Toroviruses Affecting Animals and Humans: A Review

Architecture of torovirus replicative organelles

Plus-stranded RNA viruses replicate in the cytosol of infected cells, in membrane-bound replication complexes. We previously identified double membrane vesicles (DMVs) in the cytoplasm of cells infected with Berne virus (BEV), the prototype member of Torovirus genus (Nidovirales Order). Our previous analysis by transmission electron microscopy suggested that the DMVs form a reticulovesicular network (RVN) analogous those described for the related severe acute respiratory syndrome coronavirus (SARS-CoV-1). Here, we used serial sectioning and electron tomography to characterize the architecture of torovirus replication organelles, and to learn about their biogenesis and dynamics during the infection. The formation of a RVN in BEV infected cells was confirmed, where the outer membranes of the DMVs are interconnected with each other and with the ER. Paired or zippered ER membranes connected with the DMVs were also observed, and likely represent early structures that evolve to give rise to DMVs. Also, paired membranes forming small spherule-like invaginations were observed at late time post-infection. Although resembling in size, the tomographic analysis show that these structures are clearly different from the true spherules described previously for coronaviruses. Hence, BEV shows important similarities, but also some differences, in the architecture of the replication organelles with other nidoviruses.

Characterization and Identification of a Novel Torovirus Associated With Recombinant Bovine Torovirus From Tibetan Antelope in Qinghai-Tibet Plateau of China

Toroviruses (ToVs) are enteric pathogens and comprise three species, equine torovirus (EToV), bovine torovirus (BToV), and porcine torovirus (PToV). In this study, a novel torovirus (antelope torovirus, AToV) was discovered from fecal samples of Tibetan antelopes (Pantholops hodgsonii) with viral loads of 2.10×10⁹ to 1.76×10¹⁰ copies/g. The genome of AToV is 28,438 nucleotides (nt) in length encoding six open reading frames (ORFs) with 11 conserved domains in pp1ab and a putative slippery sequence (₁₄₁₇₁UUUAAAC₁₄₁₇₇) in the overlapping region of ORF1a and ORF1b. Phylogenetic analysis illustrated strains of AToV form a unique clade within ToVs and comparative analysis showed AToV share relatively low sequence identity with other ToVs in six ORFs (68.2–91.6% nucleotide identity). These data suggested that AToV represents a novel and distinct species of ToVs. Based on the M genes, evolutionary analysis with BEAST of AToV and other ToVs led to a most recent common ancestor estimate of 366years ago. Remarkably, recombination analysis revealed AToV was the unknown parental ToV that once involving in the recombinant events of HE genes of two Dutch strains of BToV (B150 and B155), which indicated that AToV occurred cross-species transmission and existed both in the Netherlands and China. This study revealed a novel torovirus, a natural reservoir host (Tibetan antelope) of toroviruses for the first time, and appealed to further related studies to better understand the diversity of toroviruses.

Serpentoviruses: More than Respiratory Pathogens

In recent years, nidoviruses have emerged as important respiratory pathogens of reptiles, affecting captive python populations. In pythons, nidovirus (recently reclassified as serpentovirus) infection induces an inflammation of the upper respiratory and alimentary tract which can develop into a severe, often fatal proliferative pneumonia. We observed pyogranulomatous and fibrinonecrotic lesions in organ systems other than the respiratory tract during full postmortem examinations on 30 serpentovirus reverse transcription-PCR (RT-PCR)-positive pythons of varying species originating from Switzerland and Spain. The observations prompted us to study whether this not yet reported wider distribution of lesions is associated with previously unknown serpentoviruses or changes in the serpentovirus genome. RT-PCR and inoculation of Morelia viridis cell cultures served to recruit the cases and obtain virus isolates. Immunohistochemistry and immunofluorescence staining against serpentovirus nucleoprotein demonstrated that the virus infects not only a broad spectrum of epithelia (respiratory and alimentary epithelium, hepatocytes, renal tubules, pancreatic ducts, etc.), but also intravascular monocytes, intralesional macrophages, and endothelial cells. With next-generation sequencing we obtained a full-length genome for a novel serpentovirus species circulating in Switzerland. Analysis of viral genomes recovered from pythons showing serpentovirus infection-associated respiratory or systemic disease did not reveal sequence association to phenotypes; however, functional studies with different strains are needed to confirm this observation. The results indicate that serpentoviruses have a broad cell and tissue tropism, further suggesting that the course of infection could vary and involve lesions in a broad spectrum of tissues and organ systems as a consequence of monocyte-mediated viral systemic spread.IMPORTANCE During the last years, python nidoviruses (now reclassified as serpentoviruses) have become a primary cause of fatal disease in pythons. Serpentoviruses represent a threat to captive snake collections, as they spread rapidly and can be associated with high morbidity and mortality. Our study indicates that, different from previous evidence, the viruses do not only affect the respiratory tract, but can spread in the entire body with blood monocytes, have a broad spectrum of target cells, and can induce a variety of lesions. Nidovirales is an order of animal and human viruses that comprises important zoonotic pathogens such as Middle East respiratory syndrome coronavirus (MERS-CoV), severe acute respiratory syndrome coronavirus (SARS-CoV), and SARS-CoV-2. Serpentoviruses belong to the same order as the above-mentioned human viruses and show similar characteristics (rapid spread, respiratory and gastrointestinal tropism, etc.). The present study confirms the relevance of natural animal diseases to better understand the complexity of viruses of the order Nidovirales.

Advances in Diagnostic Approaches for Viral Etiologies of Diarrhea: From the Lab to the Field

The applications of correct diagnostic approaches play a decisive role in timely containment of infectious diseases spread and mitigation of public health risks. Nevertheless, there is a need to update the diagnostics regularly to capture the new, emergent, and highly divergent viruses. Acute gastroenteritis of viral origin has been identified as a significant cause of mortality across the globe, with the more serious consequences seen at the extremes of age groups (young and elderly) and immune-compromised individuals. Therefore, significant advancements and efforts have been put in the development of enteric virus diagnostics to meet the WHO ASSURED criteria as a benchmark over the years. The Enzyme-Linked Immunosorbent (ELISA) and Polymerase Chain Reaction (PCR) are the basic assays that provided the platform for development of several efficient diagnostics such as real-time RT-PCR, loop-mediated isothermal amplification (LAMP), polymerase spiral reaction (PSR), biosensors, microarrays and next generation sequencing. Herein, we describe and discuss the applications of these advanced technologies in context to enteric virus detection by delineating their features, advantages and limitations.