Third-generation flavivirus vaccines based on single-cycle, encapsidation-defective viruses

Flaviviruses are arthropod-borne pathogens that cause significant disease on all continents of the world except Antarctica. Flavivirus diseases are particularly important in tropical regions where arthropod vectors are abundant. Live-attenuated virus vaccines (LAVs) and inactivated virus vaccines (INVs) exist for some of these diseases. LAVs are economical to produce and potent, but are not suitable for use in the immunocompromised. INVs are safer, but are more expensive to produce and less potent. Despite the success of both classes of these first-generation flavivirus vaccines, problems associated with their use indicate a need for improved products. Furthermore, there are no suitable vaccines available for important emerging flavivirus diseases, notably dengue and West Nile encephalitis (WNE). To address these needs, new products, including LAVs, INVs, viral-vectored, genetically engineered LAVs, naked DNA, and subunit vaccines are in various stages of development. Here we describe the current state of these first- and second-generation vaccine candidates, and compare these products to our recently described single-cycle, encapsidation defective flavivirus vaccine: RepliVAX. RepliVAX can be propagated in C-expressing cells (or as a unique two-component virus) using methods similar to those used to produce today's economical and potent LAVs. However, due to deletion of most of the gene for the C protein, RepliVAX cannot spread between normal cells, and is unable to cause disease in vaccinated animals. Nevertheless, RepliVAX is potent and efficacious in animal models for WNE and Japanese encephalitis, demonstrating its utility as a third-generation flavivirus vaccine that should be potent, economical to produce, and safe in the immunocompromised.

West Nile virus genome amplification requires the functional activities of the proteasome

The lifecycle of intracellular pathogens, especially viruses, is intimately tied to the macromolecular synthetic processes of their host cell. In the case of positive-stranded RNA viruses, the ability to translate and, thus, replicate their infecting genome is dependent upon hijacking host proteins. To identify proteins that participate in West Nile virus (WNV) replication, we tested the ability of siRNAs designed to knock-down the expression of a large subset of human genes to interfere with replication of WNV replicons. Here we report that multiple siRNAs for proteasome subunits interfered with WNV genome amplification. Specificity of the interference was shown by demonstrating that silencing proteasome subunits did not interfere with Venezuelan equine encephalitis virus replicons. Drugs that blocked proteasome activity were potent inhibitors of WNV genome amplification even if cells were treated 12 h after infection, indicating that the proteasome is required at a post-entry stage(s) of the WNV infection cycle.

RNA interference screen for human genes associated with West Nile virus infection

West Nile virus (WNV), and related flaviviruses such as tick-borne encephalitis, Japanese encephalitis, yellow fever and dengue viruses, constitute a significant global human health problem. However, our understanding of the molecular interaction of such flaviviruses with mammalian host cells is limited. WNV encodes only 10 proteins, implying that it may use many cellular proteins for infection. WNV enters the cytoplasm through pH-dependent endocytosis, undergoes cycles of translation and replication, assembles progeny virions in association with endoplasmic reticulum, and exits along the secretory pathway. RNA interference (RNAi) presents a powerful forward genetics approach to dissect virus-host cell interactions. Here we report the identification of 305 host proteins that affect WNV infection, using a human-genome-wide RNAi screen. Functional clustering of the genes revealed a complex dependence of this virus on host cell physiology, requiring a wide variety of molecules and cellular pathways for successful infection. We further demonstrate a requirement for the ubiquitin ligase CBLL1 in WNV internalization, a post-entry role for the endoplasmic-reticulum-associated degradation pathway in viral infection, and the monocarboxylic acid transporter MCT4 as a viral replication resistance factor. By extending this study to dengue virus, we show that flaviviruses have both overlapping and unique interaction strategies with host cells. This study provides a comprehensive molecular portrait of WNV-human cell interactions that forms a model for understanding single plus-stranded RNA virus infection, and reveals potential antiviral targets.

