Comparative epidemiology of porcine circovirus type 3 in pigs with different clinical presentations

Abstract

Background: Porcine circovirus type 3 (PCV3), as an emerging circovirus species, was reported to be widely circulating in the United States, China, South Korea and Poland. Previous studies revealed that PCV3 was mainly concentrated in sick animals with respiratory disease, skin disease, reproductive disorders and so on. However, the circulating status of PCV3 in pigs with other clinical presentations (especilly asymptomatic or diarrhea) was not well established.

Findings: In this study, to conduct a comparative epidemiological survey of PCV3, 80 weaned pig serum samples with severe respiratory disease (SRD), 175 weaned pig serum samples with mild respiratory disease (MRD), 216 asymptomatic weaned pig serum samples, 35 diarrheal weaned pig samples and 35 non-diarrheal weaned pig samples were collected from eight provinces of China. Via qPCR testing, PCV3 was circulating in all sampling provinces, with total positive rates varying from 1.04% to 100%. Interestingly, the PCV3-positive rate was significantly higher in weaned pigs with SRD (63.75%, 51/80) than in those weaned pigs with MRD (13.14%, 23/175) and asymptomatic pigs (1.85%, 4/216) (P < 0.01). Similarly, the PCV3-positive rate was significantly higher in diarrheal weaned pigs (17.14%, 6/35) than in non-diarrheal weaned pigs (2.86%, 1/35) (P < 0.05). Moreover, the lower Ct values of qPCR were frequently found in those weaned pigs or fattening pigs with respiratory disease and diarrhea rather than that in asymptomatic pigs. Sequence analysis showed that low genetic diversity existed among those PCV3 sequences collected from pigs with different clinical presentations.

Conclusions: The present study further extends evidence that newly described PCV3 widely circulates in six additional provinces of Southern and Northern China and has high similarity to previously reported isolates. As an emerging virus of swine, although the present case-control study reveals that PCV3 has a potential association with swine respiratory disease and diarrhea, further investigations into the pathogenesis are needed to ascertain the role of PCV3 in swine health.

Keywords: Asymptomatic; Comparative epidemiology; Diarrhea; Porcine circovirus type 3; Respiratory disease.

Fig. 1

Geographical distribution of PCV3 in China

Fig. 2

The detection results of PCV3 in pigs with different clinical presentations. a PCV3-positive rate in pigs with different clinical presentations; b Ct value of qPCR for the detection of PCV3 in pigs with different clinical presentations. Note: SRD means severe respiratory disease, while MRD means mild respiratory disease

Fig. 3

Phylogenetic analysis based on complete genome sequences of PCV1, PCV2 and PCV3. The phylogenetic tree was constructed by the Maximum Likelihood method using MEGA 5.1 software. One thousand bootstrap replicates (1000 times) were used to assess tree topology. Only bootstrap scores of at least 50 were retained. Scale bar indicates 10% nucleotide sequence divergence. The PCV3 isolates of PCV3-CHN/GD2016 and PCV3-CHN/CC2016 obtained in this study are indicated by gray shading