Mechanisms of GII.4 norovirus persistence in human populations

Genetic analysis of the capsid region of norovirus GII.4 variants isolated in South Korea

Norovirus is one of the major causes of non-bacterial gastroenteritis in humans. The aim of this study was to analyze the amino acid variation of open reading frame 2 of GII.4 variants in South Korea during the period from November 2006 to December 2012. Sixty-nine complete nucleotide sequences of open reading frame 2 were obtained from 113 GII.4 strains. The GII.4 2006b variants were detected predominantly between 2006 and 2009; however, new GII.4 variants, which were termed the 2010 variant and the 2012 variant, emerged in 2010 and 2012, respectively. The number of GII.4 2006b variants steadily decreased until 2012, whereas the number of gastroenteritis cases caused by the new variants increased between 2010 and 2012. The amino acid sequence in the ORF2 region obtained in this study was compared with other GII.4 variants isolated in various countries. Amino acid variations were observed primarily at epitope sites and the surrounding regions. Amino acids 294, 359, 393, and 413 of the P2 subdomain were the most variable sites among the GII.4 variants. The information in this study can be useful in basic research to predict the emergence and determine the genetic functions of new GII.4 variants.

The spatial diffusion of norovirus epidemics over three seasons in Tokyo

We studied the spatial trend of norovirus (NoV) epidemics using sentinel gastroenteritis surveillance data for patients aged <15 years (n = 140) in the Tokyo area for the 2006–2007 to 2008–2009 seasons utilizing the kriging method of geographical information system (GIS). This is the first study of the spreading pattern of NoV epidemics using sentinel surveillance data. Correlations of sentinel cases between the seasons and with demographic data were examined to identify the trend and related factors. A similar pattern of diffusion was observed over the seasons, and its mean correlation between seasons was significantly high. A higher number of cases were found in the peripheral area, which surrounds the most populated central area, and showed a correlation with the ratio of the children population (r = 0·321, P < 0·01) and the ratio of residents in larger families (r = 0·263, P < 0·01). While NoV susceptibility remained, the results suggest a transmission route in the local community as a possible epidemic factor. Prevention with focus on the peripheral area is desirable.

Norovirus GII.4 detection in environmental samples from patient rooms during nosocomial outbreaks

Norovirus (NoV) is an important cause of nosocomial gastroenteric outbreaks. This 5-month study was designed to characterize NoV contamination and airborne dispersal in patient rooms during hospital outbreaks. Air vents, overbed tables, washbasins, dust, and virus traps designed to collect charged particles from the air were swabbed to investigate the possibility of NoV contamination in patient rooms during outbreaks in seven wards and in an outbreak-free ward. Symptomatic inpatients were also sampled. Nucleic acid extracts of the samples were examined for NoV RNA using genogroup I (GI) and GII real-time reverse transcription-PCR (RT-PCR). The NoV strains were characterized by RT-PCR, sequencing, and phylogenetic analysis of the RNA-dependent RNA-polymerase-N/S capsid-coding region (1,040 nucleotides [nt]). Patient strains from two outbreaks in one ward were sequenced across the RNA-dependent-RNA-polymerase major capsid-coding region (2.5 kb), including the hypervariable P2 domain. In the outbreak wards, NoV GII was detected in 48 of 101 (47%) environmental swabs and 63 of 108 patients (58%); NoV genotype II.4 was sequenced from 18 environmental samples, dust (n = 8), virus traps (n = 4), surfaces (n = 6), and 56 patients. In contrast, NoV GII was detected in 2 (GII.4) of 28 (7%) environmental samples and in 2 (GII.6 and GII.4) of 17 patients in the outbreak-free ward. Sequence analyses revealed a high degree of similarity (>99.5%, 1,040 nt) between NoV GII.4 environmental and patient strains from a given ward at a given time. The strains clustered on 11 subbranches of the phylogenetic tree, with strong correlations to time and place. The high nucleotide similarity between the NoV GII.4 strains from patients and their hospital room environment provided molecular evidence of GII.4 dispersal in the air and dust; therefore, interventional cleaning studies are justified.

Molecular epidemiology of norovirus from patients with acute gastroenteritis in northwestern Spain

The high incidence of norovirus (NoV) infections seems to be related to the emergence of new variants that evolved by genetic drift of the capsid gene. In this work, that represents a first effort to describe the molecular epidemiology of NoV in the northwest of Spain, a total of eight different NoV genotypes (GII.1, GII.3, GII.4, GII.6, GII.7, GII.12, GII.13, GII.14) were detected. The major genotypes observed were GII.4 (45·42%) and GII.14 (34·9%), being detected in all age groups. In addition, and although most of GII.4 sequences belonged to 2006b (7·2%) and 2010 (50·35%) variants, the presence of new NoV variants was observed. Phylogenetic analysis revealed that a high number of GII.4 sequences (35·24%) could be assigned to the newly emerging Sydney 2012 variant, even during late 2010. The high prevalence of NoV GII.14 observed in this study may indicate the emergence of this genotype in Spain.

Chimeric GII.4 norovirus virus-like-particle-based vaccines induce broadly blocking immune responses

There is currently no licensed vaccine for noroviruses, and development is hindered, in part, by an incomplete understanding of the host adaptive immune response to these highly heterogeneous viruses and rapid GII.4 norovirus molecular evolution. Emergence of a new predominant GII.4 norovirus strain occurs every 2 to 4 years. To address the problem of GII.4 antigenic variation, we tested the hypothesis that chimeric virus-like particle (VLP)-based vaccine platforms, which incorporate antigenic determinants from multiple strains into a single genetic background, will elicit a broader immune response against contemporary and emergent strains. Here, we compare the immune response generated by chimeric VLPs to that of parental strains and a multivalent VLP cocktail. Results demonstrate that chimeric VLPs induce a more broadly cross-blocking immune response than single parental VLPs and a similar response to a multivalent GII.4 VLP cocktail. Furthermore, we show that incorporating epitope site A alone from one strain into the background of another is sufficient to induce a blockade response against the strain donating epitope site A. This suggests a mechanism by which population-wide surveillance of mutations in a single epitope could be used to evaluate antigenic changes in order to identify potential emergent strains and quickly reformulate vaccines against future epidemic strains as they emerge in human populations.

IMPORTANCE

Noroviruses are gastrointestinal pathogens that infect an estimated 21 million people per year in the United States alone. GII.4 noroviruses account for >70% of all outbreaks, making them the most clinically important genotype. GII.4 noroviruses undergo a pattern of epochal evolution, resulting in the emergence of new strains with altered antigenicity over time, complicating vaccine design. This work is relevant to norovirus vaccine design as it demonstrates the potential for development of a chimeric VLP-based vaccine platform that may broaden the protective response against multiple GII.4 strains and proposes a potential reformulation strategy to control newly emergent strains in the human population.

Temporal dynamics of norovirus GII.4 variants in Brazil between 2004 and 2012

Noroviruses (NoVs) are the major cause of acute gastroenteritis outbreaks, and, despite a wide genetic diversity, genotype II.4 is the most prevalent strain worldwide. Mutations and homologous recombination have been proposed as mechanisms driving the epochal evolution of the GII.4, with the emergence of new variants in 1–3-year intervals causing global epidemics. There are no data reporting the dynamics of GII.4 variants along a specific period in Brazil. Therefore, to improve the understanding of the comportment of these variants in the country, the aim of this study was to evaluate the circulation of NoV GII.4 variants during a 9-year period in 3 out of 5 Brazilian regions. A total of 147 samples were sequenced, and a phylogenetic analysis of subdomain P2 demonstrated the circulation of six GII.4 variants, Asia_2003, Hunter_2004, Den Haag_2006b, Yerseke_2006a, New Orleans_2009, and Sydney_2012, during this period. The most prevalent variant was Den Haag_2006b, circulating in different Brazilian regions from 2006 to 2011. A Bayesian coalescent analysis was used to calculate the mean evolutionary rate of subdomain P2 as 7.3×10⁻³ (5.85×10⁻³–8.82×10⁻³) subst./site/year. These analyses also demonstrated that clade Den Haag_2006b experienced a rapid expansion in 2005 and another in 2008 after a period of decay. The evaluation of the temporal dynamics of NoV GII.4 in Brazil revealed a similar pattern, with few exceptions, to the worldwide observation. These data highlight the importance of surveillance for monitoring the emergence of new strains of NoV GII.4 and its impact on cases of acute gastroenteritis.

