Antigenic relatedness among the Norwalk-like agents by serum antibody rises

Molecular Diagnostic Methods for Detection and Characterization of Human Noroviruses

Human noroviruses are a group of viral agents that afflict people of all age groups. The viruses are now recognized as the most common causative agent of nonbacterial acute gastroenteritis and foodborne viral illness worldwide. However, they have been considered to play insignificant roles in the disease burden of acute gastroenteritis for the past decades until the recent advent of new and more sensitive molecular diagnostic methods. The availability and application of the molecular diagnostic methods have led to enhanced detection of noroviruses in clinical, food and environmental samples, significantly increasing the recognition of noroviruses as an etiologic agent of epidemic and sporadic acute gastroenteritis. This article aims to summarize recent efforts made for the development of molecular methods for the detection and characterization of human noroviruses.

Experimental human infection with Norwalk virus elicits a surrogate neutralizing antibody response with cross-genogroup activity

The human noroviruses (NoVs) are genetically diverse, rapidly evolving RNA viruses and are the major cause of epidemic gastroenteritis of humans. Serum antibodies that block the interaction of NoVs and NoV viruslike particles (VLPs) with host attachment factors are considered surrogate neutralizing antibodies in the absence of cell culture and small-animal replication models for the human NoVs. A serological assay for NoV-blocking antibodies was used to assess the breadth of the heterotypic antibody response in the context of an experimental challenge study with a human NoV. Heterotypic histo-blood group antigen (HBGA)-blocking activity against GI.4, GI.7, and GII.4 NoVs increased significantly in the serum of individuals (n = 18) infected with Norwalk virus (GI.1). Although the fold increases and peak titers of heterotypic antibody were more modest than titers of antibody reactive with the challenge antigen, Norwalk virus infection elicited a serological rise even against the novel Sydney variant of GII.4 NoVs. These observations indicate that the development of a broadly cross-protective NoV vaccine containing a limited number of genotypes may be possible.

Norovirus: an overview

Although noroviruses (NoVs) were the first viral agents linked to gastrointestinal disease, for a long time they have been considered secondary cause of gastroenteritis, second to rotaviruses as etiologic agents. The development of molecular techniques in diagnosing NoV provided a clearer insight into the epidemiological impact of these viruses, which are currently recognized not only as the leading cause of non-bacterial gastroenteritis outbreaks, but also as a major cause of sporadic gastroenteritis in both children and adults. This review focuses on the required knowledge to understand their morphology, genetics, transmission, pathogenesis, and control. Since no vaccine is available, prevention of NoV infection relies mainly on strict community and personal hygiene measures.

Minimum Infective Dose of the Major Human Respiratory and Enteric Viruses Transmitted Through Food and the Environment

Viruses are a significant cause of morbidity and mortality around the world. Determining the minimum dose of virus particles that can initiate infection, termed the minimum infective dose (MID), is important for the development of risk assessment models in the fields of food and water treatment and the implementation of appropriate infection control strategies in healthcare settings. Both respiratory and enteric viruses can be shed at high titers from infected individuals even when the infection is asymptomatic. Presence of pre-existing antibodies has been shown to affect the infectious dose and to be protective against reinfection for many, but not all viruses. Most respiratory viruses appear to be as infective in humans as in tissue culture. Doses of <1 TCID₅₀ of influenza virus, rhinovirus, and adenovirus were reported to infect 50% of the tested population. Similarly, low doses of the enteric viruses, norovirus, rotavirus, echovirus, poliovirus, and hepatitis A virus, caused infection in at least some of the volunteers tested. A number of factors may influence viruses' infectivity in experimentally infected human volunteers. These include host and pathogen factors as well as the experimental methodology. As a result, the reported infective doses of human viruses have to be interpreted with caution.

Immunology of Norovirus Infection

Noroviruses are the leading cause of epidemic non-bacterial gastroenteritis worldwide. Despite their discovery over three decades ago, little is known about the host immune response to norovirus infection. The purpose of this chapter is to review the field of norovirus immunology and discuss the contributions of outbreak investigations, human and animal challenge studies and population-based studies. This chapter will survey both humoral and cellular immunity as well as recent advances in norovirus vaccine development.

