| Porcine circovirus (PCV), which belongs to the family Circoviridae, genus Circovirus, is one of the smallest animal viruses with non-nenveloped, single-stranded circular genome and a size of 17 nm in diameter. Since the first report of disease caused by PCV2 in Canada in 1991, PCV2 infection has spread rapidly in a global scale. The diseases associated with PCV2 also appeared in several regions of China and have resulted in increasing problems to date. PCV2 infection was found as early as 1990s in Ningbo province and has caused huge economic impact on swine industry. To get insights of the prevalence of PCV2 associated diseases in Ningbo and to develop an efficient method to monitor and control the infections, we intended to carry out epidemiological study of PCV2 infections.Surveys on PCV2 infection were undertaken via clinical observations, necropsy and detection of pathogens in the laboratory. It was found that the infection mostly occurred in piglets aged 2-3 weeks. In its outbreak, the morbidity and mortality could be up to 60% and 25%, respectively, and the infection would last for 7-10 days or even longer. The affected pigs showed growth retardation (56%), dyspnea (43), cough (41%) and diarrhea. The characteristics of pathological findings were wasting (76%), enlarged lymph nodes (62%) and spleen, pulmonary consolidation (53%), kidneys with multiple white-foci (25%) and jaundice. Thirty-nine cases were found as PCV2 positive by PCR out of 96 samples from 7 regions of Ningbo, with a positive rate of 40.6%, indicating that PCV2 infection is prevalent in Ningbo and associated with the respiratory disease in finishing pigs.As there was no PCV2 vaccines used in preventitative strategies to date, the antibodies of PCV2 detected in sera were due to viral infection. Therefore,524 serum samples from 19 herds of 8 regions were analyzed by ELISA. Results showed that the positive rate at herd level was 84.21%(16/19). The sero-prevalence in sows and post-weaning piglets was 45.72%(123/269) and 54.18%(138/255), with an average rate of 49.81%(261/524). There were differences of positive rate within herds since Ninghai and Beilun had more PCV2 positive herds. Moreover, consistently with other studies, sero-positive rate of PCV2 infection in piglets increased from 19.12% in piglets younger than 30 days to 54.14% or 72.26% in pigs aged of 30-70 days or 70-100 days, respectively.Co-infections and secondary infections of PCV2 with other pathogens were found commonly in the fields. In this study, PCR and indirect hemagglutination test were used to study the relationships between PCV2 infection and porcine reproductive and respiratory disease (PRRS), pseudorabies (PR), classical swine fever (CSF) and WD. The results showed that PCV2 infection was found in 82.35% of herds. Co-infections with any two of the pathogens PCV2, PRRSV and PRV were 31.25%,11.46% and 23.96%. There were also 9.38% of the cases found to be infected simultaneously with PCV2, PRRSV and PRV. Furthermore, in PCV2 infected herds, the immune responses of CSFV and WD were suppressed. On the other hand, eighty-four cases out of 98 PCV2 positive tissue or serum samples (85.71%) were infected by bacteria, among which 61 cases were co-infected of PCV2 with one bacterial pathogen (62.24%) and we could find two bacterial agents in the other 23 cases (23.47%). Interestingly, it seemed to be more frequently that PCV2 infection could be accompanied by respiratory diseases, since 33 cases were found to have respiratory diseases in addition to PCV2 infection.We found that PCV2 associated diseases in the positive herds could be brought under control by comprehensive strategies, especially the use of autogenous vaccines.
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