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).