National epidemic of Lordsdale Norovirus in the UK

Evolutionary Constraints on the Norovirus Pandemic Variant GII.4_2006b over the Five-Year Persistence in Japan

Norovirus GII.4 is a major cause of global outbreaks of viral gastroenteritis in humans, and has evolved by antigenic changes under the constantly changing human herd immunity. Major shift in the pandemic GII.4 strain periodically occurs concomitant with changes in the antigenic capsid protein VP1. However, how the newly emerged strain evolves after the onset of pandemic remains unclear. To address this issue, we examined molecular evolution of a pandemic lineage, termed the GII.4_2006b, by using the full-length viral genome and VP1 sequences (n = 317) from stools collected at 20 sites in Japan between 2006 and 2011. Phylogenetic tree showed a radial diversification of the genome sequences of GII.4_2006b, suggesting a rapid genetic diversification of the GII.4_2006b population from a few ancestral variants. Impressively, amino acid sequences of the variable VP1 in given seasons remained as homogeneous as those of viral enzymes under annual increase in the nucleotide diversity in the VP1 coding region. The Hamming distances between the earliest and subsequent variants indicate strong constraints on amino acid changes even for the highly variable P2 subdomain. These results show the presence of evolutionary constraints on the VP1 protein and viral enzymes, and suggest that these proteins gain near maximal levels of fitness benefits in humans around the onset of the outbreaks. These findings have implications for our understanding of molecular evolution, mechanisms of the periodic shifts in the pandemic NoV GII.4 strains, and control of the NoV GII.4 pandemic strain.

Norovirus Narita 104 virus-like particles expressed in Nicotiana benthamiana induce serum and mucosal immune responses

Narita 104 virus is a human pathogen belonging to the norovirus (family Caliciviridae) genogroup II. Noroviruses cause epidemic gastroenteritis worldwide. To explore the potential of developing a plant-based vaccine, a plant optimized gene encoding Narita 104 virus capsid protein (NaVCP) was expressed transiently in Nicotiana benthamiana using a tobacco mosaic virus expression system. NaVCP accumulated up to approximately 0.3 mg/g fresh weight of leaf at 4 days postinfection. Initiation of hypersensitive response-like symptoms followed by tissue necrosis necessitated a brief infection time and was a significant factor limiting expression. Transmission electron microscopy of plant-derived NaVCP confirmed the presence of fully assembled virus-like particles (VLPs). In this study, an optimized method to express and partially purify NaVCP is described. Further, partially purified NaVCP was used to immunize mice by intranasal delivery and generated significant mucosal and serum antibody responses. Thus, plant-derived Narita 104 VLPs have potential for use as a candidate subunit vaccine or as a component of a multivalent subunit vaccine, along with other genotype-specific plant-derived VLPs.

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.

Comparison of age-stratified seroprevalence of antibodies against norovirus GII in India and the United Kingdom

Noroviruses are a common cause of gastroenteritis worldwide, but outbreaks appear to be more common in industrialized countries than in developing countries, possibly reflecting differences in exposure and immunity. In this study, age-stratified sera from India and UK populations were analysed for the presence of norovirus-genogroup II specific IgG by a time resolved immunofluorescence assay and relative levels of antibodies in the two populations were compared. Antibody levels were higher among all age groups in India than in UK and increased with age in India, whereas in the UK, levels of antibody decreased in adulthood. These results indicate different patterns of exposure to noroviruses in the two countries.

Mechanisms of GII.4 norovirus evolution

Since the late 1990s norovirus (NoV) strains belonging to a single genotype (GII.4) have caused at least four global epidemics. To date, the higher epidemiological fitness of the GII.4 strains has been attributed to a faster rate of evolution within the virus capsid, resulting in the ability to escape herd immunity. Four key factors have been proposed to influence the rate of evolution in NoV. These include host receptor recognition, sequence space, duration of herd immunity, and replication kinetics. In this review we discuss recent advancements in our understanding of these four mechanisms in relation to GII.4 evolution.

