| Porcine Circovirus(PCV)is a non-enveloped circular DNA virus,three different types of porcine circoviruses have been identified,namely PCV1,PCV2 and PCV3.PCV1 is considered to be non-pathogenic to pigs,and PCV2 is one of the important pathogens in the pig industry,it is the main cause of porcine circovirus associated diseases,Porcine circovirus disease is also one of the diseases that cause major economic losses in the global pig industry and is widespread worldwide.It mainly causes postweaning multisystemic wasting syndrome,porcine dermatitis and nephrotic syndrome,reproductive disorders,etc;PCV3 is a new circoviruses discovered by American scholars Palinski and Phan from the swine herds in 2016.After the discovery of PCV3 in the US swine herds,there have been reports in the literature that PCV3 is detected in swine herds in the Americas,Asia,Europe,and South America.The clinical significance of PCV3 has not been well described,and its pathogenicity need further research.In order to understand the epidemic and infection sutation of PCV2 in swine herds in Southern China,this study conducted a pathogen and serological survey of PCV2 in swine herds in Southern China.From January 2015 to December 2017,10293 pig serum samples and 877 tissue samples were collected from various large-scale pig farms in Southern China for antibody detection and pathogen detection.The results showed that the total positive rate of PCV2 serum antibody reached 90.25% in the past three years,and the positive rate of PCV2 serum antibody increased year by year,the total positive rate of antigen in the past three years was 30.33%,and the antigen positive rate in different years was quite different.Pathogenic and serological results indicate that PCV2 infection is common in swine herds in southern China.In order to further understand the major epidemic strains of PCV2 in swine herds in Southern China and their genetic variation,this study performed PCV2 whole genome sequencing on samples positive for PCV2 antigen,and obtained a total of 117 PCV2 strains.The analysis showed that the nucleotide homology of the 117 PCV2 genomes is between 93.6% and 100%.The results of genetic evolution analysis showed that 57 strains belonged to the PCV2 d branch,40 strains belonged to the PCV2 a branch,and 20 strains belonged to the PCV2 b branch.This indicates that the three genotypes PCV2 a,PCV2b and PCV2 d are common in swine herds in Southern China.The main epidemic strain is PCV2 d.The analysis of the ORF1 and ORF2 gene sequences shows that the ORF1 gene is very conserved and and the cap protein encoded by ORF2 gene had different mutation sites,mainly 52～91 aa,121～151 aa,185～215 aa.In order to understand the epidemiological characteristics and genetic evolution of PCV3 in swine herds in Southern China,this study conducted a retrospective epidemiological survey on 285 tissue materials collected from January 2015 to June 2017.The pathogen investigation showed that the positive rate of PCV3 antigen showed a trend of increasing year by year,and the total positive rate was 26.7%.PCV3 also has a co-infection with PCV2 and PRRSV.In addition,PCV3 has a wide range of tissue tropism and can detect the presence of PCV3 from lung,tonsil,lymph nodes,kidneys,and aborted fetal tissues,and has the highest detection rate in aborted fetuses.The genetic evolution and molecular characterization of the PCV3 Cap gene showed that the two amino acid sites based on the Cap protein(A24V and R27K)can separate PCV3 into three different branches(PCV3a,PCV3 b,PCV3c);Bayesian genetic evolution analysis showed that PCV3 may have originated from bat-related circovirus,and its origin time is about 1966,indicating that PCV3 may have been popular in swine herds for nearly half a century.The results of this study indicate that PCV3 is widely prevalent in swine herds in Southern China and has certain genetic diversity.In addition,based on the SYBR Green I dye method,a pair of specific primers were designed in the conserved region of the PCV3 ORF2 gene sequence,and the PCV3 SYBR Green I real-time PCR assay was successfully constructed by optimizing the reaction conditions.The sensitivity of this method is high,and the limit of detection is 7.68×101 copies/μL.The detection method is specific and has no cross-amplification with PCV2,PPV,PRV,PRRSV,CSFV and SIV genomes;the repeatability is high,and the inter-assay and intra-assay coefficient of variation are less than 1.5%.The high sensitivity,good specificity and repeatability indicate that this method can be used to detect PCV3 in clinical samples.In summary,this study conducted a systematic epidemiological survey of PCV2 and PCV3 in swine herds in Southern China,revealing the major epidemic strains and genetic variation of PCV2 in swine herds in Southern China,and confirmed that the newly discovered PCV3 is also widespread and prevalent in swine herds in Southern China,revealing the tissue distribution and genetic evolution history of PCV3 in swine herds,And the PCV3 q PCR detection method was successfully constructed,which laid a foundation for monitoring the epidemic regularity of PCV2 and PCV3,as well as the related research on prevention and control and pathogenesis of PCV2 and PCV3. |