Construction and evaluation of a chimeric pseudoinfectious virus vaccine to prevent Japanese encephalitis

Multiple vaccines exist to control Japanese encephalitis (JE), but all suffer from problems. We have developed a new type of flavivirus vaccine, a pseudoinfectious virus (RepliVAX WN) that prevents West Nile virus (WNV)-induced disease. Here, we describe production of a chimeric RepliVAX (RepliVAX JE) that expresses the JE virus (JEV) prM and E proteins. Our prototype RepliVAX JE replicated poorly in cells, but blind passage produced a better-growing derivative, and analyses of this derivative allowed us to engineer a second-generation RepliVAX (RepliVAX JE.2) that grew to high titers. RepliVAX JE.2 elicited neutralizing antibodies in both mice and hamsters and provided 100% protection from a lethal challenge with JEV or WNV, respectively. These results demonstrate the utility our RepliVAX platform for producing a JE vaccine.

Non-specific antiviral response detected in RNA-treated cultured cells of the sandfly, Lutzomyia longipalpis

Lutzomyia longipalpis is the principal vector of visceral leishmaniasis in the Americas, and can also transmit some viruses. To help develop a gene-silencing system for this sandfly, we transfected cultured embryonic cells with various double-stranded RNAs using West Nile virus (WNV) virus-like particles (VLPs) expressing luciferase as the target RNA to demonstrate effective gene knock-down. When luciferase dsRNA was introduced into these cells, they produced the expected reduction in VLP-encoded luciferase, suggesting specific silencing of the luciferase gene. Surprisingly, we found that unrelated dsRNAs, which included those specific for several L. longipalpis gene sequences and Escherichia coli beta-galactosidase, diminished replication of the VLP-encoded genome. These results are the first indication for a nucleic acid-induced, non-specific antiviral response in this important insect vector.

Persistent infections of mammals and mammalian cell cultures with West Nile virus

Before 1990, West Nile virus (WNV) was considered to be one of many arthropod-borne viruses that caused mild febrile illness in man. However, in the 1990s, the virus was associated with severe CNS disease that produced mortality in horses and man in Europe. In 1999, WNV was identified as the etiologic agent of an outbreak of human and avian encephalitis in New York City (NY, USA). Like many other Flaviviridae family members, WNV is generally considered to cause acute infections, however, persistent WNV infections have been observed in laboratory-infected animals and in human patients. These persistent infections could be facilitated by changes to the viral genome that allow the virus to evade detection by the host cell, a property that has been studied in cell culture. This review highlights our current knowledge of persistent WNV infections in vitro and in vivo, and speculates on how persistence could influence virus transmission.

Differential activation of human monocyte-derived and plasmacytoid dendritic cells by West Nile virus generated in different host cells

Dendritic cells (DCs) play a central role in innate immunity and antiviral responses. In this study, we investigated the production of alpha interferon (IFN-alpha) and inducible chemokines by human monocyte-derived dendritic cells (mDCs) and plasmacytoid dendritic cells (pDCs) infected with West Nile virus (WNV), an emergent pathogen whose infection can lead to severe cases of encephalitis in the elderly, children, and immunocompromised individuals. Our experiments demonstrated that WNV grown in mammalian cells (WNV(Vero)) was a potent inducer of IFN-alpha secretion in pDCs and, to a lesser degree, in mDCs. The ability of WNV(Vero) to induce IFN-alpha in pDCs did not require viral replication and was prevented by the treatment of cells with bafilomycin A1 and chloroquine, suggesting that it was dependent on endosomal Toll-like receptor recognition. On the other hand, IFN-alpha production in mDCs required viral replication and was associated with the nuclear translocation of IRF3 and viral antigen expression. Strikingly, pDCs failed to produce IFN-alpha when stimulated with WNV grown in mosquito cells (WNV(C7/10)), while mDCs responded similarly to WNV(Vero) or WNV(C7/10). Moreover, the IFN-dependent chemokine IP-10 was produced in substantial amounts by pDCs in response to WNV(Vero) but not WNV(C7/10), while interleukin-8 was produced in greater amounts by mDCs infected with WNV(C7/10) than in those infected with WNV(Vero). These findings suggest that cell-specific mechanisms of WNV recognition leading to the production of type I IFN and inflammatory chemokines by DCs may contribute to both the innate immune response and disease pathogenesis in human infections.