Spurious polyadenylation of Norovirus Narita 104 capsid protein mRNA in transgenic plants

Noroviruses are members of the family Caliciviridae, and cause a highly communicable gastroenteritis in humans. We explored the potential to develop a plant-based vaccine against Narita 104 virus, a Genogroup II Norovirus. In stably transgenic potato, we obtained very poor expression of Narita 104 virus capsid protein (NaVCP) despite the use of a strong constitutive promoter (dual enhancer 35S) driving the native coding sequence. We identified potentially detrimental sequence motifs that could mediate aberrant mRNA processing via spurious polyadenylation signals. Northern blots and RT-PCR analysis of total RNA revealed truncated transcripts that suggested premature polyadenylation. Site-directed mutagenesis to remove one potential polyadenylation near-upstream element resulted in an increased expression of NaVCP when transiently expressed in leaves of Nicotiana benthamiana. Further, cloning of the truncated cDNAs from transgenic NaVCP potato plants and transiently transfected N. benthamiana allowed us to identify at least ten different truncated transcripts resulting from premature polyadenylation of full length NaVCP transcripts. Comparative studies using real time PCR analysis from cDNA samples revealed lower accumulation of full length transcripts of NaVCP as compared to those from a gene encoding Norwalk Virus capsid protein (a related Genogroup I Norovirus) in transiently transfected plants. These findings provide evidence for impaired expression of NaVCP in transgenic plants mediated by spurious polyadenylation signals, and demonstrate the need to scrupulously search for potential polyadenylation signals in order to improve transgene expression in plants.

Human norovirus RNA persists in seawater under simulated winter conditions but does not bioaccumulate efficiently in Pacific Oysters (Crassostrea gigas)

Norovirus (NoV) is the principal agent of bivalve molluscan shellfish-associated gastroenteric illness worldwide. Currently, noncultivable human NoVs can be detected in bivalve molluscan shellfish by using molecular methods such as real-time reverse transcription PCR assays (qRT-PCR). In addition to infectious viruses, this methodology may also detect noninfectious NoV, including fragments of the NoV genome. This study addresses, in part, the implications of qRT-PCR results for the detection of NoV in shellfish in the absence of an infectivity assay. To evaluate environmental persistence, the stability of a short fragment of the NoV genome, spanning the qRT-PCR target in the open reading frame 1/2 junction, was assessed in seawater under artificial environmental conditions simulating winter in the United Kingdom (1 mW/cm² UV irradiation, 8°C) during a 4-week period. Detectable RNA levels decreased exponentially (T₉₀ of approximately 141 h); however, sequences were still detectable for up to 2 weeks. The ability of Pacific oysters (Crassostrea gigas) to bioaccumulate NoV particles (from human feces) and RNA fragments was also compared using qRT-PCR. Oysters exposed to NoV particles subsequently were positive for NoV by qRT-PCR at levels several orders of magnitude in excess of the theoretical limit of detection, whereas oysters exposed to similar quantities of NoV RNA were either negative or positive at significantly lower levels. Therefore, although noninfectious fragments of NoV RNA may persist in the environment under winter conditions, this type of material will not be efficiently bioaccumulated by Pacific oysters and should not significantly contribute to positive qRT-PCR results.

The role of electron microscopy in the rapid diagnosis of viral infections--review

Electron microscopy (EM) allows fast visualization of viruses in a wide range of clinical specimens. Viruses are grouped into families based on their morphology. Viruses from various families look distinctly and these morphological variances are the basis for identification of viruses by EM. The identification to the family level is often sufficient for the clinician or recognition of an unknown infectious agent. Diagnostic EM has two advantages over enzyme-linked immunosorbent assay and nucleic acid amplification tests. After a simple and fast negative staining, EM allows fast morphological identification and differential diagnosis of infectious agents contained in the specimen without the need for special considerations and/or reagents. Nevertheless, EM has the disadvantage of being unsuitable as a screening method.

