In pigs, three circovirus species within the genus Circovirus have been identified so far, including the non‐pathogenic Porcine circovirus 1 (PCV1), the pathogenic Porcine circovirus 2 (PCV2) and the recently identified Porcine circovirus 3 (PCV3). In April 2019, a new circovirus with a distinct relationship to other circoviruses was identified in several pigs with severe clinical disease in Hunan province, China. The size of the viral genome, tentatively designated as porcine circovirus type 4 (PCV4), is 1,770 nucleotides (nt). PCV4 shows the highest genomic identity to mink circovirus (66.9%) and has identities of 43.2%–51.5% to the other PCV genomes. Two major genes, a replicase (Rep) gene spanning 891 nt and a capsid (Cap) gene spanning 687 nt, were predicted. Furthermore, a TaqMan® real‐time polymerase chain reaction (PCR) targeting the replicase gene was developed to investigate the prevalence of PCV4 in 187 clinical samples from Hunan province, China. The results revealed an overall PCV4 prevalence of 12.8%, with the highest positive rates in nasal swabs (28.5%, 6/21) followed by serum samples (13.4%, 11/82). The clinical significance and pathogenesis of this virus needs further investigation.

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Porcine circovirus 4 (PCV4) is the fourth porcine circovirus newly identified in China, and it could be detected in diseased and healthy pigs. To date, the prevalence of PCV4 DNA in pig herds has been investigated in many provinces from both China and Korea, with positive rates varied from 3.28% to 25.4% in samples from different regions. However, up to now no serological data have been reported to evaluate the prevalence of PCV4 in pig herds. In this study, an indirect anti-PCV4 IgG enzyme-linked immunosorbent assay (ELISA) based on replicase protein (Rep) was developed and utilized to investigate the seroprevalence of PCV4 in pig herds of China. A total of 1790 swine serum samples from 17 provinces of China were tested including samples confirmed positive for PCV4 DNA. There was no cross-reactivity of this ELISA with PCV1, PCV2 and PCV3. PCV4 Rep antibodies have been detected in serum samples from 16 out of 17 provinces in China. The PCV4 overall seroprevalence was 43.97%, with the highest of 67.8% been detected in sows, followed by fattening and suckling pigs with positive rates of 35.0% and 14.56%, respectively, and the lowest of 12.61% been detected in nursery pigs. Moreover, from the present data, the earliest positive sample could be

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