INTRODUCTION
Members of the Circoviridae , within the order Circovirus , contain a small, circular, non-enveloped, single stranded DNA genomes that range from 1.7 to 2.5 kb in size (Stenzel et al., 2017). ThePigeon circovirus (PiCV) or the columbid circovirus (CoCV), together with porcine circoviruses (PCV) types 1 and 2, psittacine beak and feather disease virus (BFDV), duck circovirus (DuCV), goose circovirus (GoCV), canary circovirus (CaCV), raven (RaCV), starling (StCV), swan (SwCV), finch and gull (FiCV and GuCV) circoviruses belongs to the genus Circovirus and the familyCircoviridae(http://www.ictvonline.org/). PiCV is a small, circular, non-enveloped DNA virus that contains single-stranded DNA approximately 2.0 kb in size. The genome of PiCV forms two main open reading frames (ORFs). The ORF-V1, located on the virion sense strand, encodes a replication-associated protein (rep ) and the ORF–C1, located on the complementary sense strand, encodes a capsid protein (cap ) (Mankertz et al., 2000; Todd et al., 2001). In contrast to the gene forming ORF-V1, the gene forming ORF-C1 of PiCV had been demonstrated to be highly genetically diverse (Stenzel & Koncicki, 2017; Zhang et al., 2015). The other ORFs (ORF-C2, ORF-C3 and ORF-C4), located on the complementary sense strand, encode three PiCV proteins with unknown functions (Mankertz et al., 2000; Todd et al., 2008). The circovirus infection in pigeons was first diagnosed in 1993 in the USA (Woods et al., 1993) and had been considered to be strongly associated with young pigeon disease syndrome (YPDS) including weakened racing performance, weight loss, lethargy, anorexia, respiratory distress and diarrhea (Raue et al., 2005).
Various methods have been applied to detect PiCV infection in clinical specimens. The original diagnose methods including electron microscopy, histology, dot blot hybridization (DBH), and in situ hybridization (Yamamoto et al., 2015) are time consuming (Smyth et al., 2001; Soike et al., 2001; Todd et al., 2002; Tsai et al., 2014). The molecular biology techniques, such as the standard PCR, real-time PCR (Duchatel et al., 2009), NGS techniques (Wang et al., 2017) and the loop-mediated isothermal amplification method (Tsai et al., 2014), enabled the more rapid and accurate detection of PiCV infections. As a result, cases of PiCV infections in pigeons were subsequently reported in various countries, including Northern Ireland (Todd et al., 2001), Germany (Mankertz et al., 2000; Raue et al., 2005), Italy (Franciosini et al., 2005), France (Abadie et al., 2001), Czech Republic (Taras et al., 2003), Belgium (Duchatel et al., 2005; Duchatel et al., 2006), Poland (Stenzel et al., 2012; Stenzel et al., 2014c), Slovenia (Krapez et al., 2012), Hungary (Cságola et al., 2012), United Arab Emirates (Ledwoń et al., 2011), Iran (Mahzounieh et al., 2014), China (Liao et al., 2015; Wang et al., 2017; Zhang et al., 2015), Japan (Yamamoto et al., 2015) and the USA (Roy et al., 2003). The previous studies also had demonstrated that PiCV has been detected in different types of pigeons including racing, fancy, feral, meat pigeons (Stenzel et al., 2014b; Stenzel et al., 2012; Wang et al., 2017). However, all of the mentioned above birds belong toColumba livia species.
In China, the first PiCV infection was detected from meat pigeons of Zhejiang province in 2009 and the full genome was sequenced (Yu et al., 2009). In recent years, several studies had proved that PiCV was prevalent among meat pigeons in eastern and southern of China (Wang et al., 2017; Zhang et al., 2015). The competitions of racing pigeons are becoming increasingly popular in China, and even, pigeon racing is considered to be a national sport. The Chinese Association of Racing Pigeon Breeders has over 5 million members. The Chinese breeders compete in numerous races at different distances in their sections through the racing season, and more than 25 million of racing pigeons were selected to attend the competitions in a total of approximately 750 racing clubs every year. However, there is no epidemiologic data on PiCV infections among racing pigeons. In order to investigate the prevalence, evolution and genome characterization of PiCV in racing pigeons in China, an extensive epidemiological investigation and bioinformatics analysis of PiCV from racing pigeons were undertaken. A total of 571 serum samples collected from healthy pigeons and 51 tissues samples collected from sick pigeons were obtained from 39 racing pigeon clubs in 11 provinces or municipalities of China between 2016 and 2019. The purpose of our study is to provide novel epidemiologic data and genome characterization about PiCV isolated from racing pigeons in China.