Application of a modified loop-mediated isothermal amplification kit for detecting Norovirus genogroups I and II

Norovirus is a major etiologic agent in worldwide outbreaks of gastroenteritis associated with food as well as person‐to‐person transmission. The ubiquitous nature of Norovirus necessitates simple and rapid detection methods with high accuracy and sensitivity. To this end, several investigators have evaluated the usefulness of commercial reverse‐transcription loop‐mediated isothermal amplification (RT‐LAMP) kits for detecting Norovirus genogroups I (GI) and II (GII). In previous studies, the conventional Loopamp kit for Norovirus GII showed a relatively high detection rate, while that for Norovirus GI showed a relatively low detection rate. In the present study, clinical Norovirus specimens were used to compare the detection rate of a modified Loopamp kit for Norovirus GI with the rates of the conventional Loopamp kit for Norovirus GI and an “in‐house” RT‐LAMP GI primer set, methods which had a high detection rate. Results from the present study showed that the modified Loopamp kit for Norovirus GI had a higher detection rate for two viral genotypes (GI.3, GI.11). On comparison with an “in‐house” GII primer set using genotype GII.4 viruses circulating recently, the detection rate by the Loopamp kit for Norovirus GII was found to be higher, with a 98% detection rate. These results indicate the applicability of the modified LAMP kit for GI and the conventional LAMP kit for GII for detection of Noroviruses in clinical samples. J. Med. Virol. 81:2072–2078, 2009. © 2009 Wiley‐Liss, Inc.

Molecular epidemiology of genogroup II-genotype 4 noroviruses in the United States between 1994 and 2006

Human noroviruses (NoVs) of genogroup II, genotype 4 (GII.4) are the most common strains detected in outbreaks of acute gastroenteritis worldwide. To gain insight into the epidemiology and genetic variation of GII.4 strains, we analyzed 773 NoV outbreaks reported to the CDC from 1994 to 2006. Of these NoV outbreaks, 629 (81.4%) were caused by GII viruses and 342 (44.2%) were caused by GII.4 strains. The proportion of GII.4 outbreaks increased from 5% in 1994 to 85% in 2006, but distinct annual differences were noted, including sharp increases in 1996, 2003, and 2006 each associated with newly emerging GII.4 strains. Sequence analysis of the full-length VP1 gene of GII.4 strains identified in this study and from GenBank segregated these viruses into at least 9 distinct subclusters which had 1.3 to 3.2% amino acid variation between strains in different subclusters. We propose that GII.4 subclusters be defined as having >5% sequence variation between strains. Our data confirm other studies on the rapid emergence and displacement of highly virulent GII.4 strains.

Analysis of integrated virological and epidemiological reports of norovirus outbreaks collected within the Foodborne Viruses in Europe network from 1 July 2001 to 30 June 2006

The Foodborne Viruses in Europe network has developed integrated epidemiological and virological outbreak reporting with aggregation and sharing of data through a joint database. We analyzed data from reported outbreaks of norovirus (NoV)-caused gastroenteritis from 13 European countries (July 2001 to July 2006) for trends in time and indications of different epidemiology of genotypes and variants. Of the 13 countries participating in this surveillance network, 11 were capable of collecting integrated epidemiological and virological surveillance data and 10 countries reported outbreaks throughout the entire period. Large differences in the numbers and rates of reported outbreaks per country were observed, reflecting the differences in the focus and coverage of national surveillance systems. GII.4 strains predominated throughout the 5-year surveillance period, but the proportion of outbreaks associated with GII.4 rose remarkably during years in which NoV activity was particularly high. Spring and summer peaks indicated the emergence of genetically distinct variants within GII.4 across Europe and were followed by increased NoV activity during the 2002-2003 and 2004-2005 winter seasons. GII.4 viruses predominated in health care settings and in person-to-person transmission. The consecutive emergence of new GII.4 variants is highly indicative of immune-driven selection. Their predominance in health care settings suggests properties that facilitate transmission in settings with a high concentration of people such as higher virus loads in excreta or a higher incidence of vomiting. Understanding the mechanisms driving the changes in epidemiology and clinical impact of these rapidly evolving RNA viruses is essential to design effective intervention and prevention measures.