Murine noroviruses comprising a single genogroup exhibit biological diversity despite limited sequence divergence

Viruses within the genus Norovirus of the family Caliciviridae are the major cause of acute, nonbacterial gastroenteritis worldwide. Human noroviruses are genetically diverse, with up to 57% divergence in capsid protein sequences, and comprise three genogroups. The significance of such genetic diversity is not yet understood. The discovery of murine norovirus (MNV) and its ability to productively infect cultured murine macrophages and dendritic cells has provided an opportunity to determine the functional consequences of norovirus diversity in vitro and in vivo. Therefore, we compared the full-length genomes of 21 new MNV isolates with five previously sequenced MNV genomes and demonstrated a conserved genomic organization consisting of four open reading frames (ORFs) and a previously unknown region of nucleotide conservation in ORF2. A phylogenetic analysis of all 26 MNV genomes revealed 15 distinct MNV strains, with up to 13% divergence at the nucleotide level, that comprise a single genotype and genogroup. Evidence for recombination within ORF2 in several MNV genomes was detected by multiple methods. Serological analyses comparing neutralizing antibody responses between highly divergent strains suggested that the MNV genogroup comprises a single serotype. Within this single genogroup, MNV strains exhibited considerable biological diversity in their ability to grow in culture and to infect and/or persist in wild-type mice. The isolation and characterization of multiple MNV strains illustrate how genetic analysis may underestimate the biological diversity of noroviruses and provide a molecular map for future studies of MNV biology.

Pathogenesis of a genogroup II human norovirus in gnotobiotic pigs

We evaluated the gnotobiotic (Gn) pig as a model to study the pathogenesis of human norovirus (HuNoV) and to determine the target cells for viral replication. Sixty-five Gn pigs were inoculated with fecal filtrates of the NoV/GII/4/HS66/2001/US strain or with pig-passaged intestinal contents (IC) and euthanized acutely (n = 43) or after convalescence (n = 22). Age-matched Gn piglets (n = 14) served as mock-inoculated controls. Seventy-four percent (48/65) of the inoculated animals developed mild diarrhea compared to 0 of 14 controls. Pigs from postinoculation days (PID) 1 to 4 tested positive for HuNoV by reverse transcription-PCR of rectal swab fluids (29/65) and IC (9/43) and by antigen (Ag) enzyme-linked immunosorbent assay (ELISA) using antiserum to virus-like particles of HuNoV GII/4. No control pigs were positive. Histopathologic examination showed mild lesions in the proximal small intestine of only one pig (1/7). Seroconversion after PID 21 was detected by antibody ELISA in 13 of 22 virus-inoculated pigs (titers, 1:20 to 1:200) but not in controls. Immunofluorescent microscopy using a monoclonal antibody to HuNoV GII capsid revealed patchy infection of duodenal and jejunal enterocytes of 18 of 31 HuNoV-inoculated pigs with a few stained cells in the ileum and no immunofluorescence (IF) in mock-inoculated controls. Immunofluorescent detection of the viral nonstructural N-terminal protein antigen in enterocytes confirmed translation. Transmission electron microscopy of intestines from HuNoV-inoculated pigs showed disrupted enterocytes, with cytoplasmic membrane vesicles containing calicivirus-like particles of 25 to 40 nm in diameter. In summary, serial passage of HuNoV in pigs, with occurrence of mild diarrhea and shedding, and immunofluorescent detection of the HuNoV structural and nonstructural proteins in enterocytes confirm HuNoV replication in Gn pigs.

