PMWS, a new emerging and multifactorial disease in swine, was first recognized in North America in 1991 . Since then, this disease has been diagnosed globally and devastated almost every pig-producing area of the world . A role of PCV2 in the etiology of PMWS was first observed in Canada in 1991, and described in the late 1990s. Although PCV2 infection has been associated with PMWS, the role of PCV2 infection in the pathogenesis of PMWS is uncertain. To date, PCV-2 infection is regarded as a necessity in PMWS cases, other cofactors are necessary to evoke disease. PMWS-affected pigs exhibit a wide spectrum of concomitant infections with PCV2, such as porcine parvovirus [11, 12], porcine reproductive and respiratory syndrome virus (PRRSV) [13, 14], TTV  and Mycoplasma hyopneumoniae , all may enhance PCV2-associated lesions and develop whole complex clinical symptom of PMWS under experimental and field conditions .
The difficulties in reproducing the full spectrum of clinical signs and lesions associated with PMWS using PCV2 alone have resulted in a hypothesis that PMWS may be triggered by an unknown pathogen, popularly termed 'agent X', Of course, no such novel agent has been identified until now . The finding of novel viral agents in the PMWS disease is challenging since there still are many open questions concerning the aetiology of the syndrome. From all the studied animals in the present study, 3 were co-infected with both PCV2 and P1, 10 were positive for PCV2 only (data not shown). Therefore, further studies needs to be conducted in order to see if the presence of the novel agent P1 described here, contributes to the development of PMWS solely and/or in synergy with PCV-2.
PCR was proved to be a useful tool for detecting PCV2 in clinical specimens because of its speed, specificity, and sensitivity. But challenges encountered by its specificity as to P1 occurrence, most of PCR assay reported previously would target the ORF2 region of the PCV2 genome, which shared high similarity nucleotide sequence with ORF1 of P1. It is vital to determine whether those PCR amplification fragments were produced by PCV2 or by P1. The PCR assay described here should provide the ability to identify and differentiate between PCV2 and P1 and screen pigs for P1 infection. To our knowledge, this is the first report on epidemiology of P1 in China. Of course, to obtain the complete genome sequence of P1 and aid in further analysis of genomic features of P1, we used the pairs of primers F3 and R3 including an overlapping region in this study. Rather than amplify the entire 648 nt of P1, designing some primers amplifying smaller products may improve the sensitivity of the assay and be more suitable for epidemiological investigation of P1.
In a previous study, 1.8% (5/278) of studied swine sera from China was shown to be P1-positive by a PCR detecting . The difference in the prevalence of P1 (19%, 47/248) obtained in our present study indicate that the P1 infection has became popular in pig herds. Of course, very low copies of P1 DNA were also detected in some sera samples, the biological importance of the virus remains to be elucidated.
In summary, the present work contributes to the understanding of P1 epidemiology, including the establishment of PCR assay described here. Taking into account the present results, it seems evident that P1 infection is already widespread in Jiangsu and those 10-30 days old pigs seem more likely to be infected with P1. Further studies should be investigated in larger scale data in order to determine the biological importance of P1.