INTRODUCTION
Members of the Circoviridae , within the order Circovirus , contain a small, circular, non-enveloped, single stranded DNA genome ranging 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, 2, 3 and 4,Psittacine beak and feather disease virus (BFDV), Duck circovirus (DuCV),Goose circovirus (GoCV), Canary circovirus (CaCV),Raven circovirus (RaCV), Starling circovirus (StCV),Swan circovirus (SwCV), Finch circovirus (FiCV), andGull circovirus (GuCV), etc. belong to the genusCircovirus of the family Circoviridae(http://www.ictvonline.org/). PiCV is a small, circular, non-enveloped virus that contains a single-stranded DNA genome with approximately 2 kb in size. The genome of PiCV has 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). The gene forming ORF-C1 of PiCV has been demonstrated to be highly genetically diverse as compared with the gene forming ORF-V1 (Stenzel et al., 2017; Zhang et al., 2015). The other ORFs including ORF-C2, ORF-C3, and ORF-C4 located on the complementary sense strand encode three viral proteins of unknown functions (Mankertz et al., 2000; Todd et al., 2008). The circovirus infection in pigeons was first identified 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).
A variety of methods have been employed to detect PiCV infection in clinical specimens. The original diagnose methods electron microscopy, histology, dot blot hybridization, 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 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). Previous studies also demonstrated that PiCV had been detected in different types of pigeons, including racing, fancy, feral and meat pigeons (Stenzel et al., 2014b; Stenzel et al., 2012; Wang et al., 2017). However, all of the above-mentioned birds belong toColumba livia species.
In China, the first PiCV infection was detected from meat pigeons in Zhejiang province in 2009, and the full genome was sequenced (Yu et al., 2009). In recent years, several studies have proved that PiCV was prevalent among meat pigeons in eastern and southern China (Wang et al., 2017; Zhang et al., 2015). Recently, 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. Chinese breeders compete in numerous races for different distances in their sections throughout the racing season, and more than 25 million racing pigeons from approximately 750 racing clubs were selected to take part in the competitions every year. However, there are 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 bioinformatic analysis of PiCV from racing pigeons were undertaken in this study. The purpose of this study is to provide novel epidemiologic data and genome characterization for PiCV strains identified from racing pigeons in China.