Continuous evolution of porcine circovirus type 2 in Korea

Porcine circovirus type 2 (PCV2) has a circular, single-stranded DNA (ssDNA) genome, which consists of two major open reading frames (ORF1 and 2). PCV2 isolates belong to three genotypes (PCV2a, PCV2b and PCV2c), based on the difference of ORF2 sequence. In 2009, a molecular epidemiology study on Chinese PCV2 isolates identified the presence of novel PCV2d genotypes based on phylogenetic analysis. In particular, PCV2d genotype has drawn much attention from the public because novel PCV2 has emerged in the cases of suspected vaccine failure. Global molecular analysis suggested that genotype shift to PCV2d is currently an ongoing process among pig population. The objectives of this study were to develop our knowledge on the genetic diversity of Korean PCV2 isolates and to further investigate the frequent emergence of PCV2d genotype.

Since 2009, clinical samples were collected from commercial pig farms. Total DNA was extracted from clinical samples. Extracted DNA was mixed with the PCV2-specific primer set in PCR premix. ORF2 sequence or full-length PCV2 sequence was amplified with specific primers. The target band was purified and sequenced. Phylogenetic analysis were performed by the neighbor-joining method with 1000 of bootstrapping values.

Of total 78 samples, 58 samples were positive for PCV2 (75% of positive rate). Eighteen PCV2 isolates from 15 pig farms were sequenced. Phylogenetic analysis of ORF2 showed that five isolates were classified as PCV2a, seven as PCV2b and six as PCV2d genotype, repectively. On the basis of chronological order, 3/4/0 isolates in 2009 and 1/3/3 in 2012 and 1/0/3 in 2015 belonged to PCV2a/PCV2b/PCV2d, respectively. Notably, of five PCV2a, one isolate was classified within PCV2a-2C, which had been not detected worldwide since the last identification in 2006 in Europe. Our result indicated that PCV2d genotype was frequently detected in Korean pig farms. Interestingly, we found two cases of genotype shift (PCV2a to PCV2d and PCV2b to PCV2d) and one concurrent infection of two genotype (PCV2a and PCV2d) on the farm level, which implies continuous genotype shift to PCV2d in Korean pig farms.

Our study showed considerable genetic variation of PCV2 in Korea. Interestingly, continuous evolution of PCV2 directed ongoing genotype shift to PCV2d after the emergence in around 2012. Therefore, continuous surveillance of PCV2 and update on PCV2 vaccine should be needed to prepare effective strategies against emergence of novel strains from viral evolution.

Disclosure of Interest: None Declared.