4. DISSCUSSION
Senecavirus A is a vesicular disease in
pigs.
The disease, also known as porcine idiopathic vesicular disease, is
important for the health of the animals and thereby, the farm economy.
Recent SVV outbreaks have been reported in many countries with large
swine production, similar to outbreaks of other important
vesicular
viruses, including VSV, SVDV, and FMDV(Canning et al., 2016). SVV has
received much attention, with a focus on virus
pathogenesis,
immunology, and epidemiology(Segalés, Barcellos, Alfieri, Burrough, &
Marthaler, 2016). The present study has clarified the characteristics of
the
pathogenesis
of SVV infection in epithelial and epidermal cells, immunosuppression,
immune evasion, and cross-host transmission.
In 2015, the first outbreak of vesicular lesions in newborn piglets was
observed at farms in Guangdong Province of China(H. Zhang et al., 2020).
The cause of the high-mortality outbreak was identified as SVA
infection. Since then, more than half of the province has been affected
by SVV infection. The strain isolated in the present study is the first
to be reported from the northernmost province of China. SVV has first
identified 30 years ago since it was first reported in the US. A turning
point like of the outbreaks occurred in 2015, after which, several
outbreaks of SVV vesicular disease (SVA-VD) and epidemic transient
neonatal loss occurred(Canning et al., 2016; Zhu et al., 2017). Prior to
2010, isolates were not pathogenic and did not display clinical signs.
Strains isolated after 2015 are considered accompanied by the vesicular
lesion.
Our phylogenetic comparison of recent SVA isolates with isolates
obtained before 2010 revealed a marked divergence of 5.59%(Saeng-Chuto,
Stott, et al., 2018). Therefore, SVV strains isolated before 2010 are
considered “historical” (Houston, Temeeyasen, & Pineyro, 2020).
Seneca Valley virus-1 (SVV-001) was first detected in a PER.C6 fetal
retinoblast cell culture in 2002. It was likely a contaminant from the
bovine serum or porcine trypsin used in the cell culture(Venkataraman et
al., 2008). SVVs have experienced great change in their nucleotide
composition over the past ten years and have been identified in
different hosts and tumor cells. Mutational
pressure from several animal hosts
accelerates the frequency of recombinant mutations in
SVV(Canuti et al., 2020). Cross-host
transmission may have led to a rapid increase in the rate at which
mutant stress has an effect.
The pathogenesis of different types of US strains of SVV varies in pigs,
even though they have similar sequences(H. Zhang et al., 2020). However,
the replication efficiencies of the different strains were all similarly
high. These characteristics imply that SVVs have the potential to infect
various host animals. Notably, SVVs have been detected and isolated from
pigs, environmental samples, mouse feces, and mouse small-intestine, and
SVV RNA was also detected in houseflies from farms that were negative
for SVV vesicular disease(Joshi, Mohr, et al., 2016). A 2012 report
described the occurrence of SVV accompanied by vesicular lesions and a
spontaneous
outbreak from a pig purchased at the Indiana State Fair(Leme et al.,
2017). In 2015, SVV was first detected in China, an outbreak occurred in
2016. The three major evolutionary
clusters
have been identified in China, as compared to the US and Canada(Zhu et
al., 2017). Additionally, all Chinese isolates could be grouped into
clusters present in the US and Canada. As shown in Figure 2A, the
isolated strain mainly belonged to the US-like cluster.
Mink were infected with SVV in our research, the virus was detected in
oral fluid and fecal-swab samples by RT-PCR and qRT-PCR, which indicated
that the mink was an important SVV host. Mink was the fourth most
frequently infected host, following humans, swine, and mice(Feronato et
al., 2018). The qRT-PCR results indicated that
fecal
swabs had a much higher quality SVV mRNA than the oral fluid. The
pathogenesis and clinical data revealed pathologic changes in the
intestinal tract. No vesicular lesions were observed in SVV infected
mink.
Histologically,
piglets had multifocal pathological
changes, such as infiltration of inflammatory cells, necrotic
keratinocytes, and hemorrhage(Leme et al., 2016). Clinical evaluation in
finisher pigs also showed that the virus can present subclinical signs
or no clinical signs. In experimentally infected pigs, infiltration of
inflammatory cells and necrotic keratinocytes were evident. In addition,
the
histopathologic
lesions in the piglets were more serious than those in the piglets, and
they were accompanied by interstitial pneumonia and ballooning
degeneration of the urinary bladder and renal pelvis epithelium(Leme et
al., 2016). All of these histopathologic changes indicate that the SVV
can invade the epithelium and epidermis cells in mammals, such as pigs
and minks. However, we still do not clearly understand the mechanism of
SVV infection of intestinal epithelial cells and oral epithelial cells
in mink.