Multivalent norovirus vaccines induce strong mucosal and systemic blocking antibodies against multiple strains

Noroviruses are important agents of human gastroenteritis characterized by extensive sequence variation in the major capsid structural protein that likely encodes critical antigenic determinants of protective immunity. The lack of an infection model has limited detailed characterizations of viral antigenic relationships and identification of the essential components for protective immunity. This information would contribute to efficacious vaccine design against a broad array of norovirus strains. To understand the extent of heterotypic norovirus antibody specificity to inter- and intra-genogroup strains and its applicability to vaccine design, we collected sera from humans infected with different norovirus strains and from mice inoculated with alphavirus vectors expressing strain-specific recombinant norovirus-like particles (VLPs). We used VLPs that were assembled from Norwalk virus (NV), Hawaii virus (HV), Snow Mountain virus (SM) and Lordsdale virus (LV) as antigens to define and compare heterotypic antibody responses in humans and mice. We also examined if these heterotypic antibodies could block specific binding of ABH histo-blood group antigens, putative receptors for norovirus binding and entry, to norovirus VLPs. Furthermore, we examined the effect of multivalent inocula on the specificity, titer, and ligand blockade properties of systemic and mucosal norovirus-specific antibodies in mice. Our studies suggest that infection with one of several different genogroup I (GI) strains in humans induces heterotypic antibodies that block NV binding to ABH antigens, although comparable findings were not evident following infection with genogroup (GII) strains. Additionally, inoculating mice with vaccine cocktails encoding multiple norovirus VLPs enhances heterotypic and ligand attachment-blocking antibody responses against the LV strain not included in the cocktail. These data suggest that multivalent vaccination may provide better protection from a broader range of noroviruses than monovalent vaccination.

Characterization of the homo- and heterotypic immune responses after natural norovirus infection

Noroviruses (NoV) are a genetically and antigenically diverse group of viruses that are common causes of outbreaks of gastroenteritis in humans of all ages. Limited information has been obtained on type specificity of the NoV immune response. In this study, we characterized the homologous and heterologous antibody responses in adults from 13 outbreaks, representing 4 different NoV genotypes. NoV specific IgG and IgA antibodies were determined as well as the increase of antibody avidity. In addition, antibody-mediated blocking of NoV binding to its putative receptor was evaluated. Both homologous and heterologous serological responses were detected after NoV infection. The avidity of antibodies could not be used to distinguish between homologous and heterologous antibody responses. However, a homologous blocking response but not a heterologous response was detected after infection with NoV belonging to genogroup II.4 by a NoV ligand binding inhibition assay. Infection with NoV induces antibodies that can block virus ligand interactions. In contrast with all currently known antibody detection assays for NoV, this can be used as a type specific assay and may be an alternative for studying neutralizing antibodies.

Immune responses to bovine norovirus-like particles with various adjuvants and analysis of protection in gnotobiotic calves

We evaluated serum and fecal antibody responses to bovine norovirus (BoNoV) virus-like particles (VLP) and protection against virulent BoNoV in gnotobiotic calves. Calves were vaccinated with two or three doses of VLPs (250 microg/dose) coadministered with oil, mutant E. coli heat-labile toxin (R192G) (mLT) or immunostimulating complexes (ISCOM). Fecal IgA antibody was detected only in calves vaccinated intranasally with VLP+mLT and after challenge, partial protection with delayed, shortened diarrhea (1-2 days) was observed only in these calves. Diarrhea presented from 2 to 6 days in calves vaccinated with VLP with oil or ISCOM and 8 to 9 days in controls. Virus shedding was detected post-challenge by ELISA in all vaccinated calves. All calves recovered from BoNoV infection had high titers of fecal IgA antibodies and were completed protection against challenge. We conclude that only BoNoV VLP+mLT given intranasally stimulated both serum and fecal IgA antibodies and partial protection.

Identification of Genogroup I and Genogroup II broadly reactive epitopes on the norovirus capsid

Norwalk virus, a member of the family Caliciviridae, is an important cause of acute epidemic nonbacterial gastroenteritis. Norwalk and related viruses are classified in a separate genus of Caliciviridae called Norovirus, which is comprised of at least three genogroups based on sequence differences. Many of the currently available immunologic reagents used to study these viruses are type specific, which limits the identification of antigenically distinct viruses in detection assays. Identification of type-specific and cross-reactive epitopes is essential for designing broadly cross-reactive diagnostic assays and dissecting the immune response to calicivirus infection. To address this, we have mapped the epitopes on the norovirus capsid protein for both a genogroup I-cross-reactive monoclonal antibody and a genogroup II-cross-reactive monoclonal antibody by use of norovirus deletion and point mutants. The epitopes for both monoclonal antibodies mapped to the C-terminal P1 subdomain of the capsid protein. Although the genogroup I-cross-reactive monoclonal antibody was previously believed to recognize a linear epitope, our results indicate that a conformational component of the epitope explains the monoclonal antibody's genogroup specificity. Identification of the epitopes for these monoclonal antibodies is of significance, as they are components in a commercially available norovirus-diagnostic enzyme-linked immunosorbent assay.

