Bat Flies of the Family Streblidae (Diptera: Hippoboscoidea) Host Relatives of Medically and Agriculturally Important "Bat-Associated" Viruses

Bat flies (Hippoboscoidea: Nycteribiidae and Streblidae) are obligate hematophagous ectoparasites of bats. We collected streblid bat flies from the New World (México) and the Old World (Uganda), and used metagenomics to identify their viruses. In México, we found méjal virus (Rhabdoviridae; Vesiculovirus), Amate virus (Reoviridae: Orbivirus), and two unclassified viruses of invertebrates. Méjal virus is related to emerging zoonotic encephalitis viruses and to the agriculturally important vesicular stomatitis viruses (VSV). Amate virus and its sister taxon from a bat are most closely related to mosquito- and tick-borne orbiviruses, suggesting a previously unrecognized orbivirus transmission cycle involving bats and bat flies. In Uganda, we found mamucuso virus (Peribunyaviridae: Orthobunyavirus) and two unclassified viruses (a rhabdovirus and an invertebrate virus). Mamucuso virus is related to encephalitic viruses of mammals and to viruses from nycteribiid bat flies and louse flies, suggesting a previously unrecognized orthobunyavirus transmission cycle involving hippoboscoid insects. Bat fly virus transmission may be neither strictly vector-borne nor strictly vertical, with opportunistic feeding by bat flies occasionally leading to zoonotic transmission. Many "bat-associated" viruses, which are ecologically and epidemiologically associated with bats but rarely or never found in bats themselves, may actually be viruses of bat flies or other bat ectoparasites.

Pteropine Orthoreovirus in an Angolan Soft-Furred Fruit Bat (Lissonycteris angolensis) in Uganda Dramatically Expands the Global Distribution of an Emerging Bat-Borne Respiratory Virus

Pteropine orthoreovirus (PRV; Reoviridae: Spinareovirinae) is an emerging bat-borne zoonotic virus that causes influenza-like illness (ILI). PRV has thus far been found only in Australia and Asia, where diverse old-world fruit bats (Pteropodidae) serve as hosts. In this study, we report the discovery of PRV in Africa, in an Angolan soft-furred fruit bat (Lissonycteris angolensis ruwenzorii) from Bundibugyo District, Uganda. Metagenomic characterization of a rectal swab yielded 10 dsRNA genome segments, revealing this virus to cluster within the known diversity of PRV variants detected in bats and humans in Southeast Asia. Phylogeographic analyses revealed a correlation between geographic distance and genetic divergence of PRVs globally, which suggests a geographic continuum of PRV diversity spanning Southeast Asia to sub-Saharan Africa. The discovery of PRV in an African bat dramatically expands the geographic range of this zoonotic virus and warrants further surveillance for PRVs outside of Southeast Asia.

Human Respiratory Viruses☆

Viruses are the leading causes of acute lower respiratory-tract infection in infancy. Respiratory syncytial virus (RSV) is the most common pathogen in severe disease, with hMPV, PIV-3, influenza viruses, and rhinoviruses accounting for the majority of the remainder of acute viral respiratory infections. Humans generally do not develop lifelong immunity to reinfection with these viruses; rather, specific immunity protects against severe and lower respiratory-tract disease.

Genome heterogeneity of SA11 rotavirus due to reassortment with "O" agent

Derivatives of the rotavirus SA11-H96 strain, isolated in 1958 from an overtly healthy vervet monkey, have been used extensively to probe the viral life cycle. To gain insight into the phenotypic and genotypic differences among SA11 isolates, we sequenced the segmented double-stranded RNA genomes of SA11-H96 (P5B[2]:G3), two SA11-4F-like viruses (P6[1]:G3), two SA11-4F-like viruses with gene 5 rearrangements, and relevant segments of SA11 temperature-sensitive mutants and the "O" (Offal) agent (P6[1]:G8), a rotavirus isolated in 1965 from abattoir waste. This analysis indicates that the only complete genomic sequence previously reported for SA11 (Both) is instead that of a reassortant, originating like the SA11-4F-like viruses, from the introduction of an "O" agent gene into the SA11 genetic background. These results, combined with identification of mutations that correlate with altered growth properties and ts phenotype, emphasize the importance of considering segment origin and sequence variation in interpreting experimental outcomes with SA11 strains.