Detection of viral agents of gastroenteritis: Norovirus, Sapovirus and Astrovirus

Norovirus, Sapovirus and Astrovirus are causative agents of viral gastroenteritis affecting all age groups, but most frequently the young, the elderly and persons in semi-closed communities such as hospitals, nursing homes, military bases and cruise ships. The sensitive and rapid detection of causative agents of viral gastroenteritis is key to the effective implementation of infection control systems. Traditional detection methods such as electron microscopy and antigen detection assays lack sensitivity. The detection of gastrointestinal viruses by molecular methods has resulted in increased levels of detection, and enables the epidemiological investigation of viral strains. The significant diversity of gastrointestinal viruses, in particular Norovirus and Sapovirus, are compounded by increasing reports of virus recombination, and pose an ongoing challenge to the development of sensitive and specific molecular detection assays.

Real-time reverse transcription PCR detection of norovirus, sapovirus and astrovirus as causative agents of acute viral gastroenteritis

The design and development of highly sensitive real-time reverse transcription PCR assays for the detection of norovirus genogroups I, II and IV, sapovirus genogroups I, II and IV, and human astrovirus from stool samples is described. Examination of 140 stool samples from paediatric patients exhibiting symptoms of diarrhoea and/or vomiting resulted in increased detection levels as compared to examination by electron microscopy. Real-time PCR resulted in a 200% increase in the rate of detection of norovirus as compared to electron microscopy. Only genogroup II noroviruses were detected in the stool specimens and when examined using partial-genotyping primers all were identified as clustering with the genogroup II/4(Bristol/Lordsdale) cluster. Sapovirus was not detected in any of the stool specimens by electron microscopy while 11% (15/140) of specimens were sapovirus positive by real-time RT-PCR, accounting for 36% of calicivirus diarrhoea. Real-time RT-PCR resulted in a tenfold increase in the rate of detection of astrovirus when compared to detection by electron microscopy with both type 1 and type 4 human astroviruses being detected in circulation. The results highlight the importance of the introduction of molecular methods for the routine screening of stool samples for causative agents of viral gastroenteritis.

Atypical norovirus epidemic in Hong Kong during summer of 2006 caused by a new genogroup II/4 variant

An atypically high level of norovirus activity was noticed in Hong Kong beginning in early May 2006. A study was carried out to investigate whether this was caused by a new norovirus variant. Epidemiological data including monthly positivity rates and the numbers of outbreaks per month from January to July 2006 were analyzed and compared to those from 2002 to 2005. In a comparison with the epidemiological data from 2001 to 2005, an atypical peak of norovirus-associated gastroenteritis outbreak was observed beginning in May 2006, concurring with a striking increase in norovirus activity. Most of the outbreaks (>60%) were located in homes for the elderly. Phylogenetic analysis for both RdRp and 5' capsid regions showed that this epidemic was caused by a new genogroup II/4 variant. This variant was genetically distinct from the predominant variants of 2002 and 2004 but was closely related to one of the 95/96-subset variants which caused an epidemic in Hong Kong in 2001, suggesting that the 95/96 subset may be starting to recirculate.

Correlation of norovirus variants with epidemics of acute viral gastroenteritis in Hong Kong

Norovirus (NV) (formerly called Norwalk-like virus) is the most common etiological agent of acute viral gastroenteritis outbreaks worldwide. Recent reports have shown that two new GII.4 variants caused epidemics in Europe. To investigate if it is also the case in Hong Kong, a molecular epidemiological study was undertaken between January 2002 and June 2005. During this period, there was a substantial increase in acute cases of gastroenteritis caused by NV. Phylogenetic analysis showed that GII.2 and GII.4 are the major circulating genotypes. Two new GII.4 variants (variants C and D) were identified in 2002 and 2004, which quickly became the predominant strains. They were almost identical to the variants causing epidemics in Europe recently. Since geographically distinct areas were involved within a short period of time, it is possible that GII.4 has a particular propensity for causing pandemics.