Structural analysis of determinants of histo-blood group antigen binding specificity in genogroup I noroviruses

Human noroviruses (NoVs) cause acute epidemic gastroenteritis. Susceptibility to the majority of NoV infections is determined by genetically controlled secretor-dependent expression of histo-blood group antigens (HBGAs), which are also critical for NoV attachment to host cells. Human NoVs are classified into two major genogroups (genogroup I [GI] and GII), with each genogroup further divided into several genotypes. GII NoVs are more prevalent and exhibit periodic emergence of new variants, suggested to be driven by altered HBGA binding specificities and antigenic drift. Recent epidemiological studies show increased activity among GI NoVs, with some members showing the ability to bind nonsecretor HBGAs. NoVs bind HBGAs through the protruding (P) domain of the major capsid protein VP1. GI NoVs, similar to GII, exhibit significant sequence variations in the P domain; it is unclear how these variations affect HBGA binding specificities. To understand the determinants of possible strain-specific HBGA binding among GI NoVs, we determined the structure of the P domain of a GI.7 clinical isolate and compared it to the previously determined P domain structures of GI.1 and GI.2 strains. Our crystallographic studies revealed significant structural differences, particularly in the loop regions of the GI.7 P domain, altering its surface topography and electrostatic landscape and potentially indicating antigenic variation. The GI.7 strain bound to H- and A-type, Lewis secretor, and Lewis nonsecretor families of HBGAs, allowing us to further elucidate the structural determinants of nonsecretor HBGA binding among GI NoVs and to infer several contrasting and generalizable features of HBGA binding in the GI NoVs.

IMPORTANCE

Human noroviruses (NoVs) cause acute epidemic gastroenteritis. Recent epidemiological studies have shown increased prevalence of genogroup I (GI) NoVs. Although secretor-positive status is strongly correlated with NoV infection, cases of NoV infection associated with secretor-negative individuals are reported. Biochemical studies have shown that GI NoVs exhibit genotype-dependent binding to nonsecretor histo-blood group antigens (HBGAs). From our crystallographic studies of a GI.7 NoV, in comparison with previous studies on GI.1 and GI.2 NoVs, we show that genotypic differences translate to extensive structural changes in the loop regions that significantly alter the surface topography and electrostatic landscape of the P domain; these features may be indicative of antigenic variations contributing to serotypic differentiation in GI NoVs and also differential modulation of the HBGA binding characteristics. A significant finding is that the threshold length and the structure of one of the loops are critical determinants in the binding of GI NoVs to nonsecretor HBGAs.

Within-host evolution results in antigenically distinct GII.4 noroviruses

Genogroup II, genotype 4 (GII.4) noroviruses are known to rapidly evolve, with the emergence of a new primary strain every 2 to 4 years as herd immunity to the previously circulating strain is overcome. Because viral genetic diversity is higher in chronic than in acute infection, chronically infected immunocompromised people have been hypothesized to be a potential source for new epidemic GII.4 strains. However, while some capsid protein residues are under positive selection and undergo patterned changes in sequence variation over time, the relationships between genetic variation and antigenic variation remains unknown. Based on previously published GII.4 strains from a chronically infected individual, we synthetically reconstructed virus-like particles (VLPs) representing early and late isolates from a small-bowel transplant patient chronically infected with norovirus, as well as the parental GII.4-2006b strain. We demonstrate that intrahost GII.4 evolution results in the emergence of antigenically distinct strains over time, comparable to the variation noted between the chronologically predominant GII.4 strains GII.4-2006b and GII.4-2009. Our data suggest that in some individuals the evolution that occurs during a chronic norovirus infection overlaps with changing antigenic epitopes that are associated with successive outbreak strains and may select for isolates that are potentially able to escape herd immunity from earlier isolates.

IMPORTANCE

Noroviruses are agents of gastrointestinal illness, infecting an estimated 21 million people per year in the United States alone. In healthy individuals, symptomatic infection typically resolves within 24 to 48 h. However, symptoms may persist for years in immunocompromised individuals, and development of successful treatments for these patients is a continuing challenge. This work is relevant to the design of successful norovirus therapeutics for chronically infected patients; provides support for previous assertions that chronically infected individuals may serve as reservoirs for new, antigenically unique emergent strains; and furthers our understanding of genogroup II, genotype 4 (GII.4) norovirus immune-driven molecular evolution.

Comparison of human saliva and synthetic histo-blood group antigens usage as ligands in norovirus-like particle binding and blocking assays

Blocking of norovirus-like particle binding to their cellular ligands, histo-blood group antigens with immune sera, is considered a surrogate norovirus neutralization assay. We compared human secretor positive saliva and synthetic biotinylated carbohydrates as a source of histo-blood group antigens in binding and blocking assays. Six norovirus capsid-derived virus-like particles belonging to genogroup I (GI-1-2001 and GI-3-2002) and genogroup II (GII-4-1999, GII-4-2010 New Orleans, GII-4-2012 Sydney and GII-12-1998) noroviruses were produced by a recombinant baculovirus expression system and binding profile to saliva type A, B and O and to synthetic antigens (A trimer, B trimer, H type 1, H type 3, Lewis(a) and Lewis(b)) was identified. Good correlation between virus-like particle binding to saliva type A and synthetic A trimer (r = 0.66, p < 0.05) and saliva type B and synthetic B trimer (r = 0.75, p < 0.05) was observed. Binding of each norovirus virus-like particle to the selected histo-blood group antigens was blocked by convalescent sera from NoV-infected subjects or type-specific mouse antisera. Our results support the use of either saliva or synthetic antigens in blocking assay to measure the ability of norovirus antisera to block virus-like particle binding to the carbohydrate ligands.

Histo-blood group antigens: a common niche for norovirus and rotavirus

Noroviruses (NoVs) and rotaviruses (RVs), the two most important causes of viral acute gastroenteritis, are found to recognise histo-blood group antigens (HBGAs) as receptors or ligands for attachment. Human HBGAs are highly polymorphic containing ABO, secretor and Lewis antigens. In addition, both NoVs and RVs are highly diverse in how they recognise these HBGAs. Structural analysis of the HBGA-binding interfaces of NoVs revealed a conserved central binding pocket (CBP) interacting with a common major binding saccharide (MaBS) of HBGAs and a variable surrounding region interacting with additional minor binding saccharides. The conserved CBP indicates a strong selection of NoVs by the host HBGAs, whereas the variable surrounding region explains the diverse recognition patterns of different HBGAs by NoVs and RVs as functional adaptations of the viruses to human HBGAs. Diverse recognition of HBGAs has also been found in bacterial pathogen Helicobacter pylori. Thus, exploratory research into whether such diverse recognitions also occur for other viral and bacterial pathogens that recognise HBGAs is warranted.