Phylogenetic analysis of the complete genome of 18 Norwalk-like viruses

"Norwalk-like viruses" (NLV), a member of the family Caliciviridae, are the major causative agents of acute gastroenteritis and are genetically divided into two groups, genogroup I (GI) and genogroup II (GII). We have determined the complete nucleotide sequences of 10 new NLV strains. Using this information together with eight known NLV sequences, the criteria to further classify genotypes of NLV were investigated. Validation of the topological error based on the bootstrap value and the branch length (distance) allowed us to identify two potential subgenomic regions suitable for the genotyping. They were the putative 3D-like RNA-dependent RNA polymerase (polymerase) and the capsid N-terminal/Shell domains (capsid N/S domain). When the distance distribution analysis was performed, the polymerase-based classification did not separate the strains into internal clusters within the genogroup. Furthermore, a diversity plot analysis of the complete nucleotide sequences of WUG1, a NLV GI strain, and Saitama U1, a NLV GII strain, indicated that the genotype was different between the polymerase and capsid N/S domain, suggesting that these strains are the genetic recombinants. Therefore, polymerase is not suitable for genotyping. On the other hand, the clustering based on the capsid N/S domain successfully distinguished the NLV as well as the grouping based on the antigenicity, as determined by both antigen and antibody ELISAs with recombinant virus-like particles. As the nucleotide sequences of the primers for the capsid N/S domain are highly conserved among the NLV, the amplification of the unknown genotype can be easily performed. This method will facilitate global surveying as well as epidemiologic study on NLV.

Human caliciviruses in Europe

Caliciviruses are single-stranded RNA viruses, which are divided into four genera based on their morphology and genomic structure. Viruses from two genera, the Norwalk like viruses and Sapporo like viruses, are a common cause of acute, nonbacterial gastroenteritis in humans. Although the first human calicivirus discovered nearly 30 years ago, much of the epidemiological and biological character of these viruses is only now beginning to unfold. Investigation has been difficult due to a number of factors, the viruses cannot be amplified by in vitro cell culture or animal models and electron microscopy (EM) is often not sensitive enough to detect the viruses in stool samples. Recent advances in molecular diagnostic techniques and the advent of a baculovirus expression system have highlighted the clinical and public health importance of calicivirus in all age groups, their ability to cause infection via a number of transmission routes as well as their considerable genetic diversity. These characteristics, in conjunction with the inability of humans to develop long-term immunity make HuCV an important public health issue in Europe and worldwide.

Characterization of capsid genes, expressed in the baculovirus system, of three new genetically distinct strains of "Norwalk-like viruses"

"Norwalk-like viruses" (NLVs), members of a newly defined genus of the family Caliciviridae, are the most common agents of outbreaks of gastroenteritis in the United States. Two features of NLVs have hindered the development of simple methods for detection and determination of serotype: their genetic diversity and their inability to grow in cell culture. To assess the immune responses of patients involved in outbreaks of gastroenteritis resulting from infection with NLVs, we previously used recombinant-expressed capsid antigens representing four different genetic clusters, but this panel proved insufficient for detection of an immune response in many patients. To extend and further refine this panel, we expressed in baculovirus the capsid genes of three additional genetically distinct viruses, Burwash Landing virus (BLV), White River virus (WRV), and Florida virus. All three expressed proteins assembled into virus-like particles (VLPs) that contained a full-length 64-kDa protein, but both the BLV and WRV VLPs also contained a 58-kDa protein that resulted from deletion of 39 amino acids at the amino terminus. The purified VLPs were used to measure the immune responses in 403 patients involved in 37 outbreaks of acute gastroenteritis. A majority of patients demonstrated a fourfold rise in the titer of immunoglobulin G to the antigen homologous to the outbreak strain, but most seroconverted in response to other genetically distinct antigens as well, suggesting no clear pattern of type-specific immune response. Further study of the antigenicity of the NLVs by use of VLPs should allow us to design new detection systems with either broader reactivity or better specificity and to define the optimum panel of antigens required for routine screening of patient sera.

