Virion nucleic acid of Ebola virus.

The virion nucleic acid of Ebola virus consists of a single-stranded RNA with a molecular weight of approximately 4.0 x 10(6). The virion RNA did not bind to oligodeoxythymidylic acid-cellulose under conditions known to bind RNAs rich in polyadenylic acid and was not infectious under conditions which yielded infectious RNA from Sindbis virus, suggesting that Ebola virus virion nucleic acid is a negative-stranded RNA.     

Inhibition of Virion Maturation by Simultaneous Deletion of Glycoproteins E, I, and M of Pseudorabies Virus

Glycoprotein M (gM), the product of the UL10 gene of pseudorabies virus (PrV), is one of the few nonessential glycoproteins conserved throughout the Herpesviridae. In contrast to wild-type PrV strains, the UL10 gene product of the attenuated PrV vaccine strain Bartha (PrV-Ba) is not modified by N-glycans due to a mutation in the DNA sequence encoding the consensus N-glycosylation motif. To assay function of the UL10 protein in PrV-Ba, a UL10-deletion mutant (PrV-Ba-UL10(-)) was isolated. Surprisingly, in contrast to gM-deleted wild-type PrV, PrV-Ba-UL10(-) was severely impaired in plaque formation, inducing only foci of very few infected RK13, Vero, and PSEK cells and tiny plaques on MDBK cells. Since this effect was significantly more dramatic than in wild-type PrV, additional mutations known to be present in PrV-Ba were analyzed for their contribution to this phenotype. trans-complementation of the mutated PrV-Ba UL21 or gC protein by the wild-type version had no influence on the observed phenotype. In contrast, complementation of the gE/gI deletion rescued the phenotype. The synergistic effect of deletions in gE/gI and gM on plaque size was verified by construction of a gE/I/M triple mutant derived from wild-type PrV which exhibited the same phenotype. The dramatic effect of deletion of gM on plaque size in a gE/I- virus background was mainly attributable to a function of gM, and not of the gM/gN complex, as shown by analysis of a gE/I/N triple mutant. Interestingly, despite the strong effect on plaque size, penetration was not significantly impaired. In noncomplementing cells infected with the gE/I/M triple mutant, electron microscopy showed absence of secondary envelopment in the cytoplasm but occurrence of intracytoplasmic accumulations of nucleocapsids in association with electron dense material, presumably tegument proteins. These structures were not observed after infection of cells expressing either gE/I or gM. We suggest that gE/I and gM are required for late stages in virion morphogenesis prior to final envelopment in the cytoplasm.     

分子伴侣与病毒糖蛋白

分子伴侣(molecular chaperone)的概念由Lasky于1978年首先提出[1],真核细胞内质网中的分子伴侣是由多种蛋白质组成,它们可以介导新合成蛋白质的正确折叠与装配,并在真核生物的细胞中广泛存在.     

应用生物素标记的凝集素分析登革Ⅱ型病毒糖蛋白

应用十二种生物素标记的凝集素,分析了经SDS-PAGE和电转印分离的登革Ⅱ型病毒(D_2V)糖蛋白。结果表明:D_2V的包膜糖蛋白E可与三种D-甘露糖特异的凝集素(conAPSA及LCA)结合,提示E蛋白的寡糖链为高甘露糖型。D_2V的糖基化非结构蛋白NS_1可与两种D-甘露糖特异的凝集素conA及LCA结合,提示NS_1的寡糖链亦为高甘露糖型。实验结果还显示了一些可能为C6/36细胞感染D_2V后新合成、合成量增加或减少以至完全受抑制的糖蛋白成分,有关这些糖蛋白的性质、功能、意义尚不清楚。在本实验条件下,这种生物素标记凝集素印迹法证明了其敏感性和糖基特异性,可以作为分析各种微量含糖大分子的一种实用方法。     

Comparison of the Cowpox Virus and Vaccinia Virus Mature Virion Proteome: Analysis of the Species- and Strain-Specific Proteome

