A recent marine metagenomic study has revealed the existence of a novel group of viruses designated mirusviruses, which are proposed to form an evolutionary link between two realms of double-stranded DNA viruses, Varidnaviria and Duplodnaviria. Metagenomic data suggest that mirusviruses infect microeukaryotes in the photic layer of the ocean, but their host range remains largely unknown. In this study, we investigated the presence of mirusvirus marker genes in publicly available 1,901 eukaryotic genome assemblies, mainly derived from unicellular eukaryotes, to identify potential hosts of mirusviruses. Mirusvirus marker sequences were identified in 1,348 assemblies spanning 284 genera across eight supergroups of eukaryotes. The habitats of the putative mirusvirus hosts included not only marine but also other diverse environments. Among the major capsid protein (MCP) signals in the genome assemblies, we identified 85 sequences that showed high sequence and structural similarities to reference mirusvirus MCPs. A phylogenetic analysis of these sequences revealed their distant evolutionary relationships with the seven previously reported mirusvirus clades. Most of the scaffolds with these MCP sequences encoded multiple mirusvirus homologs, underscoring the impact of mirusviral infection on the evolution of the host genome. We also identified three circular mirusviral genomes within the genomic data of the oil producing thraustochytrid Schizochytrium sp. and the endolithic green alga Ostreobium quekettii. Overall, mirusviruses probably infect a wide spectrum of eukaryotes and are more diverse than previously reported.

Africa experiences frequent emerging disease outbreaks among humans, with bats often proposed as zoonotic pathogen hosts. We comprehensively reviewed virus-bat findings from papers published between 1978 and 2020 to evaluate the evidence that African bats are reservoir and/or bridging hosts for viruses that cause human disease. We present data from 162 papers (of 1322) with original findings on (1) numbers and species of bats sampled across bat families and the continent, (2) how bats were selected for study inclusion, (3) if bats were terminally sampled, (4) what types of ecological data, if any, were recorded and (5) which viruses were detected and with what methodology. We propose a scheme for evaluating presumed virus-host relationships by evidence type and quality, using the contrasting available evidence for Orthoebolavirus versus Orthomarburgvirus as an example. We review the wording in abstracts and discussions of all 162 papers, identifying key framing terms, how these refer to findings, and how they might contribute to people\'s beliefs about bats. We discuss the impact of scientific research communication on public perception and emphasize the need for strategies that minimize human-bat conflict and support bat conservation. Finally, we make recommendations for best practices that will improve virological study metadata.

The virus-to-prokaryote ratio (VPR), which reflects the numerical dominance of viruses over their hosts, has been proposed as a proxy for assessing the relationship between viruses and prokaryotes. Previous studies showed that VPR values fluctuate over six orders of magnitude within and across various benthic ecosystems, with an average value of approximately 10. We hypothesize that this high VPR value is largely due to the inaccurate enumeration of viruses and prokaryotes (e.g., centrifugation treatments may lead to a three-fourfold overestimation of VPR). In this study, we evaluated the impact of processing methods on the determination of VPR values. Using an optimized procedure, we investigated the marine benthic VPR at 31 sites, from intertidal zones through continental shelves to abyssal plains, and assessed its monthly variation in two contrasting intertidal habitats (muddy-sand and sandy). By compiling 135 VPR data points of surface sediments from 37 publications, we reveal the effect of centrifugation on published VPR values and describe the spatial distribution of VPR values on a larger scale based on reliable data. The results showed that the commonly used centrifugation method may result in an overestimation of VPR values that are approximately one order of magnitude higher than those obtained using the dilution method, while other processing steps had a limited impact on the VPR. Our analysis indicates that the benthic VPR value is low and less varied across temporal and spatial scales, fluctuating mostly within 10, and the average VPR is approximately 2 in both marine and freshwater habitats. An insignificant seasonal pattern in the VPR was observed in the intertidal zone, with lower VPR values occurring at high temperatures. The VPR spatial distribution was primarily associated with sediment phaeophytin a, suggesting that the trophic conditions of the upper water column and the sedimentation of organic matter to the bottom are the key factors affecting VPR values. The mean VPR in benthic habitats is approximately one order of magnitude lower and much less varied than that observed in pelagic habitats, indicating that the virus-host relationship and the ecological function of viruses in the two ecosystems may be very different.

Parvovirus B19 (B19V), a single-stranded DNA virus in the family Parvoviridae, is a human pathogenic virus, characterized by a selective but not exclusive tropism for erythroid progenitor cells. Widely diffuse, it is responsible for an ample range of clinical manifestations, whose characteristics and outcomes depend on the interplay between the viral properties and the physiological and immune status of the infected individuals. The complexity of virus-host relationship and the diversity of the clinical course of infection pose a diagnostic challenge that may require non-trivial solutions. Areas covered: The review includes an updated description of the course of B19V infection in its complexity and diversity of pathogenetic mechanisms, discusses the consequent requirements for different and appropriated diagnostic approaches, presents the main diagnostic techniques, more recent technical advancements, and their application to the diverse clinical situations. Expert commentary: The complex scenario of the infectious process and the diversity in possible pathogenetic mechanisms make necessary a multi-parametric approach for an accurate and informative laboratory diagnosis of B19V infection, combining as much as possible the molecular detection of viral components, mainly viral DNA, to commonly followed immunological detection of virus-specific antibodies and a critical assessment of laboratory findings.

For over a century, viruses have been known as the most abundant and diverse group of organisms on Earth, forming a virosphere. Based on extensive meta-analyses, we present, for the first time, a wide and complete overview of virus-host network, covering all known viral species. Our data indicate that most of known viral species, regardless of their genomic category, have an intriguingly narrow host range, infecting only 1 or 2 host species. Our data also show that the known virosphere has expanded based on viruses of human interest, related to economical, medical or biotechnological activities. In addition, we provide an overview of the distribution of viruses on different environments on Earth, based on meta-analyses of available metaviromic data, showing the contrasting ubiquity of head-tailed phages against the specificity of some viral groups in certain environments. Finally, we uncovered all human viral species, exploring their diversity and the most affected organic systems. The virus-host network presented here shows an anthropocentric view of the virology. It is therefore clear that a huge effort and change in perspective is necessary to see more than the tip of the iceberg when it comes to virology.