Genetic and antigenic analysis of two recently circulating genotypes of type A foot-and-mouth disease virus in India: evidence for positive selection in the capsid-coding genes.

We have analyzed isolates of two recently circulating genotypes (genotypes VI and VII) of type A foot-and-mouth disease virus (FMDV) from India. Maximum-likelihood models provided support for the presence of positively selected sites in the capsid-coding (P1) region. Positive selection was detected at a number of amino acid positions behind a background of strong purifying selection. Among the positively selected sites, four were identified at known critical antigenic residues (VP2 79, VP3 59 and 70 and, VP1 83), suggesting that FMDVs are under pressure from the immune system. Two residues (VP2 134 and VP3 59) that are part of the heparan sulfate-binding pocket in subtype A22 FMDV are also inferred to be under positive selection. Antigenic divergence was observed between and within the genotypes in neutralization tests with sera raised against the representative isolates from genotypes VI and VII. The two vaccine strains showed one-way antigenic relationships (r value) of <0.2 with 64% of the isolates, whereas, with genotypes VI and VII an r value of >0.4 was observed with 24% and 64% of the isolates, respectively. No correlation could be deduced from the amino acid substitutions at specific critical residues and lower r values in the field isolates.