Genetic diversity of porcine circovirus type 2 from pigs in Republic of Korea

Revisiting Porcine Circovirus Infection: Recent Insights and Its Significance in the Piggery Sector

Porcine circovirus (PCV), a member of the Circoviridae family within the genus Circovirus, poses a significant economic risk to the global swine industry. PCV2, which has nine identified genotypes (a-i), has emerged as the predominant genotype worldwide, particularly PCV2d. PCV2 has been commonly found in both domestic pigs and wild boars, and sporadically in non-porcine animals. The virus spreads among swine populations through horizontal and vertical transmission routes. Despite the availability of commercial vaccines for controlling porcine circovirus infections and associated diseases, the continuous genotypic shifts from a to b, and subsequently from b to d, have maintained PCV2 as a significant pathogen with substantial economic implications. This review aims to provide an updated understanding of the biology, genetic variation, distribution, and preventive strategies concerning porcine circoviruses and their associated diseases in swine.

Production of Bivalent Subunit Vaccine for Porcine via 2A-Like Sequence in Baculovirus Expression Vector System

Classical swine fever virus (CSFV) and porcine circovirus type 2 (PCV2) have caused severe diseases in swine populations worldwide. Here, a polycistronic baculovirus vector was developed to express a bivalent vaccine, consisting of the CSFV-E2 and PCV2-Cap protein, and an immunomodulator protein derived from the Flammulina velutipes, FVE-FIP, as well as the selection marker, green fluorescent protein. The simultaneous expression of the CSFV-E2 and PCV2-Cap protein was mediated by the 2A-like sequence derived from the Perina nuda virus (PnV), while the expression of the FVE-FIP was driven by the internal ribosome entry site (IRES) element derived from the Rhophalosipum padi virus (RhPV). The Western blot analysis result suggested that the CSFV-E2, PCV2-Cap, and FVE-FIP protein were successfully co-expressed by the infected Spodoptera frugiperda IPBL-Sf21 (Sf21) cell line. The extracted cell lysate containing all three recombinant proteins was administered to Balb/C mice with or without the supplementation of Freund’s adjuvant. The ELISA analysis of the serum collected from all the immunized groups showed detectable antibodies against CSFV-E2 and PCV2-Cap. Furthermore, the immunized group without the adjuvant supplementation demonstrated a similar level of antibodies to the group with adjuvant supplementation, suggesting the efficiency of the FVE-FIP in enhancing the immune response. These results demonstrated the polycistronic baculovirus vector could be employed to develop bivalent vaccines for pigs.

Identification and genotyping of porcine circovirus type II (PCV2) in Mexico

Porcine circovirus type 2 (PCV2), family Circoviridae, genus Circovirus infection in domestic pig has been associated with several pathological conditions being the most important of them the postweaning multisystemic wasting syndrome. Many studies have demonstrated the existence of three PCV2 genotypes (a, b, and c) and recently PCV3. Until now, these genotypes or subgenotypes have not been described in Mexico. We found genetic changes in ORF2 from nine strains of PCV2 obtained from samples of Jalisco, Veracruz, Estado de México, Hidalgo and Sonora states of Mexico. Our results shown the presence of two genotypes (PCV2a and PCV2b) as well as, the presence and differences between the reported subgenotypes. The subgenotype PCV2b (1A/1B, 1A) has a higher prevalence (87.5%) in comparison with PCV2a (2C) (12.5%).

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.

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

Porcine circovirus type 2 (PCV2) is the major causative agent of postweaning multisystemic wasting syndrome (PMWS) in Taiwanese pig farms. We analyzed the complete genomes of 571 Taiwanese PCV2 isolates in Taiwan from 2001 to 2011 and divided the isolates into 2 distinct genotypes (PCV2a and PCV2b) with 6 clusters (1A, 1B, 1C, 2B, 2D, and 2E). Of the 571 Taiwanese PCV2 isolates, 22.9% (131/571) belonged to PCV2a and 77.1% (440/571) to PCV2b. In this study, PCV2a isolates were the most common in 2001, and then PCV2b isolates became predominate thereafter and widely distributed in pig farms since 2003. Sequence comparisons among the 571 isolates indicated that 89.6-100% had nucleotide identity for complete genome and 87.3-100% for open reading frames 2 (ORF2). The results suggest that a higher genetic variation and shift occurred among PCV2 isolates collected from 2001 to 2011 in Taiwan.