Diagnosis of noncultivatable gastroenteritis viruses, the human caliciviruses

Gastroenteritis is one of the most common illnesses of humans, and many different viruses have been causally associated with this disease. Of those enteric viruses that have been established as etiologic agents of gastroenteritis, only the human caliciviruses cannot be cultivated in vitro. The cloning of Norwalk virus and subsequently of other human caliciviruses has led to the development of several new diagnostic assays. Antigen detection enzyme immunoassays (EIAs) using polyclonal hyperimmune animal sera and antibody detection EIAs using recombinant virus-like particles have supplanted the use of human-derived reagents, but the use of these assays has been restricted to research laboratories. Reverse transcription-PCR assays for the detection of human caliciviruses are more widely available, and these assays have been used to identify virus in clinical specimens as well as in food, water, and other environmental samples. The application of these newer assays has significantly increased the recognition of the importance of human caliciviruses as causes of sporadic and outbreak-associated gastroenteritis.

Immunity to calicivirus infection

The evolution of our understanding of immunity to calicivirus infection, using Norwalk virus as the prototype, is discussed in three stages: (1) "ancient times (1972-1978), when human volunteer studies prevailed, (2) the "middle ages (1978-1990), which were characterized by the development and implementation of solid-phase immunoassays based on native viral antigens, and (3) "modern times (1990 to present), which began with the cloning of the genome of the noncultivatable 8FIIa strain of Norwalk virus and resulted in a readily available source of recombinant virus-like particles that have revolutionized the study of caliciviruses. Throughout these stages, it has been shown repeatedly that short-term immunity develops to homologous virus. However, the search for determinants of long-term immunity continues. These studies will likely be facilitated by the newest reagents-the noninfectious recombinant virus-like particles-used in the setting of human volunteer studies and large epidemiologic studies.

Genetic classification of "Norwalk-like viruses

Reverse transcription-polymerase chain reaction has been used worldwide for the diagnosis of Norwalk-like virus (NLV) infection, yet a commonly accepted genetic classification scheme has not been established. Amino acid sequences from four regions of open-reading frame 2 (ORF2) were used to analyze 101 NLV strains, including 2 bovine strains. On the basis of this analysis, a genetic classification scheme is proposed that differentiates 99 human strains into 2 major genetic groups consisting of 5 and 10 genetic clusters, respectively. The 2 bovine strains constitute a newly defined third major genetic group composed of 2 putative clusters represented by each strain. This classification scheme is well supported by the analysis of the entire ORF2 sequences from 38 strains selected to represent the genetic diversity of the human strains used above. This scheme should provide a firm scientific basis for the unified classification of NLV strains detected around the world.

Norwalk-like virus infection in military forces: epidemic potential, sporadic disease, and the future direction of prevention and control efforts

The impact of Norwalk-like virus (NLV) infection on military forces is evaluated in this report. NLVs were a major cause of both outbreaks and sporadic disease among crowded US ground troops in the 1991 war with Iraq. NLVs also have been found to be a cause of acute gastroenteritis in other ground and shipboard deployments. Four large outbreaks of acute gastroenteritis were investigated aboard US Navy aircraft carriers between 1992 and 1997. In these outbreaks, NLVs were identified as the probable cause, and crowding was a major risk factor for transmission. An evaluation of a routine shipboard deployment also suggests that NLVs cause sporadic gastroenteritis. These data indicate that NLV infection is a major cause of acute morbidity in military forces. Because of the limitations of available prevention and control methods, development of a vaccine against these viruses may be the best solution in the military environment.