Detection and molecular characterisation of noroviruses in hospitalised children in Malawi, 1997-2007

Despite the increasing recognition of noroviruses as major pathogens associated with community-acquired diarrhoea in children, there are few studies from Africa. Long-term surveillance studies of rotavirus gastroenteritis in Malawian children have provided an opportunity to undertake a study of the importance and epidemiological features of norovirus infection in this population. Faecal specimens were collected from children <5 years of age admitted to hospital with acute diarrhoea, as well as from a comparison group of diarrhoea-free children, in Blantyre, Malawi between 1997 and 2007. Norovirus was detected using real-time PCR and strains genotyped by nucleotide sequence analysis. Norovirus was detected in 220/1,941 (11.3%) faecal specimens, comprising genogroup GI (1.8%), GII (9.4%) and mixed GI/GII (0.1%). The median age of children with norovirus was 6 months (range, 0-48 months). Norovirus was detected throughout the year, with peaks at the end of the rainy season (March) and towards the end of the dry season (August-November). Norovirus GII.4 was the most commonly detected genotype accounting for 70% of strains characterised, followed by GII.2 (6%), GII.6 (4%) and GII.12 (4%). Sub typing of GII.4 noroviruses demonstrated local circulation of strains prior to their subsequent detection in association with global epidemics of gastroenteritis. The prevalence of norovirus in children without diarrhoea was similar to the level in cases. This largest study to date of norovirus infection in African children indicates the potential role of paediatric surveillance in predicting the emergence of norovirus strains with global epidemic potential.

Emergence of the GII-4 Norovirus Sydney2012 strain in England, winter 2012-2013

Norovirus is the commonest cause of acute gastrointestinal disease and is the main aetiological agent of outbreaks of gastroenteritis, particularly in semi-closed environments. Norovirus infections in England typically peak between December and March each year. The most commonly detected norovirus strains belong to the genetically diverse genogroup-II genotype-4 (GII-4) genocluster and in the previous two norovirus winter seasons the majority of GII-4 strains in circulation worldwide have been genetically similar to the GII-4 strain New Orleans 1805/2009/USA. At the beginning of the 2012/13 season a genetically distinct GII-4 strain (Sydney 2012/NSW0514/2012/AU) was described which emerged worldwide during the winter of 2012/13. Here we describe the emergence of norovirus strains genetically related to Sydney2012 in England during the 2012/13 season to replace NewOrleans2009 strains as the most commonly detected variant of GII-4 norovirus in England. Furthermore, we demonstrate that whilst the emergence of Sydney2012 coincided with an early peak in the number of norovirus outbreaks, there was not an overall increase in norovirus activity compared to the previous season. Finally, we show that the Sydney2012 strain is associated with distinct genetic changes compared to the NewOrleans2009 strain, and these changes may have contributed to the emergence of the Sydney2012 strain.

Clinical characteristics and genetic diversity of noroviruses in adults with acute gastroenteritis in Beijing, China in 2008-2009

Norovirus (NoV) infections that cause acute gastroenteritis are commonly observed during colder months. This study was conducted to investigate the clinical features and molecular epidemiology of NoVs in adult outpatients with acute gastroenteritis in Beijing, China from August 2008 to July 2009. Five hundred nineteen patients were enrolled, their stool specimens were collected, and 136 (26.2%) were positive for NoV. The elderly were found to be more susceptible to NoVs than other age groups. The greatest number of gastroenteritis cases associated with occurred in October. Six GI and eleven GII NoV genotypes were isolated; among these, the GII.4 genotype was most prevalent (70/140 and 50% were the 2006b variant). The elderly were more susceptible to the GII.4 genotype than to other genotypes. Greater numbers of neutrophils in the peripheral blood were observed in the NoV infected group than in uninfected control group. However, the levels of neutrophils and leukocytes in the non-GII.4 patients infected with NoV were higher than those of the GII.4-infected patients. The data highlight the role of NoV as a primary agent responsible for gastroenteritis in adults in Beijing, China.

Effects and clinical significance of GII.4 Sydney norovirus, United States, 2012-2013

During 2012, global detection of a new norovirus (NoV) strain, GII.4 Sydney, raised concerns about its potential effect in the United States. We analyzed data from NoV outbreaks in 5 states and emergency department visits for gastrointestinal illness in 1 state during the 2012-13 season and compared the data with those of previous seasons. During August 2012-April 2013, a total of 637 NoV outbreaks were reported compared with 536 and 432 in 2011-2012 and 2010-2011 during the same period. The proportion of outbreaks attributed to GII.4 Sydney increased from 8% in September 2012 to 82% in March 2013. The increase in emergency department visits for gastrointestinal illness during the 2012-13 season was similar to that of previous seasons. GII.4 Sydney has become the predominant US NoV outbreak strain during the 2012-13 season, but its emergence did not cause outbreak activity to substantially increase from that of previous seasons.

Increase in genogroup II.4 norovirus host spectrum by CagA-positive Helicobacter pylori infection

BACKGROUND

Noroviruses (NoVs) represent a considerable public health burden. Despite their enormous genetic diversity, most outbreaks are due to the single GII.4 genotype, but the reasons for this are poorly understood. NoVs use histo-blood group antigens (HBGAs) as attachment factors. Since HBGAs are present in saliva, binding of strains to saliva is commonly used as a surrogate for recognition of the gut surface by specific strains, although the relationship between saliva and gut tissue expression of HBGAs is not well defined.

METHODS

The presence of fucosylated HBGAs in saliva and stomach biopsy specimens, as well as that of genogroup I.1 and genogroup II.4 virus-like particles, were compared in a series of 109 donors from Portugal.

RESULTS

An overall good concordance between HBGA expression in saliva and stomach surface mucosa was observed. However, unexpected mucosal expression of α(1,2)fucosylated epitopes in nonsecretor individuals was frequently detected, allowing for GII.4 attachment. Although all individuals were infected with Helicobacter pylori, abnormal expression of α(1,2)fucosylated motifs and binding of GII.4 virus-like particles in nonsecretors' mucosa were associated with positivity for the H. pylori CagA virulence factor.

CONCLUSIONS

Infection by CagA-positive H. pylori induces expression of GII.4 attachment factors in nonsecretors' mucosa, expanding the host range of these strains and thereby possibly contributing to their epidemiological dominance.

Genetic diversity and distribution of human norovirus in China (1999-2011)

Noroviruses (NoVs) are a leading cause of epidemic and sporadic acute gastroenteritis worldwide. However, the genetic diversity and geographical distribution of NoV isolates from China have not been well described thus far. In this study, all NoV sequences obtained in China from 1999 to 2011 (n = 983), both partial and complete genomes, were downloaded from GenBank. Genotyping and phylogenetic and recombination analyses were performed in order to gain a better understanding of the distribution and genetic diversity of NoVs in China. The results indicated that approximately 90% of NoV sequences were obtained from the coastal regions of China, and most of the NoV sequences from distinct geographical regions appeared to be closely related. GII.4 was the most prevalent genotype, accounting for 64.4% of all genotypes, followed by GII.12 (13.9%) and GII.3 (7.0%). Over the last decade, the GII.4 variants were dominated by successive circulation of GII.4/2002, GII.4/2004, GII.4/2006b, and GII.4/2008, with GII.4/2006b continuing to date. A relatively high frequency of NoV intergenotype recombinants was identified. The most common ORF1/ORF2 intergenotype recombinant was GII.12/GII.4 (n = 11), and the relative frequency was up to 30% among all the recombinant strains (n = 36). These findings may aid in the evaluation and implementation of appropriate measures for monitoring NoV infectious diseases in China.

Analysis of early strains of the norovirus pandemic variant GII.4 Sydney 2012 identifies mutations in adaptive sites of the capsid protein

Global surveillance for norovirus identified in 2012 the emergence of a novel pandemic GII.4 variant, termed Sydney 2012. In Italy, the novel pandemic variant was identified as early as November 2011 but became predominant only in the winter season 2012-2013. Upon sequencing and comparison with strains of global origin, the early Sydney 2012 strains were found to differ from those spreading in 2012-2013 in the capsid (ORF2) putative epitopes B, C and D, segregating into a distinct phylogenetic clade. At least three residues (333, 340 and 393, in epitopes B, C and D, respectively) of the VP1 varied among Sydney 2012 strains of different clades. These findings suggest that the spread of the pandemic variant in Italy during the winter season 2012-2013 was due to the introduction of strains distinct from those circulating at low frequency in the former winter season and that similar strains were also circulating elsewhere worldwide.

