Molecular Epidemic Characteristics and Genetic Variation Analysis of Porcine Circovirus 3 in Some Pig Farms in Central China from 2020 to 2022
[Objective]The epidemiological and genetic variation characteristics of porcine circovirus 3 in some parts of central China were investigated through a systematic experimental protocol,which provided the data of basis for the research of PCV3 vaccine.[Methods]A total of 3 500 clinical samples collected from 15 large-scale pig farms in central China(Hubei,Hunan and Henan)were detected by real-time PCR,and the relationship between different pig herds,different tissues and organs,different amount of detoxification and the characteristics of corresponding epidemiological symptoms was analyzed.The whole genome of PCV3 for some positive samples was amplified,sequenced and analyzed.[Result]50.86%(1 780/3 500)of the tested samples were positive for PCV3 nucleic acid,pigs at different ages were susceptible to PCV3,and the positive rates of PCV3 were higher in nursing pigs,suckling piglets and growing finishing,which were 70.44%(498/707),67.19%(596/887)and 41.75%(177/424),respectively.The positive rates of PCV3 in lymph nodes,lung,and postpartum placenta samples were 67.05%(59/88),63.79%(74/116),and 49.11%(55/112),while the positive rates of PCV3 in blood and colostrum samples were 56.09%(502/895)and 44.20%(278/629),respectively,and PCV3 was detected in nasal swabs and saliva.The positive rates of PCV3 was high in the pig with symptoms of reproductive disorder,which were 44.43%(399/898),36.43%(431/1183),and 28.04%(233/831),respectively.The length of the whole genome sequence of the 24 PCV3 was 2 000 nt,and the homology of the whole genome nucleotide sequence among the 24 sample strains was 98.4%-100%,and the homology of the whole genome with the reference strain PCV3 at home and abroad was 97.4%-99.5%.Genetic evolution analysis showed that 23 strains of PCV3 belonged to the PCV3b subtype and 1 strain belonged to the PCV3a subtype.The results of Rep amino acid sequence alignment showed that PCV3-L2,L23 and L14 strains had unique variation sites in(N124I),(A183E)and(V244I),respectively.The results of Cap protein amino acid sequence alignment showed that PCV3-L15,L21,L3 and L19 strains had unique variation sites in(T45P),(R2K),(R14K)and(F7L),respectively.[Conclusion]PCV3 can infect pigs at different ages and is distributed in different tissues and organs.PCV3 can infect pigs through vertical transmission(such as colostrum and semen)and horizontal transmission(such as oral and nasal secretions and saliva).PCV3 infection may be closely related to reproductive disorders,respiratory disorders,multiple organ inflammation and arthritis in pigs.In addition,both PCV3a and PCV3b subtypes were prevalent in large-scale pig farms in the surveyed area,and the PCV3b subtype was the dominant strain.In this study,genome-wide amino acid sequence analysis revealed that some unique variation sites were located in Cap protein,which may lead to changes in the immunogenicity conferred by Cap protein.In this study,the genetic evolution of the whole gene sequence of PCV3 is elaborated,aming to lay a foundation for future research on PCV3 vaccines.
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