Cleavage map and proteolytic processing of the murine norovirus nonstructural polyprotein in infected cells

Murine norovirus (MNV) is presently the only member of the genus Norovirus in the Caliciviridae that can be propagated in cell culture. The goal of this study was to elucidate the proteolytic processing strategy of MNV during an authentic replication cycle in cells. A proteolytic cleavage map of the ORF1 polyprotein was generated, and the virus-encoded 3C-like (3CL) proteinase (Pro) mediated cleavage at five dipeptide cleavage sites, 341E/G342, Q705/N706, 870E/G871, 994E/A995, and 1177Q/G1178, that defined the borders of six proteins with the gene order p38.3 (Nterm)-p39.6 (NTPase)-p18.6-p14.3 (VPg)-p19.2 (Pro)-p57.5 (Pol). Bacterially expressed MNV 3CL Pro was sufficient to mediate trans cleavage of the ORF1 polyprotein containing the mutagenized Pro sequence into products identical to those observed during cotranslational processing of the authentic ORF1 polyprotein in vitro and to those observed in MNV-infected cells. Immunoprecipitation and Western blot analysis of proteins produced in virus-infected cells demonstrated efficient cleavage of the proteinase-polymerase precursor. Evidence for additional processing of the Nterm protein in MNV-infected cells by caspase 3 was obtained, and Nterm sequences 118DRPD121 and 128DAMD131 were mapped as caspase 3 cleavage sites by site-directed mutagenesis. The availability of the MNV nonstructural polyprotein cleavage map in concert with a permissive cell culture system should facilitate studies of norovirus replication.

Emergence of a new norovirus genotype II.4 variant associated with global outbreaks of gastroenteritis

Norovirus (NoV) is highly infectious and is the major cause of outbreak gastroenteritis in adults, with pandemic spread of the virus being reported in 1995 and 2002. The NoV genome is genetically diverse, which has hampered development of sensitive molecular biology-based methods. In this study we report on a nested reverse transcriptase PCR (nRT-PCR) that was designed to amplify the highly conserved 3' end of the polymerase region and the 5' end of the capsid gene of NoV genogroup II (GII). The nRT-PCR was validated with strains isolated from sporadic and outbreak cases between 1997 and 2004 in New South Wales, Australia. Phylogenetic analysis identified six genotypes circulating in New South Wales, GII.1, GII.3, GII.4, GII.6, GII.7, and GII.10, with GII.4 being the predominant genotype. In 2004, there was a marked increase in NoV GII activity in Australia, with a novel GII.4 variant being identified as the etiological agent in 18 outbreaks investigated. This novel GII.4 variant, termed Hunter virus, differed by more than 5% at the amino acid level across the capsid from any other NoV strain in the GenBank and EMBL databases. The Hunter virus was subsequently identified as the etiological agent in large epidemics of gastroenteritis in The Netherlands, Japan, and Taiwan in 2004 and 2005.

Molecular epidemiology of norovirus strains circulating in Ireland from 2003 to 2004

Since 2002, the burden of norovirus (NoV) infection in Ireland has increased. Outbreaks in institutional settings are the most common causing widespread disruption to health service delivery. This is the first national study of NoV in the Republic of Ireland and its aim was to identify the major NoV strains circulating in Ireland over a 13-month period between November 2003 and November 2004, inclusive. A prospective study screened faecal samples (n = 478) for NoV RNA. Positive samples (n = 116) were further analysed by a second PCR, targeted to the orf1/orf2 junction of the virus. Phylogenetic analysis was based on sequence alignments of this domain. GII/4 viruses represented 92.2% of sequences, 2.7% were GII/2, GII/3 and GGIIB cluster-like strains. The remaining 5.2% were of GI origin. NoV was detectable throughout the study period, although two peaks of infection were observed. The majority of infections were caused by a range of closely related GII/4 NoV strains.

Confirmation of electron microscopy results by direct testing of viruses adhered to grids using nucleic acid amplification techniques

It is possible to visualize rapidly viral particles by electron microscopy (EM) in patient samples and in cell cultures, and characterize the particles on the basis of their size and morphology. In many instances, EM has contributed to the diagnosis of specific infectious agents. Four different types of viruses with different characteristics of particle size, capsid structure, the presence or absence of an envelope, genomic content and stability outside the host were screened and diagnosed by EM at the level of family/genus. The results were confirmed at the species level by elution of the sample material from the grids used for EM examination and nucleic acid amplification. This approach could be valuable in situations where the immediate diagnosis is unclear, or when new infectious agents appear.