The risk of SVA infection varies markedly between herds and farms. Risk
factors including a high number of breeding females, a higher number of
farm employees, and the time of weaning may contribute to the spread of
SVV(Tousignant et al., 2017). A serology analysis in animals detected
neutralizing antibodies to SVV in 27 of 71 porcine samples, 10 of 30
bovine samples, and five of 35 wild mouse samples, with no neutralizing
antibodies detected in more than 100 human serum samples(Reddy et al.,
2007). Taken together, these data show that SVV could naturally
replicate in farm animals and humans, and that farm animal could be
stimulated to produce neutralizing antibodies(Baker et al., 2017). In
contrast, the production of neutralizing antibodies in humans is
relatively rare.
Virus
shedding could be detected up to 28 days post-infection. However,
persistent shedding of the virus could be sustained up to 60 days
following SVV infection(Maggioli et al., 2019). Finisher pigs reportedly
produce neutralizing antibodies at 5 dpi following experimental
inoculation, with maximum antibody concentrations between 7 and 14 dpi.
However,
neutralizing
antibodies decreased incrementally during the first two weeks
post-infection(Houston et al., 2020). In a longitudinal study on
SVA-infected farms, the antibody titers of piglets were higher during
the first week of age, but disappeared in the
last
four and five weeks after born. More importantly, 20% to 40% of
piglets with neutralizing antibodies presented
viremia
and viral shedding in feces and oral fluids, which were sustained
between four and five weeks without clinical symptoms(Tousignant et al.,
2017). Another study reported high SVA genetic diversity in samples
collected over 12 months from swine and several sites in their
environments(Joshi et al., 2020). The special immune and infection
status exerted mutation pressure, which was the main driver of the
evolution of SVV, rather than natural selection.
In the present study, SVV nucleic acid was detected from swabs of
internal and external surfaces in the farm. The result indicates that
SVV poses an environmental risk. The prior detection of SVA in mice and
houseflies indicated that these may play a role in the epidemiology of
SVV, which would also increase the risk of SVV infecting wild animals as
natural hosts(Joshi, Mohr, et al., 2016). The latter may act as a
natural reservoir and thus, a potential vector. Mink are higher in the
food chain than mice (Figure4). Thus, it is possible that SVV could
infect mink and that it could evolve.
Mutational
pressure has been considered the major factor in the variation, compared
with natural selection. All the studies to date have focused on the role
of geographic distribution in contributing to the codon usage pattern of
SVA. Mutational pressure has a more important role in SVA evolution than
natural selection(Chen et al., 2017). However, no study has focused on
cross-species
transmissions, such as the complex links between physiological
differences in hosts, disease progression, and viral release. Mink
infected with SVV provide a new avenue of mutational pressure. Studies
of SVV infection in mink could increase the understanding of the
cross-species
transmission of SVV and the viral life cycle within the environment. The
collective knowledge could inform the prevention of SVA infection.
SVV exhibits
immune
evasion activity in the immune system of humans and other mammals.
Analyses involving antibodies to SVV surface antigens showed that SVV
could stimulate the immune system of mink. Antibody titers increased
with the infection of SVV in mink. In the clinical evaluation, there
were no differences in the level of the IgG antibody dynamics between
clinically affected and non-affected animals. Immune evasion was the
major characters in the identification of
SVV
as a potent oncolytic virus against tumors in medicine; other features
include the targeting and penetration of solid tumors following
intravenous administration, the inability of insertional mutagenesis,
and self-replication with selective tropism for cancer cells(Burke,
2016). A strong cellular immune response was reportedly induced by SVV
infection, which promotes the response of interferon-gamma -specific T
cells as early as 3–7 dpi(L. G. Gimenez-Lirola et al., 2016). T-cell
responses did not completely clear SVV at the first 14 dpi. However, the
evolution of SVV from the same infecting farm for one year indicated
that the evolution was not from a single host, in this case of the pig.
Likely, the multiple hosts are the mutational pressure promoting the SVV
evolution and cross-species transmission.
CONFLICT OF INTEREST STATEMENT
The authors declare no financial or commercial conflicts of interest.