Production of pseudoinfectious yellow fever virus with a two-component genome

Application of genetically modified, deficient-in-replication flaviviruses that are incapable of developing productive, spreading infection is a promising means of designing safe and effective vaccines. Here we describe a two-component genome yellow fever virus (YFV) replication system in which each of the genomes encodes complete sets of nonstructural proteins that form the replication complex but expresses either only capsid or prM/E instead of the entire structural polyprotein. Upon delivery to the same cell, these genomes produce together all of the viral structural proteins, and cells release a combination of virions with both types of genomes packaged into separate particles. In tissue culture, this modified YFV can be further passaged at an escalating scale by using a high multiplicity of infection (MOI). However, at a low MOI, only one of the genomes is delivered into the cells, and infection cannot spread. The replicating prM/E-encoding genome produces extracellular E protein in the form of secreted subviral particles that are known to be an effective immunogen. The presented strategy of developing viruses defective in replication might be applied to other flaviviruses, and these two-component genome viruses can be useful for diagnostic or vaccine applications, including the delivery and expression of heterologous genes. In addition, the achieved separation of the capsid-coding sequence and the cyclization signal in the YFV genome provides a new means for studying the mechanism of the flavivirus packaging process.

West Nile virus-induced interferon production is mediated by the double-stranded RNA-dependent protein kinase PKR

Cells carry a variety of molecules, referred to as pathogen recognition receptors (PRRs), which are able to sense invading pathogens. Interaction of PRRs with viral compounds instigates a signaling pathway(s), resulting in the activation of genes, including those for type I interferon (IFN), which are critical for an effective antiviral response. Here we demonstrate that the double-stranded RNA (dsRNA)-dependent protein kinase PKR, which has been shown to function as a PRR in cells treated with the dsRNA mimetic poly(I:C), serves as a PRR in West Nile virus (WNV)-infected cells. Evidence for PKR's role as a PRR was obtained from both human and murine cells. Using mouse embryonic fibroblasts (MEFs), we demonstrated that PKR gene knockout, posttranscriptional gene silencing of PKR mRNA using small interfering RNA (siRNA), and chemical inhibition of PKR function all interfered with IFN synthesis following WNV infection. In three different human cell lines, siRNA knockdown and chemical inhibition of PKR blocked WNV-induced IFN synthesis. Using the same approaches, we demonstrated that PKR was not necessary for Sendai virus-induced IFN synthesis, suggesting that PKR is particularly important for recognition of WNV infection. Taken together, our data suggest that PKR could serve as a PRR for recognition of WNV infection.

West Nile virus infection activates the unfolded protein response, leading to CHOP induction and apoptosis

West Nile virus (WNV)-mediated neuronal death is a hallmark of WNV meningitis and encephalitis. However, the mechanisms of WNV-induced neuronal damage are not well understood. We investigated WNV neuropathogenesis by using human neuroblastoma cells and primary rat hippocampal neurons. We observed that WNV activates multiple unfolded protein response (UPR) pathways, leading to transcriptional and translational induction of UPR target genes. We evaluated the role of the three major UPR pathways, namely, inositol-requiring enzyme 1-dependent splicing of X box binding protein 1 (XBP1) mRNA, activation of activating transcription factor 6 (ATF6), and protein kinase R-like endoplasmic reticulum (ER) kinase-dependent eukaryotic initiation factor 2alpha (eIF2alpha) phosphorylation, in WNV-infected cells. We show that XBP1 is nonessential or can be replaced by other UPR pathways in WNV replication. ATF6 was rapidly degraded by proteasomes, consistent with induction of ER stress by WNV. We further observed a transient phosphorylation of eIF2alpha and induction of the proapoptotic cyclic AMP response element-binding transcription factor homologous protein (CHOP). WNV-infected cells exhibited a number of apoptotic phenotypes, such as (i) induction of growth arrest and DNA damage-inducible gene 34, (ii) activation of caspase-3, and (iii) cleavage of poly(ADP-ribose) polymerase. The expression of WNV nonstructural proteins alone was sufficient to induce CHOP expression. Importantly, WNV grew to significantly higher viral titers in chop(-)(/)(-) mouse embryonic fibroblasts (MEFs) than in wild-type MEFs, suggesting that CHOP-dependent premature cell death represents a host defense mechanism to limit viral replication that might also be responsible for the widespread neuronal loss observed in WNV-infected neuronal tissue.

