Evolutionary and epidemiological analyses based on spike genes of porcine epidemic diarrhea virus circulating in Thailand in 2008–2015

Porcine epidemic diarrhea (PED) has been endemic causing sporadic outbreaks in Thailand since 2007. In 2014–2015, several herds had experienced severe PED outbreaks and the reason of the re-current outbreaks was unknown. Whether or not the introduction of exotic strains or continual evolution of existing PEDV, genetic analyses would provide a more understanding in its evolutionary pattern. In the study, 117 complete spike gene sequences of Thai PED virus (PEDV) collected from 2008 to 2015 were clustered along with 95 references of PEDV spike sequences, and analyzed with the US sequences dataset (n = 99). The phylogenetic analysis demonstrated that Thai PEDV spike sequences were genetically diverse and had been influenced by multiple introduction of exotic strains. Although Thai PEDV have been evolved into 6 subgroups (TH1–6), Subgroup TH1 strains with the unique 9 nucleotides (CAA GGG AAT) insertion between 688th–689th position of spike (changing amino acid from N to TREY) insertion has become the dominant subgroup since 2014. Thai PEDV spike gene have higher evolutionary rate compare to that of the US sequences. One contributing factor would be the intra-recombination between subgroups. Thailand endemic strain should be assigned into new subclade of G2 (Thai pandemic variant).