Immunoglobulin M antibody capture enzyme-linked immunosorbent assay for diagnosis of St. Louis encephalitis

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Abstract
Sera from patients with St. Louis encephalitis were tested with an IgM antibody capture enzyme immunoassay (MAC ELISA). The assay used 5 reagents: antihuman IgM, test serum, sucrose-acetone-extracted mouse brain antigen, broadly cross-reactive flavivirus monoclonal antibody conjugated to alkaline phosphatase, and substrate (P-nitrophenyl phosphate). MAC ELISA endpoint titers correlated (r = 0.893) with the absorbance value of a 1:100 dilution of patient serum. Significant (.gtoreq. 4-fold) changes in the endpoint titers between paired sera corresponded to a critical ratio (ratio of absorbance values at the 1:100 dilution) of .gtoreq. 1.3 IgM antibodies were detected in 71% of patients bled 0-3 days after the onset of illness, in 99% bled 4-21 days and in 91% bled at 22-67 days. Thereafter, the IgM seropositivity rate declined; however, 29% of sera were still positive at 115-251 days after the onset on illness. MAC ELISA titers were significantly correlated with hemagglutination inhibition (r = 0.258) and neutralization (r = 0.711) titers. Because IgM antibodies appeared early and waned rapidly, a diagnosis was made on the basis of a decrease in titer more often by MAC ELISA than by hemagglutination inhibition, complement fixation or neutralization tests. IgM antibodies generally showed a high degree of specificity; heterologous cross-reactions were, however, present in 4 of 14 sera examined. The MAC ELISA is useful for the rapid, early diagnosis of ST. Louis encephalitis.