[Bluetongue, an emerging disease in Northern Europe]

Bluetongue is a non contagious viral disease of sheep transmitted by bites of haematophagous midges. The disease is caused by an orbivirus belonging to the Reoviridae family. The genome is segmented in 10 double-strand RNA encapsidated in a non-enveloped spherical particle with a icosaedral symetry. Twenty distinct serotypes have been identified so far, each of them inducing limited cross-protection against the others. Sheep are usually the only ones showing clinical signs like pyrexia, congestion of mucosa and cyanosis of the tongue. However, cattle, goat and wild ruminants can be asymptomatically infected. Formerly restricted to the area between the 30/40th south and 40/50th north parallels, the infection has progressively extended to the south of Europe and was more recently introduced in the north. The reason for this extension might be twice: the northward spreading of the tropical vector Culicoides imicola and the adaptation of the virus to a yet unknown endemic biting midge. Control of the disease is based on the use of live-attenuated or inactivated vaccines specific of the serotype. In free area, emergency measures can also consist in the rapid detection and elimination of the outbreaks.

Mammalian reovirus, a nonfusogenic nonenveloped virus, forms size-selective pores in a model membrane

During cell entry, reovirus particles with a diameter of 70-80 nm must penetrate the cellular membrane to access the cytoplasm. The mechanism of penetration, without benefit of membrane fusion, is not well characterized for any such nonenveloped animal virus. Lysis of RBCs is an in vitro assay for the membrane perforation activity of reovirus; however, the mechanism of lysis has been unknown. In this report, osmotic-protection experiments using PEGs of different sizes revealed that reovirus-induced lysis of RBCs occurs osmotically, after formation of small size-selective lesions or "pores." Consistent results were obtained by monitoring leakage of fluorophore-tagged dextrans from the interior of resealed RBC ghosts. Gradient fractionations showed that whole virus particles, as well as the myristoylated fragment mu1N that is released from particles, are recruited to RBC membranes in association with pore formation. We propose that formation of small pores is a discrete, intermediate step in the reovirus membrane-penetration pathway, which may be shared by other nonenveloped animal viruses.

Structure of avian orthoreovirus virion by electron cryomicroscopy and image reconstruction

Among members of the genus Orthoreovirus, family Reoviridae, a group of non-enveloped viruses with genomes comprising ten segments of double-stranded RNA, only the "non-fusogenic" mammalian orthoreoviruses (MRVs) have been studied to date by electron cryomicroscopy and three-dimensional image reconstruction. In addition to MRVs, this genus comprises other species that induce syncytium formation in cultured cells, a property shared with members of the related genus Aquareovirus. To augment studies of these "fusogenic" orthoreoviruses, we used electron cryomicroscopy and image reconstruction to analyze the virions of a fusogenic avian orthoreovirus (ARV). The structure of the ARV virion, determined from data at an effective resolution of 14.6 A, showed strong similarities to that of MRVs. Of particular note, the ARV virion has its pentameric lambda-class core turret protein in a closed conformation as in MRVs, not in a more open conformation as reported for aquareovirus. Similarly, the ARV virion contains 150 copies of its monomeric sigma-class core-nodule protein as in MRVs, not 120 copies as reported for aquareovirus. On the other hand, unlike that of MRVs, the ARV virion lacks "hub-and-spokes" complexes within the solvent channels at sites of local sixfold symmetry in the incomplete T=13l outer capsid. In MRVs, these complexes are formed by C-terminal sequences in the trimeric mu-class outer-capsid protein, sequences that are genetically missing from the homologous protein of ARVs. The channel structures and C-terminal sequences of the homologous outer-capsid protein are also genetically missing from aquareoviruses. Overall, the results place ARVs between MRVs and aquareoviruses with respect to the highlighted features.

