The mean annual incidence of RMSF in these two counties was 14.59 cases/100,000 population. Identified cases occurred between the months of March and September. The overall mean annual incidence for men was 19.65/100,000 and for women 10.3/100,000. The study suggests that persons contracting RMSF are exposed to the vectors at their residences, which tend to be detached dwellings in woody, brushy areas. Children are more likely than adults both to be seen by physicians early in the illness and to be checked for ticks. Eighty-four percent of patients with serologically confirmed cases developed a rash. The epidemiology of RMSF in these two endemic counties is described.

To identify Rickettsia rickettsii antigens of immunological importance, we examined sera from patients with serologically confirmed cases of Rocky Mountain spotted fever by crossed immunoelectrophoresis for antibodies to antigens extracted from the R strain of R. rickettsii with the detergent Triton X-100. Sixteen antigens were identified in the detergent extract by crossed immunoelectrophoresis with a hyperimmune rabbit serum raised against whole rickettsiae. When the rabbit antiserum was placed in the reference gel and patient sera were placed in the intermediate gel, antibodies to one or more antigens were detected in 61 of 71 North Carolina sera, all of 7 Oklahoma sera, and 9 of 10 Montana sera obtained from 1 day to 40 years after onset of Rocky Mountain spotted fever. Antibodies to antigens 1 and 16 were found as early as 1 day after onset of illness, and antibody to 16 was found in 20 of 29 sera obtained within the first 7 days of illness. Antibodies to antigens 2 and 3 generally did not appear until the third week of illness but were found in six of seven serum samples collected 4 to 40 years after onset of Rocky Mountain spotted fever. Antibodies to R. rickettsii antigens 1, 7, 8, and 16 were found in sera from patients with illnesses caused by other etiological agents. Four of the Oklahoma and Montana sera from Rocky Mountain spotted fever patients, but none of the North Carolina sera, had antibodies to antigen 12. Sera containing antibodies against antigens 3 and 14 prevented death of mice challenged with two 50% lethal doses of R. rickettsii.