Prevalence and genetic variation of porcine circovirus type 2 in Taiwan from 2001 to 2011

Molecular epidemiology and genetic variation analyses of porcine circovirus type 2 isolated from Yunnan Province in China from 2016-2019

BACKGROUND

Porcine circovirus type 2 (PCV2) is the causative agent of porcine circovirus-associated disease (PCVAD). Its prevalence in swine herds was first reported in China in 2000. PCV2 infection causes immunosuppression that leads to multiple diseases, causing serious economic problems for the swine industry in China. Since information on the genetic variation of PCV2 in Yunnan province is limited, this study aims to investigate the molecular epidemiological and evolutionary characteristics of PCV2 from 2016 to 2019.

METHODS

A total of 279 clinical samples were collected from different regions of Yunnan between 2016 to 2019, and PCV2 was detected by PCR. We then amplified full genomes from the positive samples, and the sequences were analysed for homology and genetic evolution.

RESULTS

Overall, 60.93% (170/279) of the screened swine herd samples were positive for PCV2. We sequenced 15 Yunnan province PCV2 strains from positive samples. Analyses of the complete genomes and Cap genes led to the classification of the 15 Yunnan PCV2 strains into PCV2a (2 of 15), PCV2b (1of 15) and PCV2d (12 of 15). All strains shared 94.3-99.9% of their identities with the nucleotide sequences of complete genomes in this study and shared 94.2-99.9% identity with the reference sequences. All strains share 89.4-100% and 86.8-100% of their identities with the nucleotide and amino acid (aa) sequences of Cap, respectively.

CONCLUSIONS

The results of this study provide evidence that PCV2a, PCV2b and PCV2d genotypes coexisted in Yunnan Province from 2016 to 2019, and the priority prevalence genotype was PCV2d. The data provide evidence for the increased genetic diversity and insights into the molecular epidemiology of PCV2. This study also provides basic data for the Yunnan province PCV2 molecular epidemiological survey and accumulates effective materials for the development of PCV2 vaccines.

Phylogeographic and genetic characterization of porcine circovirus type 2 in Taiwan from 2001-2017

Porcine circovirus type 2 (PCV2) is an important pathogen that causes significant economic losses in the swine industry worldwide. Five major PCV2 genotypes have been identified, including PCV2a, PCV2b, PCV2c, PCV2d, and PCV2e. To investigate the prevalence and phylodynamics of the different PCV2 genotypes in Taiwan, 214 PCV2 ORF2 sequences from Taiwan and other countries were analyzed. Genotypic differences were observed among PCV2a, 2b, and 2d at amino acid position 89 in ORF2, with isoleucine (I), arginine (R), and leucine (L), respectively. Similar to other countries, a genotypic shift was also observed in Taiwan, where the predominant genotype shifted from PCV2b to 2d after 2010. The estimated nucleotide substitution rate of Taiwanese strains in the ORF2 region was 8.467 × 10⁻⁴ substitutions per site per year. This rapid evolution rate of PCV2 may lead to the genotypic shift observed in Taiwan. The times to the most recent common ancestor (TMRCA) for PCV2a, -2b, and -2d-2 was dated to 1970, 1992 and 2004, respectively. Thus, the PCV2a, -2b, and -2d genotypes were already present in Taiwan before the introduction of the PCV2 vaccine.

Evidence of natural co-infection with PCV2b subtypes in vivo

Porcine circovirus type 2 (PCV2) is the causative pathogen of porcine circovirus-associated diseases (PCVAD). This virus evolves mostly through point mutations and genome recombination between different PCV2 genotypes (e.g. PCV2a and PCV2b), as has been confirmed in swine herds. In the current work, the complete PCV2 genome sequences of 69 clones derived from various tissues (lymph node, spleen and lung,) of an infected individual, were subjected to phylogenetic and alignment analyses. The results not only demonstrate co-infection with distinct PCV2b subtypes (e.g. 1B and 1C) in the same animal, but also highlight another mechanism of evolution - diverse point mutations acquired during immune evasion by this virus.

In silico analysis of surface structure variation of PCV2 capsid resulting from loop mutations of its capsid protein (Cap)

Outbreaks of porcine circovirus (PCV) type 2 (PCV2)-associated diseases have caused substantial economic losses worldwide in the last 20 years. The PCV capsid protein (Cap) is the sole structural protein and main antigenic determinant of this virus. In this study, not only were phylogenetic trees reconstructed, but variations of surface structure of the PCV capsid were analysed in the course of evolution. Unique surface patterns of the icosahedral fivefold axes of the PCV2 capsid were identified and characterized, all of which were absent in PCV type 1 (PCV1). Icosahedral fivefold axes, decorated with Loops BC, HI and DE, were distinctly different between PCV2 and PCV1. Loops BC, determining the outermost surface around the fivefold axes of PCV capsids, had limited homology between Caps of PCV1 and PCV2. A conserved tyrosine phosphorylation motif in Loop HI that might be recognized by non-receptor tyrosine kinase(s) in vivo was present only in PCV2. Particularly, the concurrent presence of 60 pairs of the conserved tyrosine and a canonical PXXP motif on the PCV2 capsid surface could be a mechanism for PXXP motif binding to and activation of an SH3-domain-containing tyrosine kinase in host cells. Additionally, a conserved cysteine in Loop DE of the PCV2 Cap was substituted by an arginine in PCV1, indicating potentially distinct assembly mechanisms of the capsid in vitro between PCV1 and PCV2. Therefore, these unique patterns on the PCV2 capsid surface, absent in PCV1 isolates, might be related to cell entry, virus function and pathogenesis.

