LIQUID-PHASE BLOCKING SANDWICH ENZYME-LINKED-IMMUNOSORBENT-ASSAY FOR DETECTION OF ANTIBODIES AGAINST FOOT-AND-MOUTH-DISEASE VIRUS IN WATER-BUFFALO SERA

Objective-To develop and apply the liquid-phase blocking sandwich ELIS A (BLOCKING-ELISA) for the quantification of antibodies against foot-a nd-mouth disease virus (FMDV) strains O-1 Campos, A(24) Cruzeiro, and C-3 Indaial. Design-Antibody quantification. Sample Population-158 wat er buffalo from various premises of Sao Paulo Stale-Brazil. The sera w ere collected either from systemically vaccinated or nonvaccinated ani mals. Procedure-The basic reagents of BLOCKING-ELISA (capture and dete ctor antibodies, virus antigens, and conjugate) were prepared and the reaction was optimized and standardized to quantify water buffalo anti bodies against FMDV. An alternative procedure based on mathematical in terpolation was adopted to estimate more precisely the antibody 50% co mpetition liters in the BLOCKING-ELISA. These titers were compared wit h the virus-neutralization test (VNT) titers to determine the correlat ion between these techniques. The percentages of agreement, cutoff poi nts, and reproducibility also were determined. Results-The antibody li ters obtained in the BLOCKING-ELISA had high positive correlation coef ficients with VNT, reaching values of 0.90 for O-1 Campos and C-3 Inda ial, and 0.82 for the A(24) Cruzeiro (P < 0.0005). The cutoff points o btained by use of the copositivity and conegativity curves allowed det ermination of high levels of agreement between BLOCKLNG-ELISA and VNT antibody titers against the 3 FMDV strains analyzed. Conclusions-The r esults characterized by high cor relation coefficients, levels of agre ement, and reproducibility indicate that the BLOCKING-ELISA may replac e the conventional VNT for detection and quantification of antibodies from water buffalo sera to FMDV.