Early production of type I interferon during West Nile virus infection: role for lymphoid tissues in IRF3-independent interferon production

Infection of cells with flaviviruses in vitro is reduced by pretreatment with small amounts of type I interferon (IFN-alpha/beta). Similarly, pretreatment of animals with IFN and experiments using mice defective in IFN signaling have indicated a role for IFN in controlling flavivirus disease in vivo. These data, along with findings that flavivirus-infected cells block IFN signaling, suggest that flavivirus infection can trigger an IFN response. To investigate IFN gene induction by the very first cells infected during in vivo infection with the flavivirus West Nile virus (WNV), we infected mice with high-titer preparations of WNV virus-like particles (VLPs), which initiate viral genome replication in cells but fail to spread. These studies demonstrated a brisk production of IFN in vivo, with peak levels of over 1,000 units/ml detected in sera between 8 and 24 h after inoculation by either the intraperitoneal or footpad route. The IFN response was dependent on genome replication, and WNV genomes and WNV antigen-positive cells were readily detected in the popliteal lymph nodes (pLN) of VLP-inoculated mice. High levels of IFN mRNA transcripts and functional IFN were also produced in VLP-inoculated IFN regulatory factor 3 null (IRF3(-/-)) mice, indicating that IFN production was independent of the IRF3 pathways to IFN gene transcription, consistent with the IFN type produced (predominantly alpha).

Mutations in West Nile virus nonstructural proteins that facilitate replicon persistence in vitro attenuate virus replication in vitro and in vivo

West Nile virus (WNV) infections in vertebrates are generally acute but persistent infections have been observed. To investigate the ability of WNV to produce persistent infections, we forced subgenomic WNV replicons to replicate within a cell without causing cell death. Detailed analyses of these cell-adapted genomes revealed mutations within the nonstructural protein genes NS2A (D73H, M108K), NS3 (117Kins), NS4B (E249G) and NS5 (P528H). WNV replicons and WNVs harboring a subset of NS2A or NS3 mutations showed a reduction in genome replication, a reduction in antigen accumulation, a decrease in cytopathic effect, an increased ability to persist in cell culture and/or attenuation in vivo. Taken together, these data indicate that WNV with a defect in replication and an increased potential to persist within the host cell can be generated by point mutations at multiple independent loci, suggesting that persistent viruses could arise in nature.

Construction of an infectious cDNA clone for a Brazilian prototype strain of dengue virus type 1: characterization of a temperature-sensitive mutation in NS1

To help understand the mechanism of pathogenesis of dengue virus (DV), we set out to create an infectious cDNA of the Brazilian prototype strain of DV serotype 1 (DV1-BR/90). PCR-amplified fragments of DV1-BR/90 cDNA were readily assembled into a subgenomic cDNA that could be used to produce replicating RNAs (replicons), lacking the structural protein-encoding regions of the genome. However, assembly of a cDNA capable of producing infectious virus was only possible using a bacterial artificial chromosome plasmid, indicating that DV1 sequences were especially difficult to propagate in E. coli. While characterizing our cDNA we discovered a fortuitous temperature-sensitive mutation in the NS1 encoding region. Using our infectious cDNA and a renilla luciferase-expressing replicon we were able to demonstrate that this mutation produced a defect in RNA replication at 37 degrees C, demonstrating that the DV1 NS1 protein plays an essential role in RNA replication.