Prevalence and phylogenetic analysis of the current porcine circovirus 2 genotypes after implementation of widespread vaccination programmes in the USA

To determine the prevalence of porcine circovirus 2 (PCV2) genotypes in the USA during 2010–2011, 5 years after widespread PCV2 vaccination, serum samples from clinically normal pigs that were PCV2 vaccinated (n = 1177), non-vaccinated (n = 378) or of unknown vaccination status (n = 120), and 100 lung samples from pigs diagnosed with PCV-associated disease (PCVAD) were tested. The presence of PCV2, PCV1, PCV1-2a and porcine parvovirus (PPV) DNA was determined by PCR. Determination of the PCV2 genotype was done by differential PCR and sequencing. The prevalence of PCV2a and PCV2b in serum samples was 7.7 % (129/1675) and 8.4 % (141/1675), respectively. PCV2a DNA was only detected in non-vaccinated pigs. For the 100 PCVAD pigs, the prevalence of PCV2a and PCV2b in lung tissues was 13.0 and 65.0 %, respectively. Partial PCV2 ORF2 sequences (9–563 nt) were obtained from 85 PCV2 DNA-positive samples (24 normal pigs and 61 PCVAD cases). Phylogenetic analysis revealed that 12.9 % (11/85) of the sequences belonged to the 2E clade and the PCV2a genotype and 87.1 % (74/85) belonged to the 1B clade and the PCV2b genotype. The alignment of putative PCV2 capsid amino acid sequences revealed possible recombination or mutation between PCV2a and PCV2b genotypes. Chimeric PCV1-2a was not detected in any of the samples and the prevalence rates of PCV1 and PPV were low. Our results suggest PCV2b is more prevalent than PCV2a in PCVAD cases and in vaccinated herds PCV2b circulation is common. The data generated in this study provide novel information on the distribution of PCV2 genotypes in vaccinated pig populations.

Immunology of porcine circovirus type 2 (PCV2)

The emergence of porcine circovirus type 2 (PCV2) associated diseases and particularly postweaning multisystemic wasting syndrome (PMWS) was a shock for the swine industry and formulated a considerable challenge for researchers in the area of viral immunology in swine. The unique features of PMWS of which emaciation and lymphoid depletion were the most evident indicated a deep involvement of the immune system of the pig in the pathogenesis of this condition and indicated that PCV2 was a singular pathogen. Also, the multifactorial nature of the disease complicated the understanding of PMWS pathogenesis. Nowadays, it is known that PCV2 deeply affects the functionality of the immune system of the pig but also the industry has been able to produce efficacious vaccines. In the present paper some of the most relevant immunological features of PMWS and of PCV2 infection in general will be reviewed.

Porcine circovirus type 2 and its associated diseases in Korea

This review describes the characterization of porcine circovirus type 2 (PCV2), the field situation of porcine circovirus-associated disease (PCVAD) and the PCV2 vaccine in Korea. PCVAD has been considered the most devastating disease in Korean livestock history since its first outbreak in 1999. Postweaning multisystemic wasting syndrome (PMWS) and porcine respiratory disease complex (PRDC) are the most common clinical forms of PCVAD. Interestingly, only PCV2b strains have been isolated from pigs with PMWS since 2005, but only PCV2a strains were isolated from pigs with PMWS in 2000 to 2001. Clinically, PMWS is divided into two stages: early and late. Early PMWS primarily occurs in pigs between 4 and 8 weeks of age. This form is a typical presentation of PMWS and is characterized clinically by wasting, decreased weight gain, enlarged lymph nodes, and dyspnea. Late PMWS primarily occurs in pigs between 8 and 12 weeks of age. The main clinical manifestation is diarrhea, which is often accompanied by salmonellosis coinfection. In recent years, the PCVAD disease pattern has slightly changed. The occurrence of PMWS has decreased while PRDC cases are increasing in frequency.

Efficacy and future prospects of commercially available and experimental vaccines against porcine circovirus type 2 (PCV2)

Porcine circovirus type 2 (PCV2) is the causative agent of an economically significant collection of disease syndromes in pigs, now known as porcine circovirus associated diseases (PCVADs) in the United States or porcine circovirus diseases (PCVDs) in Europe. Inactivated and subunit vaccines based on PCV2a genotype are commercially available and have been shown to be effective at decreasing mortality and increasing growth parameters in commercial swine herds. Since 2003, there has been a drastic global shift in the predominant prevalence of PCV2b genotype in swine populations, concurrently in most but not all cases with increased severity of clinical disease. Although the current commercial vaccines based on PCV2a do confer cross-protection against PCV2b, novel experimental vaccines based on PCV2b genotype such as modified live-attenuated vaccines are being developed and may provide a superior protection and reduce vaccine costs. In this review, we discuss the current understanding of the impact of PCV2 infection on the host immune response, review the efficacy of the currently available commercial PCV2 vaccines in experimental and field conditions, and provide insight into novel experimental approaches that are useful in the development of next generation vaccines against PCV2.

