Molecular epidemiology of SAT3-type foot-and-mouth disease.

VP1 gene nucleotide sequences of 51 SAT3-type foot-and-mouth disease (FMD) viruses from seven southern and eastern African countries were used to infer a gene phylogeny. Results obtained by phylogenetic analysis of the homologous 405 nt region corresponding to the C-terminal 128 amino acids of 1D and adjacent 7 amino acids of 2A indicate that there are six distinct virus lineages evolving independently in different geographical localities in accordance with the FMD topotype concept. Topotypes I-IV occur in southern Africa, whilst topotypes V and VI are unique to East Africa. Viruses of different topotypes differ from each other at 20% or more of the nucleotide sites, specified in this study. Despite the limited geographical distribution of this serotype, the level of intratypic variation is intermediate between that of SAT1 and SAT2, both of which are widely distributed in sub-Saharan Africa. Within SAT3, 37.3% and 47.4% of sites were completely conserved on nucleotide and amino acid levels, respectively. The locality-specific grouping of viruses permits accurate determination of the sources of outbreaks, whilst the high levels of variation within the immunodominant 1D protein has implications for the control of the disease through vaccination.