Commensal and mutualistic relationships of reoviruses with their parasitoid wasp hosts

During evolution, certain endoparasitoid wasps have developed mechanisms to suppress the defence systems of their hosts. For this purpose, these species, all of which belong to the families Ichneumonidae and Braconidae, inject various kinds of virus-like particles. The most studied of these particles are classified as polydnaviruses (family Polydnaviridae) which are symbiotic viruses. Over the past decade, it has also been shown that several wasp species harbour reoviruses (family Reoviridae), and that two of these suppress host defence, allowing the development of the parasitoid eggs. In this paper, we summarize the key features of these viruses and their relationships with their wasp hosts. Five reoviruses are known that appear to be non-pathogenic for the wasps. Three of these, McRVLP, HeRV, OpRVLP, use their wasp hosts as vectors, and do not appear to be involved in host defence suppression. The fourth, DpRV-1, is a commensal reovirus detected in most field populations of the wasp, Diadromus pulchellus. This reovirus is always found associated with an ascovirus, DpAV-4a, which is indispensable for host immune suppression. Although DpRV-1 has not been shown to directly increase D. pulchellus parasitic success, it may contribute to this success by retarding DpAV-4a replication in the wasp. The fifth reovirus, DpRV-2, occurs in a specific population of D. pulchellus in which DpRV-1 and DpAV-4 are absent. This virus has a mutualistic relationship with its wasp host, as its injection by females during oviposition is essential for host immunosuppression. Interestingly, these viruses belong to several different reovirus genera.

RNA silencing of rotavirus gene expression

RNA interference (RNAi) is a double-stranded RNA (dsRNA)-triggered mechanism for suppressing gene expression, which is conserved in evolution and has emerged as a powerful tool to study gene function. Rotaviruses, the leading cause of severe diarrhea in young children, are formed by three concentric layers of protein, and a genome composed of 11 segments of dsRNA. Here, we show that the RNAi machinery can be triggered to silence rotavirus gene expression by sequence-specific short interfering RNAs (siRNAs). RNAi is also useful for the study of the virus-cell interactions, through the silencing of cellular genes that are potentially important for the replication of the virus. Interestingly, while the translation of mRNAs is readily stopped by the RNAi machinery, the viral transcripts involved in virus genome replication do not seem to be susceptible to RNAi. Since gene silencing by RNAi is very efficient and specific, this system could become a novel therapeutic approach for rotavirus and other virus infections, once efficient methods for in vivo delivery of siRNAs are developed. Although the use of RNAi as an antiviral therapeutic tool remains to be demonstrated, there is no doubt that this technology will influence drastically the way postgenomic virus research is conducted.

Directed changes in the number of double-stranded RNA genomic segments in bacteriophage phi6

Bacteriophage Phi6 has a genome of three segments of double-stranded RNA. The segments are designated S, M, and L. Each segment has a unique packaging site, pac, near the 5' end of the plus strand. The plus strands of the segments are normally packaged in the order S, M, L. Chimeras of segment M and S in which segment M is at the 5' end of the plus strand can be stably incorporated into the virion; however, an independent segment S must be included along with normal segment L, even if it contains no active genes. A chimera of segment M and S in which segment S is at the 5' end of the plus strand can be stably incorporated into the virion along with normal segment L to form a two-segment genome. A chimera of segments S, M, and L in which the packaging sequence is that of S can also form a stable nonsegmented genome. These findings are consistent with a model that we have proposed for the packaging of the Phi6 genome.

Stoichiometric packaging of the three genomic segments of double-stranded RNA bacteriophage phi6

A model that explains the stoichiometric packaging of the chromosomes of phi6, a bacteriophage with a genome of three unique double-stranded RNA segments, is proposed and supported. Ordered switches in packaging specificity and RNA synthesis are determined by the amount of RNA within the procapsid. The plus strand of segment S binds to one of several sites on the outside of the empty procapsid. The RNA enters and the procapsid expands so that the S sites are lost and M sites appear. Packaging of segment M results in the loss of the M sites and the appearance of the L sites. Packaging of L readies the particle for minus-strand synthesis. If any of the segments is less than normal size, packaging of that class of segments continues until the normal content of RNA for that segment is packaged and the binding sites then change.