Existence of multiple genotypes associated with acute gastroenteritis during 6-year survey of norovirus infection in Japan

Norovirus (NoV) is recognized as one of the most common causative agent of diarrheal disease in young children worldwide. The current study was undertaken to determine the distribution of NoV genotypes in Japan. A total of 3,864 fecal specimens from children with acute gastroenteritis in five regions (Tokyo, Maizuru, Saga, Sapporo, and Osaka) of Japan from July 1995 to June 2001 were collected and then tested for the presence of NoV by RT-PCR. Three hundred sixty four were found to be positive for NoV, accounting for 11%. The highest prevalence of NoV infection was in November, December, and January as the early winter months in Japan. NoV was subjected to be further characterized to sequencing analysis. All NoVs belonged to two different genogroups I and II and these represented 3% and 97%, respectively. This finding indicated that NoV genogroup II was the dominant group causing acute gastroenteritis in Japan. Interestingly, NoV strains were classified into 16 distinct genotypes including genogroup II genotype 9 that was firstly identified in Japan. Of these, NoV genogroup II genotypes 3 and 4 dominated over other genotypes and became the leading strains in Japanese pediatric population. In conclusion, diarrhea due to NoV infection is still a health burden in Japan. This report also stresses the great genetic diversity as well as the importance of NoV causing the diarrhea in Japan.

Norovirus classification and proposed strain nomenclature

Without a virus culture system, genetic analysis becomes the principal method to classify norovirus (NoV) strains. Currently, classification of NoV strains beneath the species level has been based on sequences from different regions of the viral genome. As a result, the phylogenetic insights of some virus were not appropriately interpreted, and no consensus has been reached to establish a uniform classification scheme. To provide a consistent and reliable scientific basis for classifying NoVs, we analyzed the amino acid sequences for the major capsid protein of 164 NoV strains by first using an alignment based on the predicted 3D structures. A Bayesian tree was generated, and the maximum likelihood pairwise distances of the aligned sequences were used to evaluate the results from the uncorrected pairwise distance method. Analyses of the pairwise distances demonstrated three clearly resolved peaks, suggesting that NoV strains beneath the species level can be classified at three levels: strain (S), cluster (C), and genogroup (G). The uncorrected pairwise distance ranges for S, C, and G were 0-14.1%, 14.3-43.8%, and 44.9-61.4%, respectively. A scheme with 29 genetic clusters [8 in genogroup 1 (G1), 17 in G2, 2 in G3, and 1 each in G4 and G5] was defined on the basis of the tree topology with the standards provided and was supported by the distance analysis. Of these, five clusters in G2 and one in G1 are newly described. This analysis can serve as the basis for a standardized nomenclature to genetically describe NoV strains.

Genetic analysis of noroviruses in Chiba Prefecture, Japan, between 1999 and 2004

Noroviruses (NVs) are common pathogens that consist of genetically divergent viruses that induce gastroenteritis in humans and animals. Between September 1999 and June 2004, 1,898 samples obtained from patients showing sporadic or outbreak gastroenteritis in Chiba Prefecture, Japan, were tested for NVs by reverse transcription-PCR. NVs were detected in 603 samples. Approximately 80% were positive for genogroup GII, 13% were positive for genogroup GI, and the remaining 7% were positive for both genogroups. Phylogenetic analysis showed that the GI and GII genogroups could be further divided into 13 and 16 genotypes (including new genotypes), respectively. The GII-4 genotype, which included five small genetic clusters (subtypes), was the most common in this study and was detected in approximately 40% of positive samples. The P2 regions of 10 strains belonging to each of the five GII-4 subtypes showed 5 to 18% amino acid diversity. The amino acid substitutions accumulated in the protruding (P) region during the 5-year study period. Our data suggest that highly variable NV strains are circulating in Chiba Prefecture, with a high rate of genetic change observed during the 5-year study period.

