The standardized methodology for performing the ELISA has been described previously [29] . The absorbance of each test sample was converted into ELISA units using a standard curve generated by serially diluting the standard in the same plate. The ELISA units of each sample were then converted to mg/ml using a conversion factor as described elsewhere [30] . The minimal detection level of the AMA1 antibody in this study was 4 mg/ml, and all responses below that limit of detection were assigned a value of 2 mg/ml for the analysis.