Salivary gland morphology and virus transmission during long-term cytopathologic West Nile virus infection in Culex mosquitoes

The effect of long-term West Nile virus (WNV) infection on Culex salivary gland morphology and viability was evaluated by transmission electron microscopy during a four week period post-blood feeding. These studies showed that apoptosis and other cytopathologic changes occurred more frequently in WNV-infected mosquitoes compared with uninfected controls. The effect of long-term infection on WNV transmission was evaluated by titering virus in saliva over the same time period. Although the mean titer of WNV in mosquito saliva did not change significantly over time, the percentage of saliva samples containing WNV decreased. Because of the importance of saliva in blood meal acquisition and virus delivery, salivary gland pathology has the potential to affect mosquito feeding behavior and virus transmission. Results from this study add to a growing body of evidence that arbovirus infections in mosquito vectors can be cytopathic, and offer a potential mechanism for virus-induced cell death in mosquitoes.

Resistance to alpha/beta interferon is a determinant of West Nile virus replication fitness and virulence

The emergence of West Nile virus (WNV) in the Western Hemisphere is marked by the spread of pathogenic lineage I strains, which differ from typically avirulent lineage II strains. To begin to understand the virus-host interactions that may influence the phenotypic properties of divergent lineage I and II viruses, we compared the genetic, pathogenic, and alpha/beta interferon (IFN-alpha/beta)-regulatory properties of a lineage II isolate from Madagascar (MAD78) with those of a new lineage I isolate from Texas (TX02). Full genome sequence analysis revealed that MAD78 clustered, albeit distantly, with other lineage II strains, while TX02 clustered with emergent North American isolates, more specifically with other Texas strains. Compared to TX02, MAD78 replicated at low levels in cultured human cells, was highly sensitive to the antiviral actions of IFN in vitro, and demonstrated a completely avirulent phenotype in wild-type mice. In contrast to TX02 and other pathogenic forms of WNV, MAD78 was defective in its ability to disrupt IFN-induced JAK-STAT signaling, including the activation of Tyk2 and downstream phosphorylation and nuclear translocation of STAT1 and STAT2. However, replication of MAD78 was rescued in cells with a nonfunctional IFN-alpha/beta receptor (IFNAR). Consistent with this finding, the virulence of MAD78 was unmasked upon infection of mice lacking IFNAR. Thus, control of the innate host response and IFN actions is a key feature of WNV pathogenesis and replication fitness.

Potential of ancestral sylvatic dengue-2 viruses to re-emerge

Dengue viruses (DENV) are the most important arboviral pathogens in tropical and subtropical regions throughout the world. DENV transmission includes both a sylvatic, enzootic cycle between nonhuman primates and arboreal mosquitoes of the genus Aedes, and an urban, endemic/epidemic cycle between Aedes aegypti, a mosquito with larval development in peridomestic water containers, and human reservoir hosts. All 4 serotypes of endemic DENV evolved independently from ancestral sylvatic viruses and have become both ecologically and evolutionarily distinct; this process may have involved adaptation to (i) peridomestic mosquito vectors and/or (ii) human reservoir hosts. To test the latter hypothesis, we assessed the ability of sylvatic and endemic DENV-2 strains, representing major genotypes from Southeast Asia, West Africa and the Americas, to replicate in two surrogate human model hosts: monocyte-derived, human dendritic cells (moDCs), and mice engrafted with human hepatoma cells. Although the various DENV-2 strains showed significant inter-strain variation in mean replication titers in both models, no overall difference between sylvatic and endemic strains was detected in either model. Our findings suggest that emergence of endemic DENV strains from ancestral sylvatic strains may not have required adaptation to replicate more efficiently in human reservoir hosts, implying that the potential for re-emergence of sylvatic dengue strains into the endemic cycle is high. The shared replication profiles of the American endemic and sylvatic strains suggest that American strains have maintained or regained the ancestral phenotype.

Production and characterization of vaccines based on flaviviruses defective in replication

To develop new vaccine candidates for flavivirus infections, we have engineered two flaviviruses, yellow fever virus (YFV) and West Nile virus (WNV), that are deficient in replication. These defective pseudoinfectious viruses (PIVs) lack a functional copy of the capsid (C) gene in their genomes and are incapable of causing spreading infection upon infection of cells both in vivo and in vitro. However, they produce extracellular E protein in form of secreted subviral particles (SVPs) that are known to be an effective immunogen. PIVs can be efficiently propagated in trans-complementing cell lines making high levels of C or all three viral structural proteins. PIVs derived from YFV and WNV, demonstrated very high safety and immunization produced high levels of neutralizing antibodies and protective immune response. Such defective flaviviruses can be produced in large scale under low biocontainment conditions and should be useful for diagnostic or vaccine applications.

