Studies On Synergetic Pathogenicity Of Co-infection With Porcine Circovirus Genotype2a/2b Strains And Porcine Parvovirus

Porcine circovirus type2(PCV2) is the main causative agent of postweaning multisystemicwasting syndrome (PMWS). According to phylogenetic analyses, PCV2can be further divided intothree genotypes which include PCV2a, PCV2b and PCV2c. Currently, PCV2a and PCV2b are widelyprevalent genotypes in the pig populations. Porcine parvovirus (PPV) is a major causative agent ofreproductive failure in sows and results in great economic loss in pig industry. Both PCV2and PPV areprevalent worldwide. Moreover, co-infection of the two viruses is common in field and has causedextensive attention. Therefore, it’s very important to investigate the synergetic pathogenicity ofco-infection with PCV2and PPV. A culture-adapted strain which named PPV/HJ strain was isolated inthis study. Synergetic pathogenicity of co-infection with PCV2and PPV was elucidated by means of pigexperimental infection. Furthermore, difference between pathogenicity of co-infection with PCVgenotype2a/2b and PPV was revealed as well. This study facilitates the control of the two virus diseases.Moreover, prevalence of porcine hokovirus (PHoV) and its mixed infection with PCV2in China wasinvestigated.First, a strain of PPV was isolated from mesenteric lymph nodes of an aborted fetus. By a series ofpassages in the swine testis cell line (ST), a culture-adapted strain, which named PPV/HJ strain, wasacquired. The virus titer of PPV/HJ strain, which had been passaged50times in ST cells, increasedremarkably since the25th passage and kept stable (107.2TCID50/mL) after the40th passage. The virusantigen mainly distributed in infected cell nucleolus by immunoperoxidase monolayer assay (IPMA).The immune complex of PPV particles with anti-PPV antibodies was observed as multiple solid virusparticles by electron microscope. The diameter of single particle was about21nm. In comparison withother13PPV NS1gene from GenBank, PPV/HJ and PPV/ZJ strain showed the highest nucleotidesimilarity which was98.4%. The50th passage PPV/HJ strain with a titer of107.2TCID50/mL was usedto inoculate35-day-old pigs. There was no evident clinical symptoms post-infection. Besides, noapparent histopathological changes were observed in the organic samples collected from theexperimental pigs post-euthanasia. However, the fact that lung, tonsil, inguinal lymph node,submandibular lymph node and small intestine were viral nucleic acid positive by PCR assay. Theresults indicated that PPV/HJ strain possessed infectivity to some degree among susceptible pigs. ThisPPV/HJ strain, which propagates stably in vitro, cultivated in the present study, facilitates furtherresearch on pathogenicity of co-infection of PCV2and PPV.In order to elucidate the synergetic pathogenicity of co-infection with PCV genotype2a/2b strainsand PPV, pig infection experiment was performed. Thirty35-day-old piglets were selected andrandomly divided into6groups: PCV2a single infection group, PCV2a+PPV co-infection group,PCV2b single infection group, PCV2b+PPV co-infection group, PPV infection group and control group.Clinical symptoms were recorded everyday post inoculation, body weight was weighed weekly andblood samples were collected regularly. Variation of CD3+CD4+CD8-, CD3+CD4-CD8+, mononuclearand granulocyte cell were determined by flow cytometry. Real-time quantitative PCR was adopted to detect viral loads in tissues, nasal and fecal swabs. Antibodies against PCV2and PPV were detected byIPMA. Enzyme linked immunosorbent assay (ELISA) was used to determine the concentration ofTNF-α和IFN-γ. Pathological changes were observed after necropsy. The results revealed that,compared with PCV2single infection, pathogenicity of PCV2and PPV co-infection was more serious.Clinical symptoms and histopathological changes induced by co-infection of PCV2and PPV were themost severe. Viral loads in the tissues, serum sample, nasal and fecal swabs collected from PCV2+PPVco-infection group were the highest. Variation of TNF-α was the most significant in this group as well.Synergetic pathogenicity of co-infection with PCV2and PPV was elucidated in the present study byexperimental infection. Furthermore, difference between pathogenicity of co-infection with PCVgenotype2a and2b, and PPV was revealed as well. This study facilitates control of the two virusdiseases.In order to investigate the prevalence of PHoV and its co-infection with PCV2in China, a total of485domestic pig samples were tested for both PHoV and PCV2by PCR. Besides, NS1gene sequencesfrom11PHoV strains were used for phylogenetic analysis. The results revealed that the prevalence ofPHoV and PCV2was51.3%and36.3%, respectively, and mixed infection occurred in20.2%. Amongall the tissue samples, the positive rates of PHoV in lymphoid nodes (79.1%) and spleen (72.1%) werethe highest. Similarly, positive rate of PCV2was47.8%in lymphoid nodes and46.5%in spleens, whichwere the highest as well. PHoVs from Chinese mainland showed a close relationship to those isolated inHong Kong. The present study provided a better understanding of PHoV prevalence and its co-infectionwith PCV2in China. In addition, the phylogenetic relationship of PHoV in domestic pigs in China andother PHoV reference strains were analyzed. The results revealed a high prevalence of PHoV in pigpopulations in China and a potential role of PHoV in the induction of PMWS. It facilitates furtherPHoV investigations.