4 | DISCUSSION
PER.C6 is a human cell line, SVV was originally used as a safe and effective oncolytic agent. It has previously been reported that SVV can be isolated from mice in pig farms and can be used as an animal model for SVV disease research. Thus, BHK-21 cells can be used for SVV research since it is a cell line of mouse origin (Liu et al., 2019; Lork et al., 2016). Both PK-15 and ST cells are porcine-origin cell lines, and most SVV strains have been isolated from pigs (Liu et al., 2019; Lork et al., 2016; Raquel et al., 2016; Vannucci et al., 2015; Wang, Prarat, Hayes, & Zhang, 2016). Virus isolation in this study was established using mouse-origin cell lines (BHK-21 and NA cells), porcine-origin cell lines (PK-15 and ST cells), monkey-origin cell lines (Vero and Marc-145 cells), bovine-origin cell line (MDBK cells), and canine-origin cell line (MDCK cells). It was found that with the exception of MDCK cells, SVV could be continuously stably passaged in all these cell lines from different species, which suggest SVV had potential of cross-species transmission. Moreover, SVV transmission was reported to be nearly 70% in China in Guangdong (Hause, Myers, Duff, & Hesse, 2016). SVV can infect pigs of different ages (Nestor et al., 2016), as well as mice; however, this was the first time that SVV was detected in the oral swab fluid of infected buffalo exhibiting clinical symptoms. Moreover, SVV antibodies could be detected in the sera of pigs, mice, and cattle (Yang, Rebekah., 2012). This study confirms the perspective in previous reports that SVV antibodies were detected in bovine serum, buffalo may also be a host of SVV (Lork et al., 2016). In addition, it was established that the buffalo on the farm were both captive and free-range. There were at least five small pig farms within a radius of 3 km around the buffalo farm, from which nearly 600 pigs were infected. This shows that the host range of the virus is expanding; thus, the media and means of transmission were diversified (Lork et al., 2016).
The genetic evolutionary tree indicated that the SVA/GD/China/2018 strain has a close relationship with the wild boar strain, Sichuan HS-01. Coincidentally, the host of these two strains is not the domestic pig and was also isolated in 2018, whereas the wild boar strain, Sichuan HS-01, was isolated in May, and the SVA/GD/China/2018 strain occurred in October. It is important to note that the SVA/GD/China/2018 and the SVV strains from pigs around the cattle farm share the highest nucleotide and higher amino acid similarity and were located on the same branch with the wild boar strain, Sichuan HS-01. From a timing perspective, the buffalo infection may be related to the infection of the surrounding pigs; however, this strain and the wild boar strain in Sichuan may have evolved from the Guangdong domestic pig strain. These findings demonstrate that there was a relatively close nucleotide relationship with SVV from non-commercial pigs. The strain shared the highest identity with the KS15-01-like strain, indicating that the strain was still new (Wang et al., 2019).
Interestingly, SVA/GD/China/2018 shared the same mutual amino acids at positions, 427K (Lysine, VP2 protein) and2104R (Arginine, 3D protein), as the first wild boar strain, Sichuan HS-01 (GenBank Accession No. MH588717), in China. In contrast, the other commercial pig strains had only one or two mutations. This finding reveals that the virus may have undergone adaptive changes in different hosts (Xu et al., 2017); however, further research is required to elucidate the role of these two sites. VP1 contains a hypervariable region with at least two antigenic sites located at the amino acid 140–160 and 200–213 sites (Saeng-Chuto, Rodtian, Temeeyasen, Wegner, & Nilubol, 2018). It has been reported that 228K in VP1, 141–143LDV, and143–148DGK in VP2 are the primary antigenic sites of FMDV (Bai et al., 2014). None of these three motif sites and antigenic sites of the SVA/GD/China/2018 strain have changed, indicating similar antigenicity and biological characteristics of this strain compared to others (Chen et al., 2017). The majority of the characteristics are unique, as the mutations were located in the VP3 protein (3/7); however, further research is required to support the effect of these changes.
In summary, we first isolated a buffalo-origin SVV strain using many of different cell lines. Genetic evolution revealed the possibility of cross-species transmission of SVV.