NEUTRALIZING ANTIBODIES TO ESCHERICHIA-COLI VERO CYTOTOXIN-1 AND ANTIBODIES TO O157-LIPOPOLYSACCHARIDE IN HEALTHY FARM FAMILY MEMBERS AND URBAN RESIDENTS

An enzyme-linked immunosorbent assay (ELISA) to detect antibodies to E scherichia coli O157 lipopolysaccharide (LPS) was developed with sera from 63 children with confirmed recent E. coli O157 infection and from 256 age-stratified urban controls. The median ELISA values for contro l and case sera were 0.05 (interquartile range, 0 to 0.20; mean +/- st andard deviation [SD], 0.15 +/- 0.22) and 1.41 (interquartile range, 1 .11 to 1.59; mean +/- SD, 1.41 +/- 0.53), respectively (P < 0.001). Wi th a breakpoint of 0.59 (mean ELISA value of the control sera + 2 SDs) , the assay had a sensitivity, specificity, and positive and negative predictive values of 95, 94, 80, and 98%, respectively, for recent E. coli O157 infection. The O157 LPS assay and Vero cytotoxin (VT) 1-neut ralizing-antibody (NAb) assay were used to compare the relative freque ncies of O157 LPS antibodies and VT1-NAbs in an age-stratified urban p opulation from Toronto, Ontario, Canada, and in 216 healthy family mem bers from dairy farms in southern Ontario. The frequency of O157 LPS a ntibodies was about threefold higher in dairy farm residents (12.5%) t han in urban residents (4.7%) (P < 0.01). Similarly, the frequency of VT1-NAbs was about sixfold higher in dairy farm residents (42.0%) than in urban residents (7.7%) (P < 0.001). These findings are consistent with a greater level of exposure of dairy farm residents to VT-produci ng E. coli (VTEC) strains. The high rate of seropositivity to VT1 in f arm residents probably reflects the booster effect of repeated VTEC ex posures and argues against a sustained generalized immunosuppressive e ffect of VT1. Seroepidemiological studies may help in assessing the le vel of exposure of different populations to VTEC strains.