Antibodies Reactive to Rickettsia rickettsii Among Children Living in the Southeast and South Central Regions of the United States

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Abstract
Background 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 withRickettsia rickettsiimay be more common. This study estimated the prevalence of antibodies reactive toR rickettsiiamong children living in the southeastern and south central United States. Study Design Approximately 300 specimens were obtained from children at each of 7 pediatric referral centers (N = 1999). Serum was tested forR rickettsiiantibodies by means of indirect immunofluorescence antibody assay. Three different cutoff titers (≥64, ≥128, and ≥256) represented increasing levels of stringency to define positive specimens. Results Overall, 12.0% of children hadR rickettsiiantibody 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. Conclusions 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 withR rickettsiior 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.

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