Seroepidemiology of norovirus-associated travelers' diarrhea

BACKGROUND

Noroviruses (NoVs) are the most common cause of epidemic gastroenteritis, responsible for at least 50% of all gastroenteritis outbreaks worldwide and were recently identified as a leading cause of travelers' diarrhea (TD) in US and European travelers to Mexico, Guatemala, and India.

METHODS

Serum and diarrheic stool samples were collected from 75 US student travelers to Cuernavaca, Mexico, who developed TD. NoV RNA was detected in acute diarrheic stool samples using reverse transcription-polymerase chain reaction (RT-PCR). Serology assays were performed using GI.1 Norwalk virus (NV) and GII.4 Houston virus (HOV) virus-like particles (VLPs) to measure serum levels of immunoglobulin A (IgA) and IgG by dissociation-enhanced lanthanide fluorescent immunoassay (DELFIA); serum IgM was measured by capture enzyme-linked immunosorbent assay (ELISA), and the 50% antibody-blocking titer (BT50 ) was determined by a carbohydrate-blocking assay.

RESULTS

NoV infection was identified in 12 (16%; 9 GI-NoV and 3 GII-NoV) of 75 travelers by either RT-PCR or fourfold or more rise in antibody titer. Significantly more individuals had detectable preexisting IgA antibodies against HOV (62/75, 83%) than against NV (49/75, 65%) (p = 0.025) VLPs. A significant difference was observed between NV- and HOV-specific preexisting IgA antibody levels (p = 0.0037), IgG (p = 0.003), and BT50 (p = <0.0001). None of the NoV-infected TD travelers had BT50  > 200, a level that has been described previously as a possible correlate of protection.

CONCLUSIONS

We found that GI-NoVs are commonly associated with TD cases identified in US adults traveling to Mexico, and seroprevalence rates and geometric mean antibody levels to a GI-NoV were lower than to a GII-NoV strain.

Epidemiology of human noroviruses and updates on vaccine development

PURPOSE OF REVIEW

Noroviruses (NoVs) are the most common cause of epidemic and sporadic cases of acute gastroenteritis worldwide. This review summarizes recent NoV disease burden estimates, epidemiology findings and provides an update on virus-like particle (VLP) vaccine studies.

RECENT FINDINGS

NoVs are the leading cause of food-borne gastroenteritis and are replacing rotavirus as the predominant gastrointestinal pathogen in pediatric populations. Genogroup II, genotype 4 NoVs (GII.4) remain the dominant genotype worldwide. Increased NoV activity reported in late 2012 was associated with the emergence of a new GII.4 variant called Sydney 2012. New studies suggest that human NoVs can bind a larger range of histoblood group antigens, a susceptibility factor for NoV illness, thus expanding the susceptible population pool for infection. Intranasal immunization with a monovalent GI NoV VLP vaccine showed proof-of-concept efficacy. Studies using intramuscular immunization with a bivalent formulation including GII.4 VLPs are now underway.

SUMMARY

The importance of NoVs as a major gastrointestinal pathogen underscores the need for well tolerated and effective vaccines. Results of VLP vaccine trials appear promising. However, the rapid evolution of NoV genotypes through antigenic drift and changing glycan specificities provide new challenges to epidemiology studies and vaccine trials.

The emergence and evolution of the novel epidemic norovirus GII.4 variant Sydney 2012

Norovirus is the leading cause of acute gastroenteritis with most infections caused by GII.4 variants. To understand the evolutionary processes that contribute to the emergence of GII.4 variants, we examined the molecular epidemiology of norovirus-associated acute gastroenteritis in Australia and New Zealand from 893 outbreaks between 2009 and 2012. Throughout the study GII.4 New Orleans 2009 was predominant; however, during 2012 it was replaced by an emergent GII.4 variant, Sydney 2012. An evolutionary analysis of capsid gene sequences was performed to determine the origins and selective pressures driving the emergence of these recently circulating GII.4 variants. This revealed that both New Orleans 2009 and Sydney 2012 share a common ancestor with GII.4 Apeldoorn 2007. Furthermore, pre-epidemic ancestral variants of each virus were identified up to two years before their pandemic emergence. Adaptive changes at known blockade epitopes in the viral capsid were also identified that likely contributed to their emergence.

Characterization of blockade antibody responses in GII.2.1976 Snow Mountain virus-infected subjects

Snow Mountain virus (GII.2.1976) is the prototype strain of GII.2 noroviruses (NoVs), which cause an estimated 8% of norovirus outbreaks, yet little is known about the immunobiology of these viruses. To define the human immune response induced by SMV infection and the antigenic relationship between different GII.2 strains that have circulated between 1976 and 2010, we developed a panel of four GII.2 variant virus-like particles (VLPs) and compared their antigenicities by enzyme immunoassay (EIA) and surrogate antibody neutralization (blockade) assays. Volunteers infected with GII.2.1976 developed a mean 167-fold increase in blockade response against the homotypic VLP by day 8 postchallenge. Blockade extended cross-genotype activity in some individuals but not cross-genogroup activity. Polyclonal sera from GII.2.1976-infected volunteers blocked GII.2.1976 significantly better than they blocked GII.2.2002, GII.2.2008, and GII.2.2010, suggesting that blockade epitopes within the GII.2 strains have evolved in the past decade. To potentially map these epitope changes, we developed mouse monoclonal antibodies (MAbs) against GII.2.1976 VLPs and compared their reactivities to a panel of norovirus VLPs. One MAb had broad cross-genogroup EIA reactivity to a nonblockade, linear, conserved epitope. Six MAbs recognized conformational epitopes exclusive to the GII.2 strains. Two MAbs recognized GII.2 blockade epitopes, and both blocked the entire panel of GII.2 variants. These data indicate that the GII.2 strains, unlike the predominant GII.4 strains, have undergone only a limited amount of evolution in blockade epitopes between 1976 and 2010 and indicate that the GII.2-protective component of a multivalent norovirus vaccine may not require frequent reformulation.

Emergence of two novel norovirus genotype II.4 variants associated with viral gastroenteritis in China

Noroviruses (NoVs) are the principal cause of epidemic viral gastroenteritis worldwide, including industrialized and developing countries. Eight hundred and fifty sporadic specimens from hospitalized children with acute gastroenteritis and 131 specimens from seven gastroenteritis outbreaks were collected during 2011-2012 in Jiangsu, China. All specimens were tested for the presence of norovirus (NoV) by real time RT-PCR, and in these, 225/850 of sporadic specimens and 76/131 of outbreak specimens were positive. By sequencing, two novel variants termed JS2011/CHN variant and JS2012/CHN variant were found. By complete genome sequencing and phylogenetic analysis confirmed that both JS2011/CHN variant and JS2012/CHN variant shared more than 98% identity with GII.4 New Orleans/2009/USA strain and GII.4 Sydney/2012/AUS. Both of them had mutations in some key sites in nucleotide sequence and amino acid sequence of ORF1-ORF3. Whether two novel variants will cause epidemic of NoV outbreaks in China deserves further attention. A national surveillance network may be needed to identify trends in molecular evolution of NoVs for prevention of future epidemics.