Cowpox virus (CPXV) causes most zoonotic orthopoxvirus (OPV) infections in Europe and Northern as well as Central Asia. The virus has the broadest host range of OPV and is transmitted to humans from rodents and other wild or domestic animals. Increasing numbers of human CPXV infections in a population with declining immunity have raised concerns about the virus’ zoonotic potential. While there have been reports on the proteome of other human-pathogenic OPV, namely vaccinia virus (VACV) and monkeypox virus (MPXV), the protein composition of the CPXV mature virion (MV) is unknown. This study focused on the comparative analysis of the VACV and CPXV MV proteome by label-free single-run proteomics using nano liquid chromatography and high-resolution tandem mass spectrometry (nLC-MS/MS). The presented data reveal that the common VACV and CPXV MV proteome contains most of the known conserved and essential OPV proteins and is associated with cellular proteins known to be essential for viral replication. While the species-specific proteome could be linked mainly to less genetically-conserved gene products, the strain-specific protein abundance was found to be of high variance in proteins associated with entry, host-virus interaction and protein processing.     

Functional Interactions of the Influenza Virus Glycoproteins

Hemagglutinin (HA) and neuraminidase (NA) are functionally related envelope glycoproteins of the influenza virus (Flu). HA interacts with terminal sialyl residues of oligosaccharides and ensures the binding of the virus particle to the cell surface. NA is a receptor-destroying enzyme that removes sialyl residues from oligosaccharides contained in cell and virus components and thereby prevents aggregation of virus particles. Analysis of reassortants combining low-functional NA of human Flu with HA of avian Flu showed that sialyl residues are not completely removed in some cases. With high HA affinity for sialyl substrates, such virus particles aggregate, aggregates accumulate on the cell surface, and virus yield decreases. Serial passaging of low-yield aggregating reassortants may lead to selection of high-yield variants, which do not aggregate. The loss of aggregation is due to a decrease in HA affinity for high-molecular-weight sialyl substrates. On evidence of sequencing of the HA gene in original reassortants and their nonaggregating variants, for different HA antigenic subtypes (H2, H3, H4, and H13) the affinity is reduced and aggregation lost through a common mechanism: an increase in the negative charge as a result of an amino acid substitution in the vicinity of the receptor-binding pocket of HA. Taken together, these findings suggest a way of postreassortment adaptation, which improves the functional match of HA and NA. The experimental system employed provides a model of natural processes associated with emergence of Flu variants having a pandemic potential.     

肾综合征出血热病毒糖蛋白的提纯和鉴定

以感染肾综合征出血热病毒(HFRSV)的Vero E6细胞为材料,用免疫亲和层析结合制备聚丙烯酰胺凝胶电泳(PAGE)从感染细胞中提纯了HFRSV两种糖蛋白。先用免疫亲和层析从感染细胞的粗制抗原中获得含有四种蛋白的混合液,用[~3H]-氨基葡萄糖在感染细胞中标记病毒糖蛋白,观察到[~3H]-氨基葡萄糖只结合入78K和57K的病毒蛋白。再用制备SDS-PAGE从HFRSV混合液中提纯78K和57K两种蛋白。实验证明这两种糖蛋白均具中和抗原决定簇,57K的糖蛋白尚具血凝活性,初步鉴定表明这两种糖蛋白相当于文献报道的HFRSV G_1和G_2。     

Mutational Analysis of the Putative Fusion Domain of Ebola Virus Glycoprotein

Ebola viruses contain a single glycoprotein (GP) spike, which functions as a receptor binding and membrane fusion protein. It contains a highly conserved hydrophobic region (amino acids 524 to 539) located 24 amino acids downstream of the N terminus of the Ebola virus GP2 subunit. Comparison of this region with the structural features of the transmembrane subunit of avian retroviral GPs suggests that the conserved Ebola virus hydrophobic region may, in fact, serve as the fusion peptide. To test this hypothesis directly, we introduced conservative (alanine) and nonconservative (arginine) amino acid substitutions at eight positions in this region of the GP2 molecule. The effects of these mutations were deduced from the ability of the Ebola virus GP to complement the infectivity of a vesicular stomatitis virus (VSV) lacking the receptor-binding G protein. Some mutations, such as Ile-to-Arg substitutions at positions 532 (I532R), F535R, G536A, and P537R, almost completely abolished the ability of the GP to support VSV infectivity without affecting the transport of GP to the cell surface and its incorporation into virions or the production of virus particles. Other mutations, such as G528R, L529A, L529R, I532A, and F535A, reduced the infectivity of the VSV-Ebola virus pseudotypes by at least one-half. These findings, together with previous reports of liposome association with a peptide corresponding to positions 524 to 539 in the GP molecule, offer compelling support for a fusion peptide role for the conserved hydrophobic region in the Ebola virus GP.     