Genetic characterization of porcine circovirus type 2 from pigs with porcine circovirus associated diseases in Argentina

Porcine circovirus type 2 (PCV-2) has been associated with syndromes grouped by the term porcine circovirus associated diseases (PCVAD). The PCV-2 isolates have been grouped into two major groups or genotypes according to their nucleotide sequence of whole genomes and/or ORF-2: PCV-2b, which have, in turn, been subdivided into three clusters (1A-1C), and PCV-2a, which has been subdivided into five clusters (2A-2E). In the present study, we obtained 16 sequences of PCV-2 from different farms from 2003 to 2008, from animals with confirmatory diagnosis of PCVAD. Since results showed an identity of 99.8% among them, they were grouped within a common cluster 1A-B. This preliminary study suggests a stable circulation of PCV-2b among the Argentinean pig population.

Genetic characterization of porcine circovirus type 2 in piglets from PMWS-affected and -negative farms in Thailand

Porcine circovirus type 2 (PCV2) is the major swine pathogen associated with Porcine circovirus associated disease (PCVAD) including post-weaning multisystemic wasting syndrome (PMWS). Currently, there are 4 subtypes of PCV2 (PCV2a, b, c and d) and some epidemiological evidences demonstrated that virulence of PCV2 may relate to its subtypes. Recently, PMWS was observed more frequently in swine farms in Thailand; however, the information regarding to PCV2 subtype involved was limited. Therefore, this study was aimed to determine the association between occurrence of PMWS and PCV2 subtypes as well as genetically characterize PCV2 in Thailand. PCV2 DNA was isolated from faecal swabs and whole blood of piglets from PMWS-affected and -negative farms. The full length ORF2 sequences were compared using multiple alignment. The results showed that PCV2 DNA was detected more frequently in PMWS-affected farms. The nucleotide identities of the ORF2 from 9 PCV2 isolates representing each PMWS-affected farm and one from the negative farm ranged from 92.4 to 99.5% suggesting that there is some genetic variation of PCV2 in Thai swine. The 10 PCV2 isolates were classified into 2 clusters, in which the 7 isolates from PMWS-positive farms were in PCV2b cluster 1 A/B. The remaining isolates were separated in the new subtype called PCV2e. The results suggest the presence of new PCV2 subtypes in addition to PCV2a and PCV2b in Asian swine population. However, correlation between subtypes and virulence of PCV2 infection is not conclusive due to limited number of the PCV2 sequences from PMWS negative farms.

Epidemiology and horizontal transmission of porcine circovirus type 2 (PCV2)

Porcine circovirus type 2 (PCV2) is a small, non-enveloped, circular, single-stranded DNA virus of economic importance in the swine industry worldwide. Based on the sequence analyses of PCV2 strains, isolates can be divided into five subtypes (PCV2a–e). PCV2 is an ubiquitous virus based on serological and viremia data from countries worldwide. In addition, PCV2 DNA was discovered in archived samples prior to the first recognition of clinical disease. Recently, a worldwide shift in PCV2 subtype from PCV2a to PCV2b occurred. PCV2 DNA can be detected in fecal, nasal, oral and tonsillar swabs as well as in urine and feces from both naturally and experimentally infected pigs. PCV2 DNA can be detected early in the infectious process and persists for extended periods of time. The effectiveness of disinfectants for reducing PCV2in vitrois variable and PCV2 is very stable in the pig environment. Limited data exist on the horizontal transmission of PCV2. Direct transmission of PCV2 between experimentally or naturally infected animals and naïve animals has been documented and the incorporation of clinical or subclinically infected animals into a population represents a risk to the herd. Indirect transmission through the oral, aerosol or vaccine routes is likely a lesser risk for the transmission of PCV2 in most swine populations but may be worth evaluating in high heath herds. The objective of this review was to discuss data on the epidemiology and horizontal transmission of PCV2.