Expression and self-assembly of recombinant capsid protein from the antigenically distinct Hawaii human calicivirus

The Norwalk and Hawaii viruses are antigenically distinct members of the family Caliciviridae and are considered to be important etiologic agents of epidemic gastroenteritis, with most studies focusing on the role of Norwalk virus. To further investigate the importance of Hawaii virus, Hawaii virus-like particles (VLPs) were produced by expression of its capsid protein in the baculovirus system and these VLPs were used as the antigen in an enzyme-linked immunosorbent assay that was efficient in the detection of a serologic response to Hawaii virus. The ready availability of Hawaii VLPs should enable larger-scale epidemiological studies to further elucidate the importance of this agent.

An epidemiological investigation of Norwalk virus infection in South Africa

A study was carried out to determine the incidence and seroprevalence of Norwalk virus (NV) in the Pretoria area, South Africa, using a recombinant NV (rNV) immunoassay for the detection of serum IgG antibodies. Maternal antibody was detectable in infants' sera up to approximately 6 months of age. Infection with NV was detected serologically in the second year of life and the seroprevalence of NV IgG rose from 37.1% at 7-11 months of age to 62.1% by the age of 40 years. No significant differences in seroprevalence of NV IgG antibody was evident between subjects of European or African ethnic origin, where overall seroprevalence rates were 56.4% and 53.9% respectively.

Enzyme immunoassay using baculovirus-expressed human calicivirus (Mexico) for the measurement of IgG responses and determining its seroprevalence in London, UK

The use of an enzyme immunoassay (EIA) employing a baculovirus-expressed recombinant human calicivirus (Mexico virus, MxV) for the detection of IgG-specific antibodies is described. MxV appeared to be related antigenically to a strain of small round structured virus, SRSV/UK4/Leeds/91, which had previously been shown by solid phase immune electron microscopy (SPIEM) to be related to Snow Mountain agent (SMA). One other outbreak which occurred in San Anita, USA in 1980 and was due to consumption of contaminated water was caused by a virus antigenically related to MxV. Volunteers and patients who developed significant IgG responses to rMxV showed anamnestic IgG responses (2 to 4-fold) in the recombinant Norwalk virus (rNV) IgG assay. Patients and volunteers who were known to have been infected with several other strains of calicivirus/small round spherical viruses (SRSV) including NV and SRSV UK3 showed no significant antibody response to rMxV in the EIA. A seroepidemiological survey of sera from 338 children in London showed that infection with MxV occurred earlier in life than NV. Primary infections with MxV were common after the age of 6 months. Over 70% of children had evidence of infection by the ages of 2 years, whereas only 12% of these children had been infected with NV. High concentrations of maternal antibody were present during the first month of life which was detected in 96% of the neonates. The results suggest that the high sensitivity of the EIA may be detecting maternal antibody throughout the first 8 months of life.

Expression, self-assembly, and antigenicity of a snow mountain agent-like calicivirus capsid protein

Virus-like particles were produced in insect cells infected with a recombinant baculovirus containing the capsid gene of MX virus, a Mexican strain of human calicivirus. These recombinant MX (rMX) particles were morphologically similar to recombinant Norwalk virus (rNV) particles as observed under an electron microscope and contained a single capsid protein with a molecular weight of 57,000, which was slightly smaller than that of rNV. This protein was immunoprecipitated by sera from volunteers infected with the Snow Mountain agent, but it reacted weakly with sera from volunteers infected with NV. This protein did not react with hyperimmune antisera from animals immunized with rNV in the rNV antigen enzyme immunoassay (EIA). Seroresponses were detected from volunteers infected with Snow Mountain agent and Hawaii agent when the rMX particles were used as antigen in an EIA. This EIA also detected an immune response in the sera of child from whom the MX virus was isolated, and a high prevalence of antibody to MX virus was found in the sera of a cohort of Mexican children.