Electrophoretic Distribution of the Proteins and Glycoproteins of Influenza Virus and Sendai Virus

The proteins of influenza (WSN) and Sendai virus have been separated by polyacrylamide gel electrophoresis into five components. In both cases, three of these components were shown to be glycoproteins containing fucose, galactose, and glucosamine. Two protein components of each virus were probably free from these sugar residues, including the structural unit of the viral ribonucleoprotein (molecular weight of about 60,000 daltons).     

Biochemical and Structural Characterization of Cathepsin L-Processed Ebola Virus Glycoprotein: Implications for Viral Entry and Immunogenicity

Ebola virus (EBOV) cellular attachment and entry is initiated by the envelope glycoprotein (GP) on the virion surface. Entry of this virus is pH dependent and associated with the cleavage of GP by proteases, including cathepsin L (CatL) and/or CatB, in the endosome or cell membrane. Here, we characterize the product of CatL cleavage of Zaire EBOV GP (ZEBOV-GP) and evaluate its relevance to entry. A stabilized recombinant form of the EBOV GP trimer was generated using a trimerization domain linked to a cleavable histidine tag. This trimer was purified to homogeneity and cleaved with CatL. Characterization of the trimeric product by N-terminal sequencing and mass spectrometry revealed three cleavage fragments, with masses of 23, 19, and 4 kDa. Structure-assisted modeling of the cathepsin L-cleaved ZEBOV-GP revealed that cleavage removes a glycosylated glycan cap and mucin-like domain (MUC domain) and exposes the conserved core residues implicated in receptor binding. The CatL-cleaved ZEBOV-GP intermediate bound with high affinity to a neutralizing antibody, KZ52, and also elicited neutralizing antibodies, supporting the notion that the processed intermediate is required for viral entry. Together, these data suggest that CatL cleavage of EBOV GP exposes its receptor-binding domain, thereby facilitating access to a putative cellular receptor in steps that lead to membrane fusion.Ebola virus (EBOV) is a member of the Filoviridae family and causes severe hemorrhagic fever in humans and nonhuman primates, with case fatality rates of up to 90%. Virus entry and attachment is mediated by a single envelope glycoprotein (GP) as a class I fusion protein, which is proteolytically processed during maturation into two subunits, GP1 and GP2. The GP1 N terminus contains a putative receptor-binding domain (RBD) (2, 9, 11, 12), and the GP2 C terminus contains a fusion peptide, two heptad-repeat regions, and a transmembrane domain. GP1 and GP2 are linked by a disulfide bond (Cys⁵³-Cys⁶⁰⁹) and form trimers of heterodimers on the surface of virions. EBOV GP is also extensively glycosylated, especially within a region of GP1 termed the mucin-like domain (MUC domain), which contains multiple N- and O-linked glycans. We and others have previously shown the MUC domain of GP1 to be cytotoxic and to induce cell rounding (17, 21), and deletion of this region increases pseudovirus infectivity compared to that of full-length GP (11). The MUC domain, however, is also known to enhance cell binding through the human macrophage C-type lectin specific for galactose and N-acetylglucosamine (hMGL) (18), suggesting that glycans in this domain may be involved in the initial cellular attachment. Several other studies have identified factors that enhance cell binding and/or infectivity, including folate receptor α (4), β integrins (19), C-type lectins DC-SIGN and L-SIGN (1), and Tyro3 family members (16). However, the critical cellular receptor(s) thought to interact directly with the GP1 RBD have yet to be identified.Following virus uptake into host cells, which is presumed to occur via receptor-mediated endocytosis (13), the virion is transported to acidified endosomes where GP is exposed to a low pH and enzymatic processing. EBOV entry is pH dependent (19); however, unlike influenza virus, for which a low pH alone induces the conformational changes that lead to membrane fusion (20), recent studies indicate that proteolysis by endosomal cathepsin L (CatL) and CatB (active only at pH 5 to 6) is a dependent step for EBOV entry (5, 14). Although the intermediate EBOV GP generated by CatL cleavage is known to have increased binding and infectivity to target cells (7), little else is known about the cleavage product, specifically where the proteolytic sites are within GP and whether the cleaved product is immunogenic. Recently, Dube and colleagues have proposed a model for CatL cleavage based on thermolysin cleavage (6). However, thermolysin is nonphysiological in this setting and is a member of the metalloenzyme-protease family, whereas CatL is a member of the cysteine-protease family and essential for EBOV entry. In this study, we have characterized the physiological CatL cleavage of the Zaire EBOV GP (ZEBOV-GP) trimer and explored the effect of cleavage on the immunological properties of the GP trimer. To generate this intermediate, we expressed and purified a recombinant form of the Ebola GP trimer ectodomain that had been stabilized with a trimerization motif derived from T4 fibritin (foldon) and purified to homogeneity. The recombinant protein was cleaved with CatL, and the stable cleavage intermediate was characterized biochemically and immunologically. We identified several sites of CatL cleavage within the ZEBOV-GP ectodomain which are different than those observed with thermolysin. The cleaved intermediate product retained binding to the EBOV-neutralizing antibody KZ52 and elicited EBOV-neutralizing antibodies in vaccinated mice. Our data, in conjunction with the recently determined structure of the ZEBOV-GP ectodomain (10), shed light on the critical role of CatL processing in GP structure and function.     

