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.