Bacteria, viruses, and parasites in an intermittent stream protected from and exposed to pasturing cattle: prevalence, densities, and quantitative microbial risk assessment

Over 3500 individual water samples, for 131 sampling times, targeting waterborne pathogens/fecal indicator bacteria were collected during a 7-year period from 4 sites along an intermittent stream running through a small livestock pasture system with and without cattle access-to-stream restriction measures. The study assessed the impact of cattle pasturing/riparian zone protection on: pathogen (bacterial, viral, parasite) occurrence, concentrations of fecal indicators, and quantitative microbial risk assessments (QMRA) of the risk of Cryptosporidium, Giardia and Escherichia coli O157:H7 infection in humans. Methodologies were developed to compute QMRA mean risks on the basis of water samples exhibiting potentially human infectious Cryptosporidium and E. coli based on genotyping Crytosporidium, and E. coli O157:H7 presence/absence information paired with enumerated E. coli. All Giardia spp. were considered infectious. No significant pasturing treatment effects were observed among pathogens, with the exception of Campylobacter spp. and E. coli O157:H7. Campylobacter spp. prevalence significantly decreased downstream through pasture treatments and E. coli O157:H7 was observed in a few instances in the middle of the unrestricted pasture. Densities of total coliform, fecal coliform, and E. coli reduced significantly downstream in the restricted pasture system, but not in the unrestricted system. Seasonal and flow conditions were associated with greater indicator bacteria densities, especially in the summer. Norovirus GII was detected at rates of 7-22% of samples for all monitoring sites, and rotavirus in 0-7% of samples for all monitoring sites; pasture treatment trends were not evident, however. Seasonal and stream flow variables (and their interactions) were relatively more important than pasture treatments for initially stratifying pathogen occurrence and higher fecal indicator bacteria densities. Significant positive associations among fecal indicator bacteria and Campylobacter spp. detection were observed. For QMRA, adjusting for the proportion of Cryptosporidium spp. detected that are infectious for humans reduces downstream risk estimates by roughly one order of magnitude. Using QMRA in this manner provides a more refined estimate of beneficial management practice effects on pathogen exposure risks to humans.

Optimization of one-step real-time reverse transcription-polymerase chain reaction assays for norovirus detection and molecular epidemiology of noroviruses in Thailand

Noroviruses (NoVs) are an important human pathogen associated with acute viral gastroenteritis worldwide. NoVs display a significant amount of genetic heterogeneity, making it difficult to develop comprehensive detection assays. In this study, primer sets and probes were designed for a TaqMan(®)-based real-time reverse transcription-polymerase chain reaction (RT-PCR) for norovirus detection purposes. The assay was optimized and utilized as a multiplex real-time RT-PCR assay for genogroup I (GI) detection, and a singleplex real-time RT-PCR assay for genogroup II (GII) detection. The assays showed high specificity for NoV detection and no cross-reactivity was observed between GI and GII. The detection limit of the assay was as low as 10 and 50 RNA copies per reaction for GI and GII, respectively. The optimized protocol was employed to assess the presence of NoV strains in clinical samples collected throughout Thailand during December 2005 to November 2006. The percentage of NoV infections among children with acute gastroenteritis (case) was 23.8% (119/500) and for children without acute gastroenteritis (control) it was 6.8% (30/441). The frequency of NoV infections varied geographically, with the highest frequency observed in the central region and the lowest frequency in the northern region (P>0.0001). Of the 149 positive case and control specimens, GII was found to be the predominant genogroup (98.6%). Partial capsid sequences were successfully obtained from 67 NoV-positive specimens and a phylogenetic analysis was performed to genotype the viral strains. GII.4 was the most common genotype detected.

Identification of immune and viral correlates of norovirus protective immunity through comparative study of intra-cluster norovirus strains

Whether or not primary norovirus infections induce protective immunity has become a controversial issue, potentially confounded by the comparison of data from genetically distinct norovirus strains. Early human volunteer studies performed with a norovirus-positive inoculum initially led to the conclusion that primary infection does not generate long-term, protective immunity. More recently though, the epidemiological pattern of norovirus pandemics has led to the extrapolation that primary norovirus infection induces herd immunity. While these are seemingly discordant observations, they may in fact reflect virus strain-, cluster-, or genogroup-specific differences in protective immunity induction. Here, we report that highly genetically related intra-cluster murine norovirus strains differ dramatically in their ability to induce a protective immune response: Primary MNV-3 infection induced robust and cross-reactive protection, whereas primary MNV-1 infection induced modest homotypic and no heterotypic protection. In addition to this fundamental observation that intra-cluster norovirus strains display remarkable differences in protective immunity induction, we report three additional important observations relevant to norovirus:host interactions. First, antibody and CD4⁺ T cells are essential to controlling secondary norovirus infections. Second, the viral minor structural protein VP2 regulates the maturation of antigen presenting cells and protective immunity induction in a virus strain-specific manner, pointing to a mechanism by which MNV-1 may prevent the stimulation of memory immune responses. Third, VF1-mediated regulation of cytokine induction also correlates with protective immunity induction. Thus, two highly genetically-related norovirus strains displayed striking differences in induction of protective immune responses, strongly suggesting that the interpretation of norovirus immunity and vaccine studies must consider potential virus strain-specific effects. Moreover, we have identified immune (antibody and CD4⁺ T cells) and viral (VP2 and possibly VF1) correlates of norovirus protective immunity. These findings have significant implications for our understanding of norovirus immunity during primary infections as well as the development of new norovirus vaccines.

Human noroviruses are a significant cause of gastroenteritis outbreaks worldwide and likely the leading cause of severe childhood diarrhea. An efficacious norovirus vaccine would have a major impact on human health but will undoubtedly be confounded by several roadblocks. First, the norovirus genus is highly genetically, and potentially antigenically, diverse. Second, it is currently unclear whether human noroviruses elicit lasting protective immunity upon natural infection. Here, we test the hypothesis that noroviruses display virus strain-specific differences in their stimulation of protective immunity. Indeed, our results reveal that two highly genetically related murine norovirus strains differ dramatically in their stimulation of protective immune responses. Moreover, we demonstrate that antibody and CD4⁺ T cells are absolutely essential to protecting from a secondary norovirus infection. Finally, we have revealed two viral correlates of protective immunity induction – VF1-mediated cytokine antagonism and VP2-dependent inhibition of antigen presenting cell maturation. Collectively, this information not only offers a potential explanation for the seemingly discordant results regarding human norovirus protective immunity but it also brings to light a previously unrecognized complexity in developing an efficacious human norovirus vaccine – individual virus strains may differ significantly in their interactions with the host immune system and thus in their immunogenicity.

Histo-blood group antigen-like substances of human enteric bacteria as specific adsorbents for human noroviruses

Histo-blood group antigens (HBGAs) have been suggested to be receptors or coreceptors for human noroviruses (HuNoVs) expressed on the intestinal epithelium. We isolated an enteric bacterium strain (SENG-6), closely related to Enterobacter cloacae, bearing HBGA-like substances from a fecal sample of a healthy individual by using a biopanning technique with anti-HBGA antibodies. The binding capacities of four genotypes of norovirus-like particles (NoVLPs) to Enterobacter sp. SENG-6 cells were confirmed by enzyme-linked immunosorbent assay (ELISA). Transmission electron microscopy demonstrated that NoVLPs bound mainly to extracellular polymeric substances (EPS) of Enterobacter sp. SENG-6, where the HBGA-like substances were localized. EPS that contained HBGA-like substances extracted from Enterobacter sp. SENG-6 was shown by enzyme-linked immunosorbent assay (ELISA) to be capable of binding to NoVLPs of a GI.1 wild-type strain (8fIIa) and a GII.6 strain that can recognize A antigen but not to an NoVLP GI.1 mutant strain (W375A) that loses the ability to bind to A antigen. Enzymatic cleavage of terminal N-acetyl-galactosamine residues in the bacterial EPS weakened bacterial EPS binding to the GI.1 wild-type strain (8fIIa). These results indicate that A-like substances in the bacterial EPS play a key role in binding to NoVLPs. Since the specific binding of HuNoVs to HBGA-positive enteric bacteria is likely to affect the transmission and infection processes of HuNoVs in their hosts and in the environment, further studies of human enteric bacteria and their binding capacity to HuNoVs will provide a new scientific platform for understanding interactions between two types of microbes that were previously regarded as biologically unrelated.

