Phylogenetic Analysis
Based on the evaluation of MEGA-X, an ML phylogenetic tree based on the full VP2 gene was constructed with the Tamura 3-parameter model plus gamma distribution with five rate categories (T92+G+I).
According to the phylogenetic tree, the 27 strains fell into three clades(Figure 2). Clade CPV-2c Ⅰ included all the CPV-2c variants in this study, and Asian CPV-2c strains, along with one Italian CPV-2c strain. This clade was separated from clade CPV-2c Ⅱ, which contained CPV-2c strains circulating in European and American countries. CPV-2c strains belong to this clade were characterized by four mutations: Ala5Gly, Phe267Tyr, Tyr324Ile, Gln370Arg. Six New CPV-2a strains in this study were classified into another clade, New CPV-2a/b Ⅰ, containing Asian New CPV-2a/b strains and two American New CPV-2a strains. All the strains located within this clade shared three mutations, Phe267Tyr, Tyr324Ile, and Thr440Ala, clustering away from other clades containing New CPV-2a variants with different mutations. Two FPLV strains were located in clade FPLV which included all the FPLV strains.