Genetic Determinants of Virulence between Two Foot-and-Mouth Disease Virus Isolates Which Caused Outbreaks of Differing Severity

Polymerase Fidelity Contributes to Foot-and-Mouth Disease Virus Pathogenicity and Transmissibility In Vivo

The low fidelity of foot-and-mouth disease virus (FMDV) RNA-dependent RNA polymerase allows FMDV to exhibit high genetic diversity. Previously, we showed that the genetic diversity of FMDV plays an important role in virulence in suckling mice. Here, we mutated the amino acid residue Phe257, located in the finger domain of FMDV polymerase and conserved across FMDV serotypes, to a cysteine (F257C) to study the relationship between viral genetic diversity, virulence, and transmissibility in natural hosts. The single amino acid substitution in FMDV polymerase resulted in a high-fidelity virus variant, rF257C, with growth kinetics indistinguishable from those of wild-type (WT) virus in cell culture, but it displayed smaller plaques and impaired fitness in direct competition assays. Furthermore, we found that rF257C was attenuated in vivo in both suckling mice and pigs (one of its natural hosts). Importantly, contact exposure experiments showed that the rF257C virus exhibited reduced transmissibility compared to that of wild-type FMDV in the porcine model. This study provides evidence that FMDV genetic diversity is important for viral virulence and transmissibility in susceptible animals. Given that type O FMDV exhibits the highest genetic diversity among all seven serotypes of FMDV, we propose that the lower polymerase fidelity of the type O FMDV could contribute to its dominance worldwide.IMPORTANCE Among the seven serotypes of FMDV, serotype O FMDV have the broadest distribution worldwide, which could be due to their high virulence and transmissibility induced by high genetic diversity. In this paper, we generated a single amino acid substitution FMDV variant with a high-fidelity polymerase associated with viral fitness, virulence, and transmissibility in a natural host. The results highlight that maintenance of viral population diversity is essential for interhost viral spread. This study provides evidence that higher genetic diversity of type O FMDV could increase both virulence and transmissibility, thus leading to their dominance in the global epidemic.

Genetic Determinants of Altered Virulence of Type O Foot-and-Mouth Disease Virus

FMD is probably the most important livestock disease in the world due to the severe economic consequences caused. The alteration of several viral genes may give the virus selective advantage to maintain its prevalence in nature. Here, we identified that a 70-nucleotide deletion in the S fragment combined with a single leucine insertion in the leader protein (L^pro) is a novel determinant of restricted growth on bovine cells, which significantly contributes to the altered virulence of serotype O FMDV in cattle. A synergistic and additive effect of the 70-nucleotide deletion in the S fragment and the single leucine insertion in L^pro on the virulence and host specificity of the virus was determined. These results will benefit efforts to understand the vial pathogenicity mechanism and molecular characteristics of FMDV.

Under different circumstances, the alteration of several viral genes may give an evolutionary advantage to the virus to maintain its prevalence in nature. In this study, a 70-nucleotide deletion in the small fragment (S fragment) of the viral 5′-untranslated region (5′-UTR) together with one amino acid insertion in the leader protein (L^pro) that naturally occurred in several serotype O foot-and-mouth disease virus (FMDV) strains in China was identified. The properties of two field serotype O FMDV strains, with or without the 70-nucleotide deletion in the S fragment and the amino acid insertion in L^pro, were compared in vitro and in vivo. Clinical manifestations of FMD were clearly observed in cattle and pigs infected by the virus without the mutations. However, the virus with the mentioned mutations caused FMD outcomes only in pigs, not in cattle. To determine the role of the 70-nucleotide deletion in the S fragment and the single amino acid insertion in L^pro in the pathogenicity and host range of FMDV, four recombinant viruses, with complete genomes and a 70-nucleotide deletion in the S fragment, a single amino acid insertion in L^pro, or both mutations, were constructed and rescued. It showed that deletion of 70 nucleotides in the S fragment or insertion of one amino acid (leucine) at position 10 of L^pro partly decreased the viral pathogenicity of Mya-98 lineage virus in cattle and pigs. However, the virus with dual mutations caused clinical disease only in pigs, not in cattle. This suggested that the S fragment and L^pro are significantly associated with the virulence and host specificity of FMDV. The naturally occurring dual mutation in the S fragment and L^pro is a novel determinant of viral pathogenicity and host range for serotype O FMDV.

IMPORTANCE FMD is probably the most important livestock disease in the world due to the severe economic consequences caused. The alteration of several viral genes may give the virus selective advantage to maintain its prevalence in nature. Here, we identified that a 70-nucleotide deletion in the S fragment combined with a single leucine insertion in the leader protein (L^pro) is a novel determinant of restricted growth on bovine cells, which significantly contributes to the altered virulence of serotype O FMDV in cattle. A synergistic and additive effect of the 70-nucleotide deletion in the S fragment and the single leucine insertion in L^pro on the virulence and host specificity of the virus was determined. These results will benefit efforts to understand the vial pathogenicity mechanism and molecular characteristics of FMDV.

Pathogenesis of a senecavirus a isolate from swine in shandong Province, China

Senecavirus A (SVA), previously called Seneca Valley virus, can cause vesicular lesions in sows and a sharp decline in neonatal piglet production. In this study, a SVA strain was isolated from a pig herd in Shandong Province in China and identified as SVV-CH-SD. The full genome was 7286 nucleotides (nt) in length and contained a single open reading frame (ORF) of 6546 nt, encoding a 2182 amino acid (aa). A phylogenetic analysis showed that the isolate shares highest sequence homology (98.52 %) with SVA strain USA-GBI26-2015. A genetic comparison of virulent and weakly virulent SVA strains showed that some amino acid residues may be associated with virulence. Animal challenge experiments showed that 90-100-day-old pigs inoculated with SVV-CH-SD intraorally and intranasally, intranasally, or intramuscularly developed low fever, blisters, and lameness. They had similar levels of neutralizing antibodies against SVA and viral loads in the serum and organs at 28 days post-CHallenge. However, 30-35- and 55-65-day-old pigs challenged with SVV-CH-SD showed no clinical signs, although anti-SVA neutralizing antibodies were detected. Our findings provide useful data for studying the pathogenesis and transmission of SVA in pigs.