Expression and antigenicity of virus-like particles of norovirus and their application for detection of noroviruses in stool samples

Human noroviruses (NoVs), members of the genus Norovirus in the family Caliciviridae, are the leading agents of nonbacterial acute gastroenteritis worldwide. Human NoVs are currently divided into at least two genogroups, genogroup I (GI) and genogroup II (GII), each of which contains at least 14 and 17 genotypes. To explore the genetic and antigenic relationship among NoVs, we expressed the capsid protein of four genetically distinct NoVs, the GI/3 Kashiwa645 virus, the GII/3 Sanbu809 virus, the GII/5 Ichikawa754 virus, and the GII/7 Osaka10-25 virus in baculovirus expression system. An antigen enzyme-linked immunosorbent assay (ELISA) with hyperimmune serum against the four recombinant capsid proteins and characterized previously three capsid proteins derived from GI/1, GI/4, and GII/12 was developed to detect the NoVs antigen in stools. The antigen ELISA was highly specific to the homotypic strains, allowing assignment of a strain to a Norovirus genetic cluster within a genogroup.

Mutation in a Lordsdale norovirus epidemic strain as a potential indicator of transmission routes

An increase in norovirus outbreaks was reported internationally during 2002 and 2003 and was also observed in Oxfordshire (United Kingdom) hospitals. To understand their epidemiological relationships, viruses from 22 outbreaks (15 from one hospital) were subjected to nucleotide sequencing. The 3'-terminal 3,255 nt or complete genomes were determined for 49 viruses. All outbreaks were caused by a genogroup II norovirus related to the Lordsdale virus (GII 4), common in healthcare settings. The norovirus mutation rate was sufficiently high that the 3,255-nucleotide sequences allowed separate and potentially connected outbreaks to be identified, since all outbreaks with identical sequences were temporally or geographically linked. The high mutation rate was further indicated by four mutations and three microheterogeneities in 3,255 nucleotides during 17 days of norovirus shedding by an immunocompromised patient. The data suggested that multiple virus introductions from the community, occasional transmission among wards, and one instance of ongoing environmental contamination had occurred. The accumulation, or lack, of mutations within an outbreak was also used to indicate the predominant transmission route. In an outbreak where person-to-person spread was thought to predominate, six mutations were detected throughout the genome, whereas one mutation was detected when point source infection was suspected. This norovirus epidemic strain differed from its closest previously described relative by 11.4 to 13.6% in the outer P2 domain of the capsid, which also had a single-amino-acid insertion. Alterations to the capsid structure compared to previous noroviruses may explain the increased number of outbreaks during 2002 and 2003.

Human norovirus infection and the lessons from animal caliciviruses

PURPOSE OF REVIEW

Human noroviruses are a major cause of infectious intestinal disease, particularly in the health sector, with considerable knock-on effects on care provision through ward closures and staff sickness. This review will describe recent advances in our understanding of human noroviruses. In addition, we will consider related nonhuman caliciviruses to highlight some potential difficulties in the control of caliciviral disease.

RECENT FINDINGS

Using more sensitive reverse transcriptase polymerase chain reaction based assays, noroviruses are now recognized as the most common cause of infectious intestinal disease in the community, as well as outbreaks of the infectious intestinal disease. After recovery from acute disease, some individuals continue shedding norovirus, particularly if immunosuppressed. The noroviruses are extremely variable, which has important implications for protection following challenge, and for future vaccination. From amongst this variability, new strains have emerged with the potential to spread widely. Recently a mouse norovirus has been identified which will afford new insights into the biology of these important viruses. Studies on human susceptibility have identified some resistant individuals in the population and a potential virus receptor, which may lead to the development of novel antiviral therapies.

SUMMARY

Lack of cell culture systems for the human noroviruses is being overcome by molecular technologies. Such studies have provided new insight into the significance and epidemiology of these viruses and opened the possibility of disease control through vaccination. Work on nonhuman caliciviruses has interesting parallels with human noroviruses, and provides new insights into the understanding of these important human pathogens.