Noroviruses and histo-blood groups: the impact of common host genetic polymorphisms on virus transmission and evolution

Noroviruses (NoVs) are recognized as a leading cause of human gastroenteritis worldwide. Infection occurs following the ingestion of contaminated food or, most often, through direct contact from person to person. However, not all individuals are equally sensitive to these viruses. Indeed, NoVs use glycans of the ABH and Lewis histo-blood group antigen family (HBGAs) as attachment factors. At the epithelial level, the synthesis of these HBGAs requires the action of several glycosyltransferases that are encoded by the ABO, FUT2, and FUT3 genes. The combined polymorphism at these three loci dictates sensitivity to NoV infection because the attachment profile to these glycans varies among strains. Structural analysis of the capsid protein interaction with HBGAs reveals distinct modes of binding for strains of genogroups I and II but high conservation within each genogroup, whereas minor amino acid changes are sufficient to generate modifications of HBGA-binding specificities or affinities. Such modifications therefore induce changes in the spectrum of susceptible individuals. Studies of NoV-HBGA interactions together with phylogenetic analyses and the epidemiologic survey of strains indicate that NoV transmission and evolution depend on both the establishment of herd immunity and the genetic resistance of many individuals, which confers herd innate protection by restricting NoV circulation.

Norovirus virus-like particle vaccines for the prevention of acute gastroenteritis

Noroviruses (NoVs) are the most common cause of nonbacterial acute gastroenteritis in humans worldwide. These highly infectious viruses were, until recently, commonly thought to cause a mild, self-limiting disease in healthy individuals, but increasing epidemiology shows that the incidence and severity of illness due to NoV infection is substantial and similar to diseases where immunization is widely recommended. Human NoV challenge studies have identified carbohydrate histo-blood group antigen expression as an important human susceptibility factor for many strains and correspondingly, that antibodies which block carbohydrate virus binding represent a potential correlate of protection against NoV infection and illness. Since human NoVs do not replicate in cell culture, there are numerous challenges to the development of a vaccine to prevent illness or infection. However, the development of NoV virus-like particles (VLPs) has enabled significant progress toward effective vaccine candidates designed to protect against multiple circulating NoV strains. Vaccination with NoV VLP vaccines has been shown to both induce antibodies that block virus-derived VLP carbohydrate binding and protect against homologous viral challenge in a human clinical study.

Emergence of new pandemic GII.4 Sydney norovirus strain correlates with escape from herd immunity

BACKGROUND

GII.4 noroviruses are a significant source of acute gastroenteritis worldwide, causing the majority of human norovirus outbreaks. Evolution of the GII.4 major capsid protein occurs rapidly, resulting in the emergence of new strains that produce successive waves of pandemic disease. A new pandemic isolate, GII.4 2012 Sydney, largely replaced previously circulating strains in late 2012. We compare the antigenic properties of GII.4 2012 Sydney with previously circulating strains.

METHODS

To determine whether GII.4-2012 Sydney is antigenically different from recently circulating strains GII.4-2006 Minerva and GII.4-2009 New Orleans in previously identified blockade epitopes, we compared reactivity and blockade profiles of GII.4-2006, GII.4-2009, and GII.4-2012 virus-like particles in surrogate neutralization/blockade assays using monoclonal antibodies and human polyclonal sera.

RESULTS

Using monoclonal antibodies that map to known blockade epitopes in GII.4-2006 and GII.4-2009 and human outbreak polyclonal sera, we demonstrate either complete loss or significantly reduced reactivity and blockade of GII.4.2012 compared to GII.4-2006 and GII.4-2009.

CONCLUSIONS

GII.4-2012 Sydney is antigenically different from GII.4-2006 Minerva and GII.4-2009 New Orleans in at least 2 key blockade epitopes. Viral evolution in key potential neutralization epitopes likely allowed GII.4-2012 to escape from human herd immunity and emerge as the new predominant strain.

Genetic and evolutionary perspectives on genogroup III, genotype 2 bovine noroviruses

Bovine noroviruses are enteric pathogens that are detected in stool samples from cattle. Five genogroups are currently described in the genus Norovirus (family Caliciviridae), and within the genogroups, sequences are further divided into genotypes according to genetic homology and phylogenetic relationships. In this study, stool specimens from Belgian cattle were screened by RT-PCR. All of the sequences that were detected were phylogenetically related to genogroup III genotype 2 bovine noroviruses, confirming their higher prevalence in comparison with strains from genotype 1. When other sequences from around the world were introduced, phylogenetic inferences allowed neither the determination of phylogenetic lineages over time nor the deduction of topotypes for genotype 2 bovine noroviruses. Three complete genotype 2 bovine norovirus sequences were also compared genetically (Newbury2/1976 /UK, Dumfries/1994/UK and B309/2003/BE). Interestingly, the genetic divergence of the complete genomes of these three strains was relatively low, but a region of the N-terminal protein encoded by ORF1, the hypervariable region of the capsid gene encoded by ORF2, and a region of the minor structural protein encoded by ORF3 seem to be the most exposed to genetic evolution. Bayesian inference also showed that genetic evolution of genogroup III, genotype 2 bovine noroviruses over a 30-year period seemed to be lower than that already reported for noroviruses from the genotypes 3 and 4 in genogroup II.

Molecular characterization of noroviruses and HBGA from infected Quilombola children in Espirito Santo State, Brazil

Noroviruses (NoV) are the main etiological agents of gastroenteritis outbreaks worldwide and susceptibility to NoV infection has been related to the histo-blood group antigen (HBGA). This study aimed to determine the prevalence of NoV strains and to evaluate the HBGA phenotype and genotype of children from semi-isolated Quilombola communities, descendents of black slaves in Brazil. A total of 397 children up to eleven years old, with and without diarrhea, from Quilombola Communities in the Espirito Santo State, Brazil, were investigated for the presence of NoV from August 2007 to September 2009. Feces were collected from all the children, and blood from the NoV positive children. NoV was screened by reverse transcription-PCR with primers for the RNA-dependent RNA polymerase region; genogroup was determined by PCR with primers for the C and D regions and genotyped by sequencing. HBGA phenotype was performed by gel-spinning and FUT2 and FUT3 were analyzed by PCR or sequencing analysis. NoV were detected in 9.2% (12/131) of diarrheic and 1.5% (4/266) of non-diarrheic children (p<0.05, Fisher's exact test). GI and GII genogroups were present in 12.5% and 87.5% of the samples, respectively. The following genotypes were characterized: GII.4 (25%), GII.12 (25%), GII.6 (12.5%) and GI.1 (6.3%), GI.3 (12.5%) and GI.4 (6.3%). Children infected with NoV showed the A (n = 6), O (n = 6), and B (n = 2) HBGA phenotypes, and 13 of them were classified as secretors (Se) and one as a non secretor (se). Mutations of Se (40), (171,216,357,428,739,960) were found for the FUT2 gene and mutations of Le (59, 202, 314) for the FUT3 gene. The only se child was infected by NoV GI, whereas the Se children were indiscriminately infected by GI or GII. This study showed rates of NoV infection in symptomatic and asymptomatic Quilombola children consistent with other studies. However, children under 12 months were seven times more affected than those between 1 and 5 years old. GII.12 was as frequent as GII.4 and GI.1 and GI.4 were described for the first time in Brazil. Owing to the small number of cases studied, no clear pattern of susceptibility and/or HBGA resistance could be inferred.