Evaluation of replicative capacity and genetic stability of West Nile virus replicons using highly efficient packaging cell lines

A stable cell system for high-efficiency packaging of West Nile virus (WNV) subgenomic replicons into virus-like particles (VLPs) was developed. VLPs could be propagated on these packaging cells and produced infectious foci similar to foci produced by WNV. Focus size correlated with the replicative capacity of WNV replicons, indicating that genome copy number, rather than amount of trans-complementing structural proteins, was rate-limiting in packaging of VLPs. Comparison of VLP production from replicon genomes encoding partial or complete C genes indicated that portions of C downstream of the cyclization sequence could improve genome replication or that cis expression of C could enhance packaging. Interestingly, a rapid loss of replicon-encoded reporter gene activity was detected within two serial passages of reporter gene-containing VLPs. The loss of reporter activity correlated with gene deletion and better VLP growth, indicating a powerful selection pressure for WNV genomes lacking reporter genes.

West Nile virus replication interferes with both poly(I:C)-induced interferon gene transcription and response to interferon treatment

West Nile virus (WNV), the leading cause of viral encephalitis in the United States, is an arthropod-transmitted member of the family Flaviviridae. We have explored the interaction of this positive-strand RNA virus with signaling pathways involved in induction of the host's innate immune response. Phosphorylation of STAT-1 in response to interferon (IFN) treatment and the ability of IFN to establish an antiviral state were reduced in WNV replicon-bearing cell lines. Similarly, the activation of IRF3 and stimulation of IFN-beta transcription in response to the double-stranded RNA (dsRNA) mimetic poly(I:C) were inhibited in replicon-bearing and WNV-infected HeLa cells. In contrast, WNV replicons did not affect IRF3 activation by Sendai virus infection, suggesting that not all IRF3 activating pathways are inhibited by WNV. Taken together, these findings demonstrate that WNV replication in cultured cells interferes with both the response to IFN and synthesis of IFN-beta in response to dsRNA.

Adaptation of West Nile virus replicons to cells in culture and use of replicon-bearing cells to probe antiviral action

Flaviviruses are emerging threats to public health worldwide. Recently, one flavivirus, West Nile virus (WNV), has caused the largest epidemic of viral encephalitis in US history. Like other flaviviruses, WNV is thought to cause a persistent infection in insect cells, but an acute cytopathic infection of mammalian cells. To study adaptation of WNV to persistently replicate in cell culture and generate a system capable of detecting antiviral compounds in the absence of live virus, we generated subgenomic replicons of WNV and adapted these to persistently replicate in mammalian cells. Here we report that adaptation of these replicons to cell culture results in a reduction of genome copy number, and demonstrate that hamster, monkey, and human cells that stably carry the replicons can be used as surrogates to detect the activity of anti-WNV compounds. Additionally, we have used these cells to investigate the interaction of WNV genomes with interferon (IFN). These studies demonstrated that IFN can cure cells of replicons and that replicon-bearing cells display lower responses to IFN than their IFN-cured derivatives.

trans-Packaged West Nile virus-like particles: infectious properties in vitro and in infected mosquito vectors

