4. DISCUSSION
The first strain of SVV was detected from swine in Canada in 2007 (Pasma et al., 2008); since then, several SVV-infected swine outbreaks have been reported. Since 2015, many countries have reported SVV epidemics or small-scale outbreaks (Leme et al., 2019). As a new infectious disease, SVV may result in huge economic losses. The clinical symptoms of SVV in swine are fever, lethargy, and anorexia; in addition, blisters on the skin or mucous membranes of the nose, mouth, tongue and hoof, are common. It is difficult to distinguish between SVV and VESV, VSV, SVDV, and FMDV according to clinical symptoms (Zhang et al., 2018). Studies have shown that SVV can be detected in infected swine for a period of 7-21 days; the clinical symptoms last 12-14 days and viremia can last 1-10 days (Maggioli et al., 2018). In addition, SVV has been detected in the intestines and feces of mice, and also in house flies, thus implying that these species may carry pathogens for propagation (Joshi et al., 2016).
In a previois study, Chinese isolates of SVV were divided into five clusters (Wang et al., 2019). Previous genetic analysis indicated that SVV isolates from the USA, Brazil, and Canada, were all clustered onto one branch of an evolutionary tree and that Chinese isolates spread out over one branch with isolates from the US, Brazil, or Canada (Xu et al., 2017, Chen et al., 2018). The genetic analysis carried out in the present study indicated that the SVV isolates from China can be divided into 3 clusters. Based on this, we can infer that the origins of the SVV strains currently circulating in China may be diverse. We also analyzed the genetic evolution of the Chinese strains in order to provide a foundation with which to prevent and control SVV in the future.
The first infection caused by SVV was reported in the Guangdong province of China in 2015 (Wu et al., 2017, Wu et al., 2016). Since then, cases of SVV infection have been detected and reported in several provinces of China (Zhang et al., 2019). Therefore, it is vital that we analyze the geographical distribution of different viral subtypes. So far, most Chinese isolates of SVV isolates are distributed in the south and central regions of China. Furthermore, our results indicate that different SVV genotypes co-exist in China. Furthermore, our results showed that the classification of the Chinese isolates have changed from clusters 1 and 3 to cluster 2. In China, the swine population is very dense and highly mobile; collectively, these factors have contributed to the diversity of the Chinese SVV isolates (Guo et al., 2020). These data provide further understanding with regards to the path of SVV propagation in China.
Four cases of SVV recombination events have been reported in China: HeN-1/2018 (960-2354 bp), HB-CH-2016 (position 1563 bp), SVA/CHN/10/2017 (4145-5620 bp) and HeNNY-1/2018 (4190-5808 bp). These recombination events predominantly occurred in the P1, P2, and P3 regions. Combined with our data, these results showed that the current recombinant regions are concentrated in the P1, P2 and P3 regions; however, the current dataset is limited and cannot fully represent the full extent of recombination in the SVV strain. The recombination events involving Chinese SVV isolates indicate that SVV recombination events may have occurred as early as 2016 (Wang et al., 2018, Guo et al., 2020). CH-GX-01-2019 (Cluster 3) is a strain that represents a recombination of Colombia-2016 (Cluster 2) and HB-CH-2016 (Cluster 1). It is possible that different clusters of virus strains could recombine in the pig breeding environment to form new strains. Viral recombination between different clusters may accelerate phenotypic variation, thus allowing the recombinant virus to escape surveillance by the host’s immune system and thus survive for longer in the host. Whether the new strain of virus (CH-GX-01-2019) exhibits variation in terms of pathogenicity and antigenicity needs to be ascertained as a matter of urgency. Furthermore, recombination events and the effects of mutation on the final amino acid sequence may lead to diversity and add to the complexity of SVV, thus changing the phenotype and functionality of the virus (Guo et al., 2020).