The prevalence of novel porcine circovirus type 3 isolates in pig farms in China.

The emerging porcine circovirus type 3 (PCV3) has been reported in Chinese swine herds since 2017. We performed a nationwide investigation on the prevalence of PCV3 in pig breeding farms and slaughterhouses in China. A total of 4,040 tonsil samples were collected from 89 farms in 25 provinces, and 1,419 lymph node samples were collected from 50 slaughterhouses in 27 provinces. The PCR results showed that in pig breeding farms, the positive rate was 41.6% (37/89) at the farm level and 5.0% (201/4040) at the individual level. In the slaughterhouses, the positive rate was 62.0% (31/50) at the farm level and 8.0% (114/1419) at the individual level. The PCR-positive samples were further sequenced, and 19 new PCV3 isolates were identified. The complete genomes of the 19 virus isolates showed 97.4%-99.7% nucleotide identity with other PCV3 isolates. The phylogenetic analysis revealed that the 19 isolates were divided into PCV3a and PCV3b genotype clusters based on the PCV3 complete genome sequences. This study indicated that PCV3 has spread extensively in both pig breeding farms and slaughterhouses. The positive rate of PCV3 was higher in eastern China compared to other regions in China. Furthermore, this study will help us understand the prevalence and genetic variation of PCV3 in Chinese swine herds.