Antigenic and genetic analyses of foot-and-mouth disease virus type A isolates for selection of candidate vaccine strain reveals emergence of a variant virus that is responsible for most recent outbreaks in India.

Recent reports indicated presence of two antigenic and genetic groups (genotypes VI and VII) of foot-and-mouth disease virus (FMDV) type A in India and are divergent from the vaccine strains. In order to choose suitable field isolate as candidate vaccine strain, anti-sera against representative isolates from both the genotypes and two in-use vaccine strains are tested in neutralization assay. Two candidate vaccine strains from both the genotypes are selected with close antigenic match to the field isolates. From the result it is evident that IND 81/00 (genotypes VII), gave a better antigenic coverage (antigenic relationship (r)-value>0.40 with 79% of isolates of 2002--2003) than IND 258/99 (genotype VI; r-value>0.40 with 42% of 2002--2003 isolates). Phylogenetic analysis based on P1 genomic region placed all the recent isolates (2001--2003) into genotype VII, with emergence of a new variant virus (VIIb--VP3(59)del) having amino acid deletion at an antigenically critical residue (VP3(59)), indicating a major evolutionary jump probably due to immune selection. Though very limited in its extent, this data indicates an apparent dominance of genotype VII over genotype VI and underscores the need to continue further molecular epidemiological investigations to substantiate this finding.