Characterization of SRSVs using RT-PCR and a new antigen ELISA

Stool samples from 451 patients involved in volunteer studies, 26 outbreaks and approximately 175 sporadic cases of acute gastroenteritis from different geographical locations in the world were tested for Norwalk virus (NV) using a newly developed antigen ELISA and RT-PCR. NV was detected in most outbreaks previously characterized as being of NV origin. Overall, a low number of positives for NV was obtained using either RT-PCR with primers that amplified a unique region of the genome, or an ELISA with hyperimmune antisera made to the baculovirus-expressed recombinant NV capsid. However, a significant number of positives was obtained when these samples were tested by RT-PCR using primers that amplified the more highly conserved regions of the genome. Sequence analysis of the amplified viral cDNAs indicated that small round structured viruses (SRSVs) with a wide range of variable genomic sequences (44-87% nucleotide and 31-99% amino acid similarity to the 8Flla NV genome sequence) were responsible for these outbreaks. Several recent outbreaks from the US, Japan and the UK were related to the Snow Mountain Agent (SMA) by sequence analyses. Continued accumulation of sequence information will facilitate the design of new primers for virus detection and increase our understanding of the relationships and epidemiology of these viruses from different sources.

Sequence diversity of small, round-structured viruses in the Norwalk virus group

We have determined the nucleotide sequences of a highly conserved region of the RNA-dependent RNA polymerase of the prototype Snow Mountain agent (SMA) and of four other small, round-structured viruses (antigenically Norwalk virus [NV]-like or SMA-like) following reverse transcription-PCR amplification of viral RNA obtained from human stools. The stool samples were either from volunteers administered SMA or from sporadic cases and outbreaks of gastroenteritis that occurred in Japan and the United Kingdom between 1984 and 1992. The GLPSG and YGDD RNA polymerase motifs were in the proper locations in the sequences of the five SRSVs, but each sequence was distinct from the 8FIIa prototype NV sequence and from each other. Analysis of the sequences and reactivities in a new NV antigen enzyme-linked immunosorbent assay showed that the five viruses could be divided into two groups (serogroups) with NV and SMA, respectively, being the prototypes. The sequences of the capsid region and a nonstructural region (2C) were determined from one strain from each group. One virus (SRSV-KY-89/89/J), isolated in Japan and antigenically similar to the prototype NV (isolated 21 years earlier in Ohio), showed a remarkable level of sequence similarity to NV. KY-89 and the 8FIIa NV showed 87.2% nucleotide similarity over 2,516 continuous nucleotides amounting to 96 to 98.9% amino acid similarity in three distinct domains in two open reading frames. Between the prototype SMA and NV, the polymerase region showed 63% nucleotide and 59% amino acid similarity, respectively. Two other antigenically SMA-like isolates (SRSV-925/92/UK and SRSV-OTH-25/89/J), from the United Kingdom and Japan, showed 80% nucleotide and 88 to 92% amino acid similarity in the polymerase region to the prototype SMA isolated 16 and 13 years earlier in the United States. The capsid region of the antigenically SMA-like OTH-25 virus showed 53% nucleotide and 65% amino acid similarity to the prototype NV capsid region. Domains of sequence diversity and conversation were identified within the capsid protein of these two distinct prototype serotypes of virus. These results indicate that NV-like and SMA-like agents are still circulating, and sequence comparisons will be useful to identify and classify distinct viruses in the NV group.

Measurement of IgA responses following Norwalk virus infection and other human caliciviruses using a recombinant Norwalk virus protein EIA

An enzyme immunoassay employing recombinant Norwalk virus capsid protein was evaluated for the measurement of IgA responses. Tests on 23 volunteers and patients known to have been infected with Norwalk virus (NV) showed that 19 developed significant IgA responses, 2 had unchanging levels of IgA and 2 failed to respond. There was no evidence of IgA responses to NV following infection with Hawaii or Snow Mountain-like viruses. Tests on sera from patients involved in outbreaks associated with eating contaminated shellfish suggest that some patients may have been infected with more than one strain of calicivirus. The use of the rNV EIA for measuring IgA and IgG responses in patients involved in a major outbreak of food poisoning affecting hospital staff indicated that the causative agent was probably NV.

