The reported annual incidence of Rocky Mountain spotted fever in the United States is 2.2 per million, but studies have suggested that human infection with Rickettsia rickettsii may be more common. This study estimated the prevalence of antibodies reactive to R rickettsii among children living in the southeastern and south central United States.
Approximately 300 specimens were obtained from children at each of 7 pediatric referral centers (N = 1999). Serum was tested for R rickettsii antibodies by means of indirect immunofluorescence antibody assay. Three different cutoff titers (≥64, ≥128, and ≥256) represented increasing levels of stringency to define positive specimens.
Overall, 12.0% of children had R rickettsii antibody titers of at least 64; 7.3%, at least 128; and 4.3%, at least 256. Strong relationships were seen between increasing age and seroprevalence at each cutoff titer. Remarkably, 6.4% of children aged 13 to 17 years had titers of at least 256. Age-adjusted seroprevalence rates at titers of at least 64 varied from 21.9% in Little Rock, Ark, to 3.5% in Louisville, Ky. At titers of at least 256, seroprevalence ranged from 7.7% in Nashville, Tenn, to 1.8% in Winston-Salem, NC. Only site and age group were strong predictors of seropositivity; a weak association was seen with nonurban residence.
To our knowledge, this is the largest serosurvey of rickettsial infection in children in the United States. Within the limitations of the immunofluorescence antibody assay, these data suggest that infections with R rickettsii or antigenically related spotted-fever group rickettsiae may be common and subclinical. The results also have implications for the interpretation of single immunofluorescence antibody assay titers in children with suspected Rocky Mountain spotted fever.