A trans-packaging system for West Nile virus (WNV) subgenomic replicon RNAs (repRNAs), deleted for the structural coding region, was developed. WNV repRNAs were efficiently encapsidated by the WNV C/prM/E structural proteins expressed in trans from replication-competent, noncytopathic Sindbis virus-derived RNAs. Infectious virus-like particles (VLPs) were produced in titers of up to 10(9) infectious units/ml. WNV VLPs established a single round of infection in a variety of different cell lines without production of progeny virions. The infectious properties of WNV and VLPs were indistinguishable when efficiencies of infection of a number of different cell lines and inhibition of infection by neutralizing antibodies were determined. To investigate the usefulness of VLPs to address biological questions in vivo, Culex pipiens quinquefasciatus mosquitoes were orally and parenterally infected with VLPs, and dissected tissues were analyzed for WNV antigen expression. Antigen-positive cells in midguts of orally infected mosquitoes were detected as early as 2 days postinfection and as late as 8 days. Intrathoracic inoculation of VLPs into mosquitoes demonstrated a dose-dependent pattern of infection of secondary tissues and identified fat body, salivary glands, tracheal cells, and midgut muscle as susceptible WNV VLP infection targets. These results demonstrate that VLPs can serve as a valuable tool for the investigation of tissue tropism during the early stages of infection, where virus spread and the need for biosafety level 3 containment complicate the use of wild-type virus.

Procedures for preventing transmission of foot-and-mouth disease virus (O/TAW/97) by people

The aim of this study was to determine personal hygiene protocols and animal avoidance periods needed to prevent transmission of FMDV (O/TAW/97). Forty-six, 9-week-old barrows free of FMDV were randomly allocated to five treatment groups and a control group. Investigators contacted and sampled FMDV-inoculated pigs for approximately 40 min and then contacted and sampled sentinel pigs after using no biosecurity procedures, washing hands and donning clean outerwear, or showering and donning clean outerwear. Personnel were sampled for nasal carriage of FMDV for 85.43 h. Contaminated personnel did not transmit FMDV to susceptible pigs after handwashing or showering, and donning clean outerwear. FMDV was transmitted when biosecurity procedures were not used. FMDV was not detected in nasal secretions of investigators. Thus, extended animal avoidance periods do not appear to be necessary to prevent transmission of FMDV (O/TAW/97) by people to pigs when organic material is removed through handwashing/showering and donning clean outerwear. This study supports similar findings in a previous publication using FMDV (O/UK/35/2001).

West Nile virus: where are we now?

Since the publication of a comprehensive review on West Nile virus (WNV) in 2002, there has been substantial progress in understanding of transmission, epidemiology, and geographic distribution of the virus and manifestations of disease produced by the infection. There have also been advances in development of diagnostic and therapeutic agents and vaccines. Nevertheless, many questions about the epidemic remain unanswered, and several new issues have arisen--for example: whether the epidemic will increase as the virus spreads to the Pacific coast of North America; whether arthropods other than mosquitoes will act as vectors for the infection; whether WNV will spread to South America and cause an epidemic there; whether the distribution of WNV in Asia and Europe will increase; and whether adaptation of WNV to new ecosystems will produce viruses with altered genetic and phenotypic properties. This review aims to provide an update on knowledge of WNV biology that can be used to highlight the advances in the field during the past 2 years and help to define the questions that academic, industrial, and public-health communities must address in development of measures to control WNV disease.

Studies of genetically defined chimeras of a European type A virus and a South African Territories type 2 virus reveal growth determinants for foot-and-mouth disease virus

The three South African Territories (SAT) types of foot-and-mouth disease virus (FMDV) display great genetic and antigenic diversity, resulting from the independent evolution of these viruses in different geographical localities. For effective control of the disease in such areas, the use of custom-made vaccines is required. To circumvent the tedious process of vaccine strain selection, an alternative in the control process is being investigated. Specifically, it is proposed to replace the antigenic determinants of an infectious genome-length cDNA copy of a good SAT vaccine strain with those of appropriate field strains, producing custom-made FMDV chimeras for use in vaccine production. Here the construction of an infectious genome-length cDNA copy of the SAT2 vaccine strain, ZIM/7/83, is described, created utilizing an exchange-cassette strategy with an existing A(12) genome-length cDNA clone. The virus derived from this cDNA (designated vSAT2) displayed excellent growth properties in cell culture, indicating its potential usefulness in the production of custom-made vaccine strains. Evaluation of the growth of various SAT2/A12 chimeras created during the derivation of SAT2 infectious cDNA suggested incompatibilities between the non-structural proteins of ZIM/7/83 and the 5' UTR of A(12).