Molecular epidemiological analysis of porcine circovirus type 2 in East China from 2010 to 2015

Abstract:

[Objective] Porcine circovirus type 2 (PCV2) is one of the major etiological agents of porcine circovirus-associated diseases (PCVAD) and could cause severe economic loss to the swine industry worldwide. However, the mechanism of pathogenesis, epidemiology and molecular genetic evolution are not fully understood. Samples obtained from East China between 2010 and 2015 were detected for PCV2. In addition, 42 of 158 PCV2 positive samples were used for the whole genome amplification and sequencing to explore the genetic relationship, molecule epidemiology and genetic evolution of PCV2. [Methods] All of 384 field samples from the clinical diseased pigs between 2010 and 2015 in Jiangsu, Anhui and Zhejiang provinces of China, were screened by specific PCR to detect PCV2. The whole genome of PCV2 positive samples selected randomly was sequenced and analyzed. [Results] PCV2 is distributed widely among swine populations in East China and 158 clinical samples were positive for PCV2, indicating that the prevalence of PCV2 infection in pigs was about 41.15%. The phylogenetic tree constructed by the neighbor-joining method based on the whole genome or ORF2 gene was highly similar. The nucleotide homology and amino acid homology between the ORF2 gene of 42 isolates and 11 reference strains was between 87% to 100% and 82.5% to 100%, respectively. Phylogenetic analysis of the ORF2 genome showed that 53 strains were presented in 4 sub-genotypes (PCV2a, PCV2b, PCV2c and PCV2d). Based on comparing the nucleotides and amino acid sequences of ORF2, 42 isolates obtained in this study shared 89.6% to 100% and 86.8% to 100% identity, respectively. Genetic evolution analysis indicated that 42 isolates could be divided into three genotypes as PCV2a, PCV2b and PCV2d (69.05%), and PCV2b did not appear in Anhui and Zhejiang. Among the three genotypes, PCV2d was the predominant genotype circulating in the swine population of three provinces. Comparisons of the amino acid sequence of Cap protein revealed that 42 isolates in this study have a low variation, four major hypervariable genome regions and special substitutions among the different PCV2 genotypes. There were some aa changes in the reported antibody epitope regions of the Cap protein. However, whether those mutations could result in antigenic variations or change the pathogenicity of PCV2 and the vaccine effect should be explored further. [Conclusion] The results demonstrated that genetic evolution of PCV2 was relatively stable and PCV2b genotype was dominant in the porcine population in East China from 2010 to 2015. According to amino acid alignment data of Cap protein, different substitutions were observed in different genotypes and some representative substitutions exist in each genotype. This study provides a reference for investigating the genetic relationship, genetic evolution and pathogenesis of PCV2 in East China.