Comparison of a liquid-phase blocking sandwich ELISA and a serum neutralization test to evaluate immunity in potency tests of foot-and-mouth disease vaccines.

Sera from cattle vaccinated against either foot-and-mouth disease virus (FMDV) strains A10 Holland, O1 BFS, or C1 Detmold were tested in a serum neutralization test (SNT) and a liquid-phase blocking sandwich ELISA (LBE), and the titers were compared with the results of intradermolingual challenge tests. The LBE test results were significantly more reproducible (P less than 0.005) than the SNT results. The correlation coefficients between SNT and LBE were 0.91 for FMDV strains A10 Holland and O1 BFS, and 0.82 for FMDV strain C1 Detmold (P less than 0.0005). The regression coefficient for strain A10 Holland was 0.80, for strain O1 BFS the value was 0.87, and for strain C1 Detmold it was 0.64. In probit analysis, titers at which 95% of the cattle were protected against challenge with the homologous strain were determined for the SNT and the LBE. In the SNT the 95% protection levels for strains A10 Holland were greater than or equal to 0.84, for O1 BFS greater than or equal to 1.59, and for C1 Detmold greater than or equal to 0.83. In the LBE they were greater than or equal to 1.28, greater than or equal to 1.71, and greater than or equal to 1.74, respectively. Because the SNT and the LBE are highly significantly correlated, and the LBE is more reproducible, the LBE is likely to predict protection more reliably than the SNT.