Development of a serological based vaccine matching technique for Foot-and-Mouth Disease (FMD) SAT 2 viruses

Abstract

Foot-and-mouth disease (FMD) is a highly contagious viral disease that primarily affects domestic animals, including cattle, sheep, goats, and pigs. FMD virus is an RNA virus with seven serotypes, six of which are found in most of Africa. Africa relies on vaccination to control FMD, but antigenic variability among serotypes and subtypes limits cross-protection. Therefore, selection of vaccine strain that is antigenically close to the field virus is crucial for FMD control. Utilizing the relationship coefficient r1-value, serological assays including virus neutralizing test (VNT), liquid phase blocking ELISA (LPBE), and solid phase competition ELISA (SPCE) have been crucial in vaccine matching. Since the year 2000, the antigenically diverse SAT 2 serotype, which has been responsible for the majority of FMD outbreaks, has not been effectively neutralized by trivalent FMD vaccines in Southern Africa, particularly in South Africa. Pentavalent FMD vaccines have been developed by the Agricultural Research Council-Onderstepoort Veterinary Research (ARC-OVR) in an effort to reduce and increase antigenic coverage against circulating and emerging SAT 2 strains. The study examined the FMD pentavalent vaccine's ability to protect against emerging and circulating SAT 2 strains using r1-value antigenic matching. Firstly, SAT 2 virus prevalence in South Africa and Zimbabwe was determined, yielding six strains (SAR/1/2001; SAR/1/2003; SAR/1/2013; SAR/15/2013, KNP/12/2008 and ZIM/2/2013). Secondly, the SPCE assay was optimised for vaccine matching due to the assay being robust in comparison to LPBE. Thirdly, involves assessing the cross-reactivity of SAT 2 viruses with sera (serum1-7) acquired from cattle. Serum sample 1-5 originated for cattle induced with pentavalent vaccine, while serum sample 6-7 where from not induced cattle. The serum sample 1-3 indicated high cross-reactivity resulting in high titre ranging log10 2.8 and log10 3.1 against the SAT 2 field viruses, while the serum sample 4-5 cross -reactivity were moderate with average titre ranging between log10 1.4 and log10 1.8 against the SAT 2 field viruses. Serum titre of serum 6-7 were between log10 0.3 to log10 0.8. and was expected. The titres resulted in r1 -values; for SAR/1/2001 with the vaccine strain ranged between 0.65 to 1.19; SAR/1/2013 ranged between 0.54 to 1.17; ZIM/2/2013 ranged between 0.60 to 1.20; SAR/15/2013 ranged between 0.59 to 1.18; SAR/1/2003 range between 0.50 to 1.14; and KNP/12/2008 ranged between 0.54 to 1.15. R1-values of the vaccine strain/homologous strain (SAR/03/2004) were 1.00 which was expected due to antigenic similarity existence between the strains. The r1-values indicate a strong antigenic similarity between the SAT 2 field viruses and the pentavalent vaccine, suggesting that the pentavalent vaccine has the potential to provide protection against the selected SAT 2 field viruses. The optimised SPCE was instrumental in cross-reactivity of SAT 2 field viruses against sera. SPCE indicated the capability to determine the r1-values which were significant in demonstrating antigenic similarity between SAT 2 field viruses and FMD pentavalent vaccine. Availability of such a SPCE assay as a vaccine matching tool will be valuable for the swift assessment of vaccine matching between a vaccine strain and circulating SAT 2 virus strains. This will provide immediate crucial information regarding the antigenic spectrum of the vaccine strain and whether the vaccine strain has the potential to provide for cross-protection or require optimization.