美国防控埃博拉疫情科研部署情况分析

对美国卫生与公众服务部所属的国立卫生研究院(NIH)、生物医学高级研究与发展管理局(BARDA)及国防部化学和生物防御项目(CBDP)中针对埃博拉病毒的相关科研部署情况进行了分析.同时,对美国发表的埃博拉病毒相关科研论文及主要药品、疫苗研发受资助情况进行了分析;对应对生物威胁加强政府投入、资源合理分配、发展广谱性应对措施、加强军地协作等建议进行了阐述。     

Comparison of Vesicular Stomatitis Virus Intracellular and Virion Ribonucleoproteins

Vesicular stomatitis virus ribonucleoproteins (RNP) obtained by a detergent treatment of purified virus (vRNP) or from infected HeLa cell cytoplasm (icRNP) were examined by sedimentation in sucrose or Renografin gradients in the presence or absence of EDTA. It was shown that vRNP and icRNP sediment at the same rate in sucrose and Renografin in the absence of EDTA; however, icRNP sedimented more slowly in the presence of EDTA than did vRNP. Polyacrylamide gel electrophoresis of the proteins of vRNA and icRNP recovered from EDTA-containing gradients demonstrated that both RNP structures contained L, N, and NS proteins in the same proportion. Electron microscopy of both RNP structures, in the absence of EDTA, demonstrated that both exist as helical structures ~20 by 700 nm. However, in the presence of EDTA the icRNP was completely uncoiled with a mean length of 4,095 nm, whereas vRNP was hardly affected. The addition of excess Mg²⁺ or Mn²⁺ to uncoiled icRNP preparations partially restored the coiled configuration. These observations suggest that the change in sedimentation of icRNP in the presence of EDTA is due to a change from a coiled to an uncoiled conformation, that icRNP and vRNP are not structurally identical, and that icRNP must undergo a conformational change during maturation of VSV from the 20-by-700-nm intracellular form to the 50-by-175-nm form found in intact virus. The icRNP containing L, N, and NS proteins (icRNP^L,N,NS) and icRNP containing only N protein (icRNP^N), prepared by centrifugation of icRNP^L,N,NS in CsCl to remove L and NS, were compared by cosedimentation in sucrose gradients. There was a decrease in sedimentation rate of icRNP^N due to loss of L and NS. This sedimentation difference was also apparent in the presence of EDTA; however, both icRNP^L,N,NS and icRNP^N sedimented at a much slower rate in the presence of EDTA, and by electron microscopy both were completely uncoiled. These observations suggest that N protein alone is responsible for the 20-by-700-nm coiled structure and that the divalent cation interactions disrupted by EDTA are N-N or N-RNA interactions. These results are discussed with regard to vesicular stomatitis virus maturation.     