Host genetic factors affect susceptibility to norovirus infections in Burkina Faso

Norovirus (NoV) constitutes the second most common viral pathogen causing pediatric diarrhea after rotavirus. In Africa, diarrhea is a major health problem in children, and yet few studies have been performed regarding NoV. The association of histo-blood group antigens (HBGA) and susceptibility to NoV infection is well established in Caucasian populations with non-secretors being resistant to many common NoV strains. No study regarding HBGA and NoV susceptibility has yet been performed in Africa. We collected 309 stool and 208 saliva samples from diarrheal children in Ouagadougou, Burkina Faso; May 2009 to March 2010. NoV was detected using real-time PCR, and genotyped by sequencing. Saliva samples were ABO, Lewis and secretor phenotyped using in house ELISA assays. NoV was detected in 12% (n = 37) of the samples. The genotype diversity was unusually large; overall the 37 positive samples belonged to 14 genotypes. Only children <2 years of age were NoV positive and the GII.4 NoVs were more frequent in the late dry season (Jan-May). NoV infections were observed less in children with the secretor-negative phenotype or blood group A (OR 0.18; p = 0.012 and OR 0.31; p = 0.054; respectively), with two non-secretors infected with genotypes GII.7 and GII.4 respectively. Lewis-negative (Le^a−b−) children, representing 32% of the study population, were susceptible to GII, but were not infected with any NoV GI. GII.4 strains preferentially infected children with blood group B whereas secretor-positive children with blood group O were infected with the largest variety of genotypes. This is the first study identifying host genetic factors associated with susceptibility to NoV in an African population, and suggests that while the non-secretor phenotype provides protection; the Lewis b antigen is not necessary for GII infection.

Complete genome analysis of a novel norovirus GII.4 variant identified in China

The complete genome sequence of a novel norovirus strain GZ2010-L87 identified in Guangzhou was analyzed phylogenetically in this study. The RNA genome of the GZ2010-L87 strain is composed of 7,559 nucleotides. The phylogenetic analysis based on open reading frame (ORF) 2 revealed that the strain belongs to the GII.4 genotype, forming the new cluster GII.4-2009 which was also identified in Asia and the USA since 2009. Furthermore, phylogenetic analyses of the full genome and the different open reading frame sequences of GZ2010-L87 and other representative strains suggested that the novel strain did not undergo recombination. Comparative analysis with the consensus sequence of 31 completely sequenced norovirus GII.4-2009 genomes showed 86 mismatched nucleotides (56 in ORF1, 16 in ORF2, and 14 in ORF3), resulting in 19 amino acid changes (9 in ORF1, 3 in ORF2, and 7 in ORF3). Furthermore, 12 variable sites were found on the capsid protein of norovirus GII.4-2009, and most were located at the P2 domain. Meanwhile, based on comparison with other GII.4 clusters, 14 sites were shown specific to the novel cluster. In summary, the genome of the new GII.4-2009 variant GZ2010-L87, which was first identified in China, was extensively characterized with a large panel of genetically diverse noroviruses. The genomic information obtained from the novel variant can be used not only as a full-length norovirus sequence standard in China but also as reference data for future evolution research.

Virus-like Particle Vaccines for Norovirus Gastroenteritis

Gastroenteritis (GE) and its associated diarrheal diseases remain as one of the top causes of death in the world. Noroviruses (NoVs) are a group of genetically diverse RNA viruses that cause the great majority of nonbacterial gastroenteritis in humans. However, there is still no vaccine licensed for human use to prevent NoV GE. The lack of a tissue culture system and a small animal model further hinders the development of NoV vaccines.

Virus-like particles (VLPs) that mimic the antigenic architecture of authentic virions, however, can be produced in insect, mammalian, and plant cells by the expression of the capsid protein. The particulate nature and high-density presentation of viral structure proteins on their surface render VLPs as a premier vaccine platform with superior safety, immunogenicity, and manufacturability. Therefore, this chapter focuses on the development of effective NoV vaccines based on VLPs of capsid proteins. The expression and structure of NoV VLPs, especially VLPs of Norwalk virus, the prototype NoV, are extensively discussed. The ability of NoV VLPs in stimulating a potent systemic and mucosal anti-NoV immunity through oral and intranasal delivery in mice is presented.

The advantages of plant expression systems as a novel production platform for VLP-based NoV vaccines are discussed in light of their cost-effectiveness, production speed, and scalability. Recent achievements from the first successful demonstration of NoV VLP production in plant expression system under the current Good Manufacture Practice (cGMP) regulation by the US Food and Drug Administration (FDA) are detailed.

Moreover, results of human clinical trials demonstrating the safety and efficacy of insect and plant-derived NoV VLPs are also presented. Due to the diversity of capsid protein among different NoV strains and its rapid antigenic drift, we speculate that vaccine development should focus on multivalent VLP vaccines derived from capsid proteins of the most prevalent strains. With the very recent approval of the first plant-made biologics by the FDA, we also speculate that plant-based production systems will play an important role in manufacturing such multivalent VLP-based NoV vaccines.

Next-generation whole genome sequencing identifies the direction of norovirus transmission in linked patients

BACKGROUND

Noroviruses are a highly transmissible and major cause of nosocomial gastroenteritis resulting in bed and hospital-ward closures. Where hospital outbreaks are suspected, it is important to determine the routes of spread so that appropriate infection-control procedures can be implemented. To investigate a cluster of norovirus cases occurring in children undergoing bone marrow transplant, we undertook norovirus genome sequencing by next-generation methods. Detailed comparison of sequence data from 2 linked cases enabled us to identify the likely direction of spread.

METHODS

Norovirus complementary DNA was amplified by overlapping polymerase chain reaction (PCR) from 13 stool samples from 5 diagnostic real-time PCR-positive patients. The amplicons were sequenced by Roche 454, the genomes assembled by de novo assembly, and the data analyzed phylogenetically.

RESULTS

Phylogenetic analysis indicated that patients were infected by viruses similar to 4 distinct GII.4 subtypes and 2 patients were linked by the same virus. Of the 14 sites at which there were differences between the consensus sequences of the 2 linked viral genomes, 9 had minor variants present within one or the other patient. Further analysis confirmed that minor variants at all 9 sites in patient B w ere present as the consensus sequence in patient A.

CONCLUSIONS

Phylogenetic analysis excluded a common source of infection in this apparent outbreak. Two of 3 patients on the same ward had closely related viruses, raising the possibility of cross-infection despite protective isolation. Analysis of deep sequencing data enabled us to establish the likely direction of nosocomial transmission.

Emergence of a new norovirus GII.4 variant and changes in the historical biennial pattern of norovirus outbreak activity in Alberta, Canada, from 2008 to 2013

The public health impact of the emergence of new norovirus (NoV) strains is uncertain. A biennial pattern of alternating quiescent and epidemic levels of NoV outbreak activity associated with the emergence of new GII.4 variants was observed in Alberta, Canada, between July 2000 and June 2008. In this study, NoV genogroup I (GI) and GII strains isolated from 710 outbreak specimens in Alberta between July 2008 and January 2013 were characterized to update historical data. The seasonality and annual variation in NoV outbreak burden were analyzed over a 10-year period (July 2002 to June 2012). We found that GII.4-2006b had persisted as the predominant variant over three observation periods (July 2006 to June 2009) during which the biennial NoV outbreak pattern continued. The emergence of GII.4-2010 (winter 2009) was not associated with increased outbreak activity, and outbreak activity between July 2009 and June 2012 when GII.4-2010 predominated (67.5 to 97.7%) did not follow a biennial pattern. GII.4-2012 first emerged in Alberta in September 2011 and became predominant in observation period July 2012 to June 2013. NoV GI, relatively rare in past years, had a higher activity level (37.3%) as represented by GI.6 and GI.7 in the winter of 2012 to 2013. A higher proportion of GI outbreaks occurred in non-health care facility settings compared to GII. Our study suggests that factors other than new variants emergence contribute to the levels of NoV outbreak activity in Alberta.

