Post-vaccination serological status predicts protection against heterologous SAT1 foot-and-mouth disease virus challenge in goats.

Foot-and-mouth disease (FMD) is a contagious transboundary animal disease of many cloven-hooved livestock and wildlife species readily transmitted by multiple routes of infection. Goats are susceptible to FMD and have been implicated in previous FMD outbreaks, but limited data are available for this species. The objective of the current study was to estimate the vaccinal antibody titer at the time of exposure that would protect goats from developing FMD lesions after experimental infection with a heterologous Southern African Territories 1 (SAT1) FMD virus (FMDV). Forty FMD-free goats were obtained for the evaluation of a pentavalent vaccine including an experimental challenge with a pool of heterologous FMDV SAT1. On the day of challenge, anti-FMDV structural protein antibodies were detected using virus neutralization tests (VNT) and multiple formats of a solid-phase competition ELISA (SPCE). Antibodies against non-structural proteins were measured using a commercial ELISA (NSPE). Goats were examined daily over two weeks to identify secondary FMD lesions that developed in the oral cavity and on the feet. Predictive modeling using logistic and probit regression as well as receiver-operating characteristic (ROC) analysis were performed to identify serological titers that were predictive of preventing FMD. Simulation modeling was also performed to describe the shape of the protection curve for VNT titers. Six vaccinated and challenged goats (6/29; 21%) developed secondary FMD lesions during the follow-up period with zero (0/5), one (1/9; 11%), one (1/7; 14%), and four (4/8; 50%) within a 2 ml (full dose), 0.67 ml, 0.33 ml, and 0.17 ml vaccine groups, respectively. The VNT and all evaluated SPCE formats could predict protection, with the VNT having better predictive ability based on Akaike information criterion (AIC) but an optimized format for the SPCE had slightly better area under the ROC curve (AUC; 0.862 versus 0.859). The NSPE results were not predictive of protection and 93% (27/29) of goats were positive on this assay at the time of experimental challenge. A log10 titer of 2.35 was required to protect 75% (95% CI, 50-90%) of experimentally challenged goats. Simulation modeling estimated a parabolic response curve rather a sigmoid shape as is generated from logistic and probit modeling. Post-vaccinal serological monitoring using VNT or SPCE is theoretically possible in goats to determine the proportion of the population at higher risk for clinical FMD. More research is necessary to determine the relationship between vaccine match and serological titer when modeling vaccinal protection from FMD in goats.