Analytical Performance Characteristics of the Cepheid GeneXpert Ebola Assay for the Detection of Ebola Virus

BackgroundThe recently developed Xpert® Ebola Assay is a novel nucleic acid amplification test for simplified detection of Ebola virus (EBOV) in whole blood and buccal swab samples. The assay targets sequences in two EBOV genes, lowering the risk for new variants to escape detection in the test. The objective of this report is to present analytical characteristics of the Xpert® Ebola Assay on whole blood samples.ConclusionIn summary, we found the Xpert® Ebola Assay to have high analytical sensitivity and specificity for the detection of EBOV in whole blood. It offers ease of use, fast turnaround time, and remote monitoring. The test has an efficient viral inactivation protocol, fulfills inclusivity and exclusivity criteria, and has specimen stability characteristics consistent with the need for decentralized testing. The simplicity of the assay should enable testing in a wide variety of laboratory settings, including remote laboratories that are not capable of performing highly complex nucleic acid amplification tests, and during outbreaks where time to detection is critical.     

ISCB Ebola Award for Important Future Research on the Computational Biology of Ebola Virus

Speed is of the essence in combating Ebola; thus, computational approaches should form a significant component of Ebola research. As for the development of any modern drug, computational biology is uniquely positioned to contribute through comparative analysis of the genome sequences of Ebola strains as well as 3-D protein modeling. Other computational approaches to Ebola may include large-scale docking studies of Ebola proteins with human proteins and with small-molecule libraries, computational modeling of the spread of the virus, computational mining of the Ebola literature, and creation of a curated Ebola database. Taken together, such computational efforts could significantly accelerate traditional scientific approaches. In recognition of the need for important and immediate solutions from the field of computational biology against Ebola, the International Society for Computational Biology (ISCB) announces a prize for an important computational advance in fighting the Ebola virus. ISCB will confer the ISCB Fight against Ebola Award, along with a prize of US$2,000, at its July 2016 annual meeting (ISCB Intelligent Systems for Molecular Biology [ISMB] 2016, Orlando, Florida).     

Biochemical, Conformational, and Immunogenic Analysis of Soluble Trimeric Forms of Henipavirus Fusion Glycoproteins

The henipaviruses, Hendra virus (HeV) and Nipah virus (NiV), are paramyxoviruses discovered in the mid- to late 1990s that possess a broad host tropism and are known to cause severe and often fatal disease in both humans and animals. HeV and NiV infect cells by a pH-independent membrane fusion mechanism facilitated by their attachment (G) and fusion (F) glycoproteins. Here, several soluble forms of henipavirus F (sF) were engineered and characterized. Recombinant sF was produced by deleting the transmembrane (TM) and cytoplasmic tail (CT) domains and appending a glycosylphosphatidylinositol (GPI) anchor signal sequence followed by GPI-phospholipase D digestion, appending a trimeric coiled-coil (GCNt) domain (sF_GCNt), or deleting the TM, CT, and fusion peptide domain. These sF glycoproteins were produced as F₀ precursors, and all were apparent stable trimers recognized by NiV-specific antisera. Surprisingly, however, only the GCNt-appended constructs (sF_GCNt) could elicit cross-reactive henipavirus-neutralizing antibody in mice. In addition, sF_GCNt constructs could be triggered in vitro by protease cleavage and heat to transition from an apparent prefusion to postfusion conformation, transitioning through an intermediate that could be captured by a peptide corresponding to the C-terminal heptad repeat domain of F. The pre- and postfusion structures of sF_GCNt and non-GCNt-appended sF could be revealed by electron microscopy and were distinguishable by F-specific monoclonal antibodies. These data suggest that only certain sF constructs could serve as potential subunit vaccine immunogens against henipaviruses and also establish important tools for further structural, functional, and diagnostic studies on these important emerging viruses.     