Norwalk Virus Minor Capsid Protein VP2 Associates within the VP1 Shell Domain

The major capsid protein of norovirus VP1 assembles to form an icosahedral viral particle. Despite evidence that the Norwalk virus (NV) minor structural protein VP2 is present in infectious virions, the available crystallographic and electron cryomicroscopy structures of NV have not revealed the location of VP2. In this study, we determined that VP1 associates with VP2 at the interior surface of the capsid, specifically with the shell (S) domain of VP1. We mapped the interaction site to amino acid 52 of VP1, an isoleucine located within a sequence motif IDPWI in the S domain that is highly conserved across norovirus genogroups. Mutation of this isoleucine abrogated VP2 incorporation into virus-like particles without affecting the ability for VP1 to dimerize and form particles. The highly basic nature of VP2 and its location interior to the viral particle are consistent with its potential role in assisting capsid assembly and genome encapsidation.

Proposal for a unified norovirus nomenclature and genotyping

Noroviruses belong to a genus of genetically diverse viruses within the family Caliciviridae and cause acute gastroenteritis in humans and animals. They are subdivided into genogroups, each of which further segregates into genotypes. Until recently, a new genotype was based on a defined pairwise distance cutoff of complete VP1 sequences, but with the increasing number of available norovirus sequences, this cutoff is no longer accurate, and sequences in the public database have been misclassified. In this paper, we demonstrate that the pairwise distance cutoff method can no longer be used and outline a phylogenetic approach to classify noroviruses. Furthermore, we propose a dual nomenclature using both ORF1 and VP1 sequences, as recombination is common and recognizing recombinant viruses may be relevant. With the continuing emergence of new norovirus lineages, we propose to coordinate nomenclature of new norovirus genotypes through an international norovirus working group.

Recombination within the pandemic norovirus GII.4 lineage

Norovirus (NoV) is the leading cause of viral gastroenteritis globally. Since 1996, NoV variants of a single genetic lineage, GII.4, have been associated with at least six pandemics of acute gastroenteritis and caused between 62 and 80% of all NoV outbreaks. The emergence of these novel GII.4 variants has been attributed to rapid evolution and antigenic variation in response to herd immunity; however, the contribution of recombination as a mechanism facilitating emergence is increasingly evident. In this study, we sought to examine the role that intragenotype recombination has played in the emergence of GII.4 variants. Using a genome-wide approach including 25 complete genome sequences generated as part of this study, 11 breakpoints were identified within the NoV GII.4 lineage. The breakpoints were located at three recombination hot spots: near the open reading frame 1/2 (ORF1/2) and ORF2/3 overlaps, as well as within ORF2, which encodes the viral capsid, at the junction of the shell and protruding domains. Importantly, we show that recombination contributed to the emergence of the recent pandemic GII.4 variant, New Orleans 2009, and a newly identified GII.4 variant, termed Sydney 2012. Reconstructing the evolutionary history of the GII.4 lineage reveals the widespread impact of both inter- and intragenotype recombination on the emergence of many GII.4 variants. Lastly, this study highlights the many challenges in the identification of true recombination events and proposes that guidelines be applied for identifying NoV recombinants.

Prevalence and genetic diversity of norovirus infection in Tunisian children (2007-2010)

Viral gastroenteritis can be a life-threatening disease in infants and young children, especially in developing countries. The aim of this study was to continue the epidemiological surveillance of norovirus (NoV) infections in Tunisian children suffering from acute gastroenteritis. Surveillance was initiated in January 2003, to monitor potential variations in strains over time, in terms of frequency and diversity of NoV genotypes, and more particularly the potential emergence of new GII.4 variants following the 2004 Hunter variant. From April 2007 to April 2010, a total of 407 stool specimens were collected from sporadic cases (238 inpatients and 169 outpatients). Furthermore, 28 stool samples were collected from children involved in 3 gastroenteritis outbreaks. Stool specimens were screened for NoV genogroup I (GI) and II (GII) by RT-PCR. NoV strains were genotyped, and variants identified, based on sequence and phylogenetic analyses of the polymerase and capsid genes. NoVs were detected in 38 sporadic cases (9.3%) and 21 epidemic cases (75%). Great diversity was observed throughout the period, with seven distinct NoV genotypes characterized in sporadic cases, and three in outbreaks. GIIb/II.3 and GII.4 were predominant globally, with fluctuations of their prevalence over time. Interestingly, the Hunter variant, which was the unique GII.4 variant observed from 2003 to April 2007 in the region of Monastir, was replaced by the 2006b variant. NoV is an important enteropathogen responsible for viral gastroenteritis among infants and children in Tunisia, and the infecting strains between 2007 and 2010 were different from those in previous years.

An in silico virtual screening study for the design of norovirus inhibitors: fragment-based molecular docking and binding free energy calculations

Gastrointestinal infections caused by noroviruses may be prevented by the inhibition of their binding to histo-blood group carbohydrate antigens. A fragment-based virtual screening approach was used, employing docking followed by molecular dynamics simulations in order to enable binding free energy calculations using the linear interaction energy method. The resulting structures, composed of high-affinity fragments, can be a good starting point for lead optimizations and four molecules that pass both REOS and SYLVIA filters, which can remove known toxic features and assess the synthetic accessibility, respectively, are proposed as inhibitors.

Norovirus and medically attended gastroenteritis in U.S. children

BACKGROUND

Cases of rotavirus-associated acute gastroenteritis have declined since the introduction of rotavirus vaccines, but the burden of norovirus-associated acute gastroenteritis in children remains to be assessed.

METHODS

We conducted active surveillance for laboratory-confirmed cases of norovirus among children younger than 5 years of age with acute gastroenteritis in hospitals, emergency departments, and outpatient clinical settings. The children resided in one of three U.S. counties during the years 2009 and 2010. Fecal specimens were tested for norovirus and rotavirus. We calculated population-based rates of norovirus-associated acute gastroenteritis and reviewed billing records to determine medical costs; these data were extrapolated to the U.S. population of children younger than 5 years of age.

RESULTS

Norovirus was detected in 21% of young children (278 of 1295) seeking medical attention for acute gastroenteritis in 2009 and 2010, with norovirus detected in 22% (165 of 742) in 2009 and 20% (113 of 553) in 2010 (P=0.43). The virus was also detected in 4% of healthy controls (19 of 493) in 2009. Rotavirus was identified in 12% of children with acute gastroenteritis (152 of 1295) in 2009 and 2010. The respective rates of hospitalization, emergency department visits, and outpatient visits for the norovirus were 8.6, 146.7, and 367.7 per 10,000 children younger than 5 years of age in 2009 and 5.8, 134.3, and 260.1 per 10,000 in 2010, with an estimated cost per episode of $3,918, $435, and $151, respectively, in 2009. Nationally, we estimate that the average numbers of annual hospitalizations, emergency department visits, and outpatient visits due to norovirus infection in 2009 and 2010 among U.S. children in this age group exceeded 14,000, 281,000, and 627,000, respectively, with more than $273 million in treatment costs each year.

CONCLUSIONS

Since the introduction of rotavirus vaccines, norovirus has become the leading cause of medically attended acute gastroenteritis in U.S. children and is associated with nearly 1 million health care visits annually. (Funded by the Centers for Disease Control and Prevention.).

Heterogeneity in norovirus shedding duration affects community risk

Norovirus is a common cause of gastroenteritis in all ages. Typical infections cause viral shedding periods of days to weeks, but some individuals can shed for months or years. Most norovirus risk models do not include these long-shedding individuals, and may therefore underestimate risk. We reviewed the literature for norovirus-shedding duration data and stratified these data into two distributions: regular shedding (mean 14-16 days) and long shedding (mean 105-136 days). These distributions were used to inform a norovirus transmission model that predicts the impact of long shedders. Our transmission model predicts that this subpopulation increases the outbreak potential (measured by the reproductive number) by 50-80%, the probability of an outbreak by 33%, the severity of transmission (measured by the attack rate) by 20%, and transmission duration by 100%. Characterizing and understanding shedding duration heterogeneity can provide insights into community transmission that can be useful in mitigating norovirus risk.