In silico analyses of antigenicity and surface structure variation of an emerging porcine circovirus genotype 2b mutant, prevalent in southern China from 2013 to 2015

Porcine circovirus type 2 (PCV2) is the pivotal pathogen causing porcine circovirus-associated diseases. In this study, 62 PCV2 isolates were identified from seven farms in southern China from 2013 to 2015 and phylogenetic trees were reconstructed based on whole-genome sequences or the cap gene. In this investigation, PCV2b was the main genotype in circulation throughout these farms. Furthermore, an emerging mutant (PCV2b-1C), isolated from PCV2-vaccinated farms, was the predominant strain prevalent on these farms. In addition, we isolated a new cluster that may represent evolution of the virus through recombination of PCV2b-1A/1B and PCV2b-1C. Finally, we discuss evidence that antigenicity and surface structure variation of the capsid resulted from mutation of the C-terminal loop (Loop CT) of the PCV2b-1C Cap in silico.

A novel subunit vaccine co-expressing GM-CSF and PCV2b Cap protein enhances protective immunity against porcine circovirus type 2 in piglets

Porcine circovirus type 2 (PCV2) causes porcine circovirus-associated disease. Capsid (Cap) protein of PCV2 is the principal immunogenic protein that induces neutralizing antibodies and protective immunity. GM-CSF is an immune adjuvant that enhances responses to vaccines. In this study, recombinant baculoviruses Ac-Cap and Ac-Cap-GM-CSF expressing the Cap protein alone and co-expressing the Cap protein and porcine GM-CSF, respectively, were constructed successfully. The target proteins were analyzed by western blotting and IFA. Further, these proteins were confirmed by electron microscopy, which showed that Cap proteins could self-assemble into virus-like particles having diameters of 17-25nm. Animal experiments showed that pigs immunized with Cap-GM-CSF subunit vaccine showed significantly higher levels of PCV2-specific antibodies and neutralizing antibodies than pigs immunized with the Cap subunit vaccine and a commercial vaccine (Ingelvac CircoFLEX; P<0.05). After PCV2 wild strain challenged, Pigs receiving the Cap-GM-CSF subunit vaccine showed significantly higher average daily weight gain after wild-type PCV2 challenge than pigs receiving the other three vaccines (P<0.05). None of PCV2 DNA was detected in all immunized animals, except control animals immunized with phosphate-buffered saline. These results indicated that GM-CSF was a powerful immunoadjuvant for PCV2 subunit vaccines because it enhanced humoral immune response and improved immune protection against PCV2 infection in pigs. Thus, the novel Cap-GM-CSF subunit vaccine has the potential to be used as an effective and safe vaccine candidate against PCV2 infection.

Porcine circovirus type 2 in China: an update on and insights to its prevalence and control

Currently, porcine circovirus type 2 (PCV2) is considered the major pathogen of porcine circovirus associated-diseases (PCVAD) that causes large economic losses for the swine industry in the world annually, including China. Since the first report of PCV2 in 1998, it has been drawing tremendous attention for the government, farming enterprises, farmers, and veterinary practitioners. Chinese researchers have conducted a number of molecular epidemiological work on PCV2 by molecular approaches in the past several years, which has resulted in the identification of novel PCV2 genotypes and PCV2-like agents as well as the description of new prevalence patterns. Since late 2009, commercial PCV2 vaccines, including the subunit vaccines and inactivated vaccines, have already been used in Chinese swine farms. The aim of this review is to update the insights into the prevalence and control of PCV2 in China, which would contribute to understanding the epidemiology, control measures and design of novel vaccines for PCV2.

Genetic characterization of porcine circovirus type 2 in the Korean wild boar population

The porcine circovirus type 2 (PCV2) has reached very high levels in the pig population in South Korea and throughout the world since it was first described in the late 1990s. In this study, we found that the prevalence of PCV2 in the Korean wild boar population was 4.98% (91/1825). Interestingly, 19 PCV2 ORF2 sequences that could be completely sequenced showed that they belonged only to genotype PCV2b, subgroup 1A/B (n=16) and 1C (n=3). We suggest that sites potentially under positive selection are responsible for the antigenicity changes and phenotypic switch patterns in the capsid gene of 55 PCV2s from Korean domestic pigs, but the sites potentially under positive selection in the 19 PCV2 ORF2 genes from Korean wild boar are not responsible for antigenicity.