Characterizing the Transmission Dynamics and Control of Ebola Virus Disease

Carefully calibrated transmission models have the potential to guide public health officials on the nature and scale of the interventions required to control epidemics. In the context of the ongoing Ebola virus disease (EVD) epidemic in Liberia, Drake and colleagues, in this issue of PLOS Biology, employed an elegant modeling approach to capture the distributions of the number of secondary cases that arise in the community and health care settings in the context of changing population behaviors and increasing hospital capacity. Their findings underscore the role of increasing the rate of safe burials and the fractions of infectious individuals who seek hospitalization together with hospital capacity to achieve epidemic control. However, further modeling efforts of EVD transmission and control in West Africa should utilize the spatial-temporal patterns of spread in the region by incorporating spatial heterogeneity in the transmission process. Detailed datasets are urgently needed to characterize temporal changes in population behaviors, contact networks at different spatial scales, population mobility patterns, adherence to infection control measures in hospital settings, and hospitalization and reporting rates.Ebola virus disease (EVD) is caused by an RNA virus of the family Filoviridae and genus Ebolavirus. Five different Ebolavirus strains have been identified, namely Zaire ebolavirus (EBOV), Sudan ebolavirus (SUDV), Tai Forest ebolavirus (TAFV), Bundibugyo ebolavirus (BDBV), and Reston ebolavirus (RESTV). The great majority of past Ebola outbreaks in humans have been linked to three Ebola strains: EBOV, SUDV, and BDBV [1]. The Ebola virus ([EBOV] formerly designated Zaire ebolavirus) derived its name from the Ebola River, located near the epicenter of the first outbreak identified in 1976 in Zaire (now the Democratic Republic of Congo). EVD outbreaks among humans have been associated with direct human exposure to fruit bats—the most likely reservoir of the virus—or through contact with intermediate infected hosts, which include gorillas, chimpanzees, and monkeys. Outbreaks have been reported on average every 1.5 years [2]. Past EVD outbreaks have occurred in relatively isolated areas and have been limited in size and duration (Fig. 1). It has been recently estimated that about 22 million people living in areas of Central and West Africa are at risk of EVD [3].Open in a separate windowFigure 1Time series of the temporal progression of four past EVD outbreaks in Congo (1976, 1995, 2014) [4–6] and Uganda (2000) [7].An epidemic of EVD (EBOV) has been spreading in West Africa since December 2013 in Guinea, Liberia, and Sierra Leone [8]. A total of 18,603 cases, with 6,915 deaths, have been reported to the World Health Organization as of December 17, 2014 [9]. While the causative strain associated with this epidemic is closely related to that of past outbreaks in Central Africa [10], three key factors have contributed disproportionately to this unprecedented epidemic: (1) substantial delays in detection and implementation of control efforts in a region characterized by porous borders; (2) limited public health infrastructure including epidemiological surveillance systems and diagnostic testing [11], which are necessary for the timely diagnosis of symptomatic individuals, effective isolation of infectious individuals, contact tracing to rapidly identify new cases, and providing supportive care to increase the chances of survival to EVD infection; and (3) cultural practices that involve touching the body of the deceased and the association of illness with witchcraft or conspiracy theories.EBOV is transmitted by direct human-to-human contact via body fluids or indirect contact with contaminated surfaces, but it is not spread through the airborne route. Individuals become symptomatic after an average incubation period of 10 days (range 2–21 days) [12], and infectiousness is increased during the later stages of disease [13]. The characteristic symptoms of EVD are nonspecific and include sudden onset of fever, weakness, vomiting, diarrhea, headache, and a sore throat, while only a fraction of the symptomatic individuals present with hemorrhagic manifestations [14]. The case fatality risk (CFR), calculated as the proportion of deaths among the total number of EVD cases with known outcomes, has been estimated from data of the first 9 months of the epidemic in West Africa at 70.8% (95% CI 68.6–72.8), in broad agreement with estimates from past outbreaks [12].Two important quantities to understand in the transmission dynamics of EVD are the serial interval and the basic reproduction number. The serial interval is defined as the time from illness onset in a primary case to illness onset in a secondary case [15] and has been estimated at 15 days on average for the ongoing epidemic [12]. The basic reproduction number, R ₀, quantifies transmission potential at the beginning of an epidemic and is defined as the average number of secondary cases generated by a typical infected individual during the early phase of an epidemic, before interventions