Development of a drtqPCR Assay for Differentiation of PCV-2 a and 2 b in Shanghai Area

A dual real-time quantitative polymerase chain reaction assay (drtqPCR) was established to detect and differentiate between Porcine circovirus-2a (PCV-2a) and Porcine circovirus-2b (PCV-2b). Genotype-specific primer sets and probes were designed by using sequence data published for different PCV-2 strains. Specificity and sensitivity of the drtqPCR were examined by using PCV-2 isolates with known genotype. Among 367 tissue samples, 44.69% (164/367) were PCV-2 positive. From 164 PCV-2 positive samples, 10.98% (18/164), 92.56% (137/164), and 3.31% (9/164) were positive for PCV-2a, PCV-2b, and both genotypes, respectively. These results suggest that the differential drtqPCR can be used to detect PCV-2 and to differentiate the 2 genotypes from field samples. The PCV-2 infection is quite common in swine of Shanghai area. Furthermore, the PCV-2b infective ratio is far higher than PCV-2a, and PCV-2a/2b mixed infections are also observed but at a lower prevalence in Shanghai area.


Introduction
Porcine circovirus-2 (PCV-2) is a very small, circular, single stranded DNA virus about 1.76 kb in length [1].Viral DNA possesses at least 3 functional open reading frames (ORFs): ORF1 encodes the Rep proteins involved in virus replication; ORF2 encodes the nucleocapsid (Cap) protein; and ORF3 encodes a protein that induces apoptosis and is involved in viral pathogenesis in vivo [2].The virus has been associated with several dis-ease syndromes in pigs, including postweaning multisystemic wasting syndrome (PMWS), porcine dermatitis and nephropathy syndrome, reproductive failure, and porcine respiratory disease complex.These syndromes together are considered porcine circovirus-associated disease (PCVAD) [1] [3].
In general, PCV-2 strains from different geographical areas were also shown to be differentiated into genetic groups 1 and 2. PCV-2 groups 1 and 2 were found to contain 1767 and 1768 nucleotides, respectively [4] [5].In phylogenetic trees, group 1 viruses were classified in the same cluster of PCV-2b, whereas group 2 viruses were classified in the cluster of PCV-2a.PCV-2 genotypes have been classified by several methods, including viral genomic sequencing, restriction fragment length polymorphism (RFLP) analysis, situ hybridization assay, nested polymerase chain reaction assay, and genotype-specific PCR [6]- [8].Therefore, a dual real-time quantitative polymerase chain reaction (drtqPCR) assay was developed to detect with high sensitivity PCV-2 strains present in tissue samples of pigs, as well as this PCR assay was able to identify PCV-2 genotypes by targeting the ORF2 gene.Each of these methods has advantages and disadvantages in obtaining the results.It is generally accepted that drtqPCR is less time consuming and labor consuming than RFLP, nPCR, and sequencing methods.In this study, we will use the drtqPCR assay to show the PCV-2a and PCV-2b infection in pigs of Shanghai area.

Reference Strains
Download reference strains sequences from GenBank (Table 1).The 12 gene sequences of PCV-2 strains in GenBank were from different countries in North America, Europe, and Asia.

Porcine Circovirus-2 Recombinant Plasmids
Extraction of KF850469 and KF850463 isolates of DNA was performed using the Qiagen QIAamp DNA Mini Kit according to the manufacturer's instructions.The PCR products encompassing the partial viral genome of both strains were produced using the primers: C-F (5'-CCA, GAA, TTC, AAC, YTT, AAC, CTT, YCT, TAT-3'), C-R (5'-GRC, RGT, GGA, CAT, GMT, GAG, A-3').The PCR products were subsequently cloned in the plasmid vector pMD18-T.Finally, the recombinant plasmids (pMD18-T/PCV-2a and pMD18-T/PCV-2b) were sequenced to verify the accuracy of the nucleotide sequences and were used as DNA templates and positive samples to validate the new drtqPCR diagnostic assay.

drtqPCR Assays
In the dual reaction, 2 µl of sample containing the DNA template was added to a 28-µl reaction mixture (Final concentration: dATP, dTTP, dGTP and dCTP 0.24 mM respectively; Mg 2+ 2.4 mM; C-F and C-R 0.32 µM; C-2a-Prob and C-2b-Probe 5 µM; Taq1 U/µL; Taq protection 0.2 U/µL).The PCR reaction started with an initial denaturation and polymerase-activating step of 94˚C for 3 min, followed by 40 amplification cycles of a 3-step PCR (94˚C for 15 sec; 60˚C for 40 sec and detecting fluorescent light).Fluorescence channels are FAM and HEX.

Standard Curves
The standard curve were assayed for assessing the stability of the drtqPCR.The recombinant plasmids pMD18-T/PCV-2a and pMD18-T/PCV-2b were 10-fold serial diluted with the original concentrations 5.4 ng/µl and 7.2 ng/µl by Thermo NANODROP 2000 respectively.

Application of drtqPCR Assay on Tissue Samples
A total of 367 tissue samples from pigs were randomly selected from Shanghai swine farms.173 samples were from dead pigs tissue for disease, and 194 samples were healthy pigs tissue (Table 2).

Standard Curves
The standard curves of drtqPCR were performed, and R 2 were 0.998 for PCV-2a and 0.984 for PCV-2b.The slope values were −3.578 and −3.477, and calculated E value were 90.3% and 93.9% respectively (Figure 1).

Discussion
Porcine circovirus-2 is now considered a global disease and the cause of significant economic losses in the swine industry [9].Nonetheless, it strongly suggests that PCV-2b virus could be more virulent than PCV-2a virus.Moreover, others have reported that genetic variations may have a great impact on the virulence of PCV-2 [10].One of the main objectives of the swine industry and health scientists is to establish if there is virulence variation between PCV-2a and PCV-2b viruses.Consequently, it has become critical for swine producers and veterinarians to establish which PCV-2 genotypes are present in their herds [11].In the present study, the established drtqPCR was believed that it combines specificity and sensitivity, and reduces the labor and time intensiveness of diagnosing PCV-2a and PCV-2b.The drtqPCR was verified by various samples in this study.
In the present study, the ratio of PCV-2 in dead pigs was 54.34%, as 34.02% in healthy.The ratio in dead pigs was higher than in healthy, and both of them raised in recent years of Shanghai area.This suggests that dead pigs for infectious diseases was related of PCV-2, but not all pigs infected with PCV-2 virus may lead to death, and infected PCV-2a/2b could be no any clinics.9 PCV-2a/2b co-infections were also observed but at a lower prevalence in Shanghai area (2.45%).It was remarkable that the co-infection ratio was rising in recent years.The ratio of PCV-2b was much higher than PCV-2a.The ratio of PCV-2a/2b co-infected rose from 2009 to 2011, whereas PCV-2a was low.This suggests PCV-2b was the most viruses in Shanghai area, and PCV-2a/2b co-infected would impact the farms of Shanghai in the future.

Conclusion
In conclusion, the new drtqPCR diagnostic assay is suitable for the sensitive identification and differentiation of PCV-2 and is a more convenient approach than RFLP [12], partial or entire viral genome sequencing, and conventional PCR techniques [13].Furthermore, it could efficiently quantify PCV-2 in submitted samples and provide a risk assessment in regard to the odds of developing death.

Table 2 .
Origin of samples.