Rocky Mountain spotted fever. Serological evidence of previous subclinical infection in children.

Serological tests for Rocky Mountain spotted fever were performed on single serum specimens from sixth graders from Forsyth County, North Carolina, an area highly endemic for this disease. Five of 508 *.098%) sera were positive (titer greater than or equal to 1:64) using the indirect fluorescent antibody method. Elevated Proteus agglutination antibody titers (greater than 1:160) to OX-19, but not ot OX-2, were common (19.6%) in these children. No correlation of those with positive OX-19 titers could be made with the cases demonstrating elevated indirect fluorescent antibody titers or with a history of urinary tract infection. These data suggest that the increased prevalence of elevated Proteus OX-19 antibody levels makes a single high titer unreliable in the diagnosis of Rocky Mountain spotted fever and that naturally acquired mild or asymptomatic subclinical cases of Rickettsia rickttsii infection occur.