are put in place [16]. If R ₀ < 1, transmission is not sufficient to generate a major epidemic. In contrast, a major epidemic is likely to occur whenever R ₀ > 1. When transmission potential is measured over time t, the effective reproduction number R_t, can be helpful to quantify the time-dependent transmission potential resulting from the effect of control interventions and behavior changes [17]. Estimates of R ₀ for the ongoing epidemic in West Africa have fluctuated around 2 with some uncertainty (e.g., [12, 18–22]), which are in good agreement with estimates from past EVD outbreaks [23]. R ₀ could also vary across regions as a function of the local public health infrastructure (e.g., availability of health care settings and infection control protocols), such that an outbreak may be very unlikely to unfold in developed countries simply as a result of baseline infection control measures in place (i.e., R ₀ < 1) while poor countries with extremely weak or absent public health systems may be unable to control an Ebola outbreak (i.e., R ₀ > 1).Mathematical models of disease transmission have proved to be useful tools to characterize the transmission dynamics of infectious diseases and evaluate the effects of control intervention strategies in order to inform public health policy [16, 24, 25]. There are a limited number of mathematical models for the transmission and control of EVD, but a number of efforts are underway in the context of the epidemic in West Africa. The transmission dynamics of EVD have been modeled on the basis of the simple compartmental susceptible-exposed-infectious-removed (SEIR) model that assumes a homogenously mixed population [23]. The modeled population can be structured according to the contributions of community, hospital, and unsafe burials to transmission as EVD transmission has been amplified in health care settings with ineffective infection control measures and during unsafe burials [23]. A schematic representation of the main transmission pathways of EVD is shown in Fig. 2.Open in a separate windowFigure 2Schematic representation of the transmission dynamics of Ebola virus disease.A recent study published in PLOS Biology by Drake and colleagues [26] presents an interesting and flexible modeling framework for the transmission and control of EVD in Liberia. Their framework is based on a multi-type branching process model in which “multi-type” refers to the consideration of two types of settings where transmission can occur, while “branching process” is the mathematical term to specify a probabilistic model. For instance, in the case of a single-type branching process, the transmission dynamics are simply described using a single reproduction number, i.e., the average number of secondary cases produced by a single primary case. However, when two types of hosts are considered in the transmission process, two reproduction numbers are needed to characterize within-group mixing (e.g., within-hospital and within-community transmission) and two reproduction numbers characterize transmission between groups (e.g., transmission from hospital to community and vice versa).Drake and colleague’s elegant modeling approach describes EVD transmission according to infection generations by calculating probability distributions of the number of secondary cases that arise in the community via nursing care or during unsafe burials and in health care settings via infections to health care workers and visitors. The model explicitly accounts for the hospitalization rate—the fraction of infectious individuals in the community seeking hospitalization (estimated in this study at 60%). However, the number of effectively isolated infectious individuals is constrained by the number of available beds in treatment centers—which are assumed in this study to operate at twice their regular capacity. It is important to note that the number of beds available to treat EVD patients was severely limited in Liberia prior to mid August 2014 (Fig. 1 in [26]). Moreover, the rate of safe burials that reduces the force of infection is included in their model as an increasing function of time. The model was calibrated by tuning six parameters to fit the trajectories of the number of reported cases in the community and among health care workers during the period 4 July to 2 September 2014 for a total of four infection generations during which the effective reproduction number was estimated to decline on average from about 2.8 to 1.4. The model was able to effectively capture heterogeneity in transmission of EVD in both the community and hospital settings.Drake and colleagues [26] employed their calibrated model to forecast the epidemic trajectory in Liberia from 3 September to 31 December 2014 under different scenarios that account for an increasing fraction of cases seeking hospitalization and a surge in the number of beds available to isolate and treat EVD patients. Their results indicate that allocating 1,700 additional beds (100 new beds every 4 days) in new Ebola treatment centers committed by US aid reduces the mean epidemic size to ~51,000 (60% reduction with respect to the baseline scenario), while epidemic