Isolation and evolutionary analysis of Senecavirus A isolates from Guangdong province, China

Senecavirus A (SVA), an emerging swine pathogen, has been reported in many provinces of China since the first outbreak in 2015 in Guangdong province. In this study, 10 lymph nodes positive for SVA, collected between 2018 and 2019 from slaughterhouses in Guangdong province, were subjected to virus isolation. Rapid and evident cytopathic effects (CPEs) were observed in SVA-infected PK-15 cells, including shrinking, rounding and detaching, with peak titers being reached at 24 h post infection (hpi). Electron microscopy showed that SVA particles are spherical and approximately 30 nm in diameter, and exist as crystalline lattices in cytoplasm revealed by ultra-thin sectioning. Phylogenetic analysis based on the whole genome sequences of all available isolates showed that SVA globally can be divided into two groups with each being further divided into two subgroups (Ia-b and IIa-b), and with the Guangdong isolates obtained here and other Chinese strains belonging to subgroups IIa and IIb. Evolutionary analysis showed that the mean substitution rate of SVA was 2.696 × 10^-3 per site per year based on whole genomic sequences, with subgroup IIb isolates having evolved faster than those of subgroup IIa. Analysis of efficient population size showed that the outbreak point of SVA worldwide occurred at the end of 2013 with that of subgroup IIb, the current dominant group, in mid 2014.