control by mid-March is only plausible through a 4-fold increase in the number of beds committed by US aid and enhancing the hospitalization rate from 60% to 99% for a final epidemic size of 12,285. Moreover, an additional epidemic forecast incorporating data up to 1 December 2014 indicated that containment could be achieved between March and June 2015.Other interventions were not explicitly incorporated in their model because it is difficult to parameterize them in the absence of datasets that permit statistical estimation of their impact on the transmission dynamics. These additional interventions include the use of household protection kits, designed to reduce transmission in the community; improvements in infection control protocols in health care settings that reduce transmission among health care workers; and the impact of rapid diagnostic kits in Ebola treatment centers, which reduce the time to isolation for infectious individuals seeking hospitalization. Increasing awareness and education of the population about the disease could have also yielded further reductions in case incidence by reducing the size of the at-risk susceptible population (Fig. 3) [27]. Nevertheless, some of these effects could have been indirectly captured implicitly by the time-dependent safe burial rate parameter in their model.Open in a separate windowFigure 3Contrasting epidemic growth in the presence and absence of behavior changes that reduce the transmission rate.Importantly, prior models of EVD transmission [23, 28, 29] and the model by Drake and colleagues have not incorporated spatial heterogeneity in the transmission dynamics. In particular, the EVD epidemic in West Africa can be characterized as a set of asynchronous local (e.g., district) epidemics that exhibit sub-exponential growth, which could be driven by a highly clustered underlying contact network or population behavior changes induced by the accumulation of morbidity and mortality rates (see Fig. 4 and [30]). EVD contagiousness is most pronounced in the later and more severe stages of Ebola infection when infectious individuals are confined at home or health care settings and mostly exposed to caregivers (e.g., health care workers, family members) [30]. This characterization would lead to EVD transmission over a network of contacts that is highly clustered (e.g., individuals are likely to share a significant fraction of their contacts), which is associated with significantly slower spread relative to the common random mixing assumption as illustrated in Fig. 5. The development of transmission models that incorporate spatial heterogeneity (e.g., by modeling spatial coupling or human migration) is currently limited by the shortage of detailed datasets from the EVD-affected areas about the geographic distribution of households, health care settings, reporting and hospitalization rates across urban and rural areas, and patterns of population mobility in the region. Some of these limitations may be overcome in the near future. For instance, cell phone data could provide a basis to characterize population mobility in the region at a refined spatial scale.Open in a separate windowFigure 4Representative time series of the cumulative number of EVD cases (in log scale) at the district level in Guinea, Sierra Leone, and Liberia.Open in a separate windowFigure 5Epidemic growth in two populations characterized by two different underlying contact networks.The ongoing epidemic in West Africa offers a unique opportunity to improve our current understanding of the transmission characteristics of EVD in humans. To achieve this goal, it is crucial to collect spatial-temporal data on population behaviors, contact networks, social distancing measures, and education campaigns. Datasets comprising detailed demographic, socio-economic, contact rates, and population mobility estimates in the region (e.g., commuting networks, air traffic) need to be integrated and made publicly available in order to develop highly resolved transmission models, which could guide control strategies with greater precision in the context of the EVD epidemic in West Africa. Although recent data from Liberia indicates that the epidemic is on track for eventual control, the epidemic in Sierra Leone continues an increasing trend, and in Guinea, case incidence roughly follows a steady trend. The potential impact of vaccines should also be incorporated in future modeling efforts as these pharmaceutical interventions are expected to become available in the upcoming months.     

Glycoproteins of Sindbis Virus: Preliminary Characterization of the Oligosaccharides

The carbohydrate content of Sindbis virus was determined by gas chromatographic analysis. The two viral glycoproteins were found to be approximately 8% carbohydrate by weight. Mannose is the sugar present in the largest amount. Smaller amounts of glucosamine, galactose, sialic acid, and fucose were also detected. Each of the two viral glycoproteins appears to contain two structurally unrelated oligosaccharides. Two of the three Sindbis-specific glycoproteins found in infected chick cells were shown to contain short, unfinished oligosaccharides.