Sequence similarity of human caliciviruses and small round structured viruses

The application of reverse transcription-polymerase chain reaction (RT-PCR) using primers directed to the RNA dependent RNA polymerase region within ORF1 of Norwalk virus (NV) showed that 31 percent of morphologically typical human caliciviruses (HuCV) and 57% of small round structured viruses (SRSVs) produced a product of 470 bp similar to the NV control, NV 8FIIa/68/US. Alignment of the amino acid sequences of morphologically typical HuCVs with previously published sequences for SRSVs, NV, and Snow Mountain agent (SMA) showed a high degree of homology (90-92%) with SMA and a lesser extent of homology with NV (60-61%). The amino acid sequence of two strains of HuCV, HuCV/3C/92/UK, and HuCV/5C/92/UK differed by only one or two amino acids respectively in the RNA dependent RNA polymerase region from that of two strains of SRSV obtained from children in the United Kingdom, SRSV/4S/90/UK and Japan, SRSV/OTH-25/89/J which were found to have identical amino acid sequences. The use of an EIA for detection of NV antigen employing antisera raised to recombinant NV protein indicated that HuCVs and SRSVs obtained from children and adults in the United Kingdom were antigenically distinct from the prototype Norwalk virus, NV/8fIIa/68/US.

Detection of antibody to recombinant Norwalk virus antigen in specimens from outbreaks of gastroenteritis

Norwalk virus and other small round-structured viruses are commonly associated with outbreaks of gastroenteritis. We used a recently described recombinant-expressed Norwalk virus (rNV) capsid protein in enzyme immunoassays to quantitatively measure immunoglobulin G (IgG) and IgA to Norwalk virus in serum pairs from patients involved in outbreaks of gastroenteritis. The outbreaks previously were classified, on the basis of the results of a blocking antibody assay, as Norwalk virus negative, serologically intermediate, or Norwalk virus positive. The rNV IgG assay was more sensitive than the blocking assay for detecting IgG to Norwalk virus in serum from patients in all outbreak classes. There was 79% concordance between seroconversions detected by the blocking antibody assay and those detected by the rNV IgG assay. The rNV IgA assay detected seroconversions to Norwalk virus primarily in patients involved in outbreaks previously classified as Norwalk virus positive.

Viruses and diarrhoea--where are we now?

Viruses are important causes of diarrhoea, with the major mortality occurring in the poor tropical overcrowded parts of the world. A successful vaccine, to rotavirus at least, may be developed, but how widely it would be used is uncertain. Even if successful, it would not remove all virus-associated diarrhoea and vomiting any more than a successful influenza vaccine would remove all viral respiratory disease. Perhaps the one aspect that needs most attention is the host. It is evident that not all infections lead to disease and that this is not simply related to the amount of virus in the faeces. This could be an indicator of the amount of damage--more virus coming from more infected cells--but there appear to be similar amounts of virus in "normal" stools as in diarrhoeal ones. Why is it then that some host, some babies, and not others have diarrhoea and vomiting? Is there an important, and as yet unrecognized, difference? If there is and it can be identified, then finding how to induce it or increase it in young babies could be as effective as a vaccine.

Subclass-specific serum antibody responses to recombinant Norwalk virus capsid antigen (rNV) in adults infected with Norwalk, Snow Mountain, or Hawaii virus

Subclass-specific antibody responses to the Norwalk virus capsid protein in adults challenged with Norwalk, Snow Mountain, or Hawaii virus were evaluated by solid-phase enzyme immunoassay using recombinant Norwalk virus capsid antigen (rNV). Fourfold or greater serum immunoglobulin G (IgG) antibody responses to rNV were detected in 15 of 20 volunteers challenged with Norwalk virus, and serum IgA and IgM antibody responses to rNV were seen in almost all subjects who had rNV IgG responses. Serum rNV IgG antibody responses also were detected in 6 of 15 volunteers challenged with Snow Mountain virus and 2 of 12 volunteers challenged with the Hawaii virus. However, the magnitude of antibody response and the geometric mean postchallenge rNV IgG antibody titers were lower in subjects challenged with Snow Mountain or Hawaii virus, and serum IgA and IgM responses generally did not occur.

Prevalence of antibodies to Norwalk virus in England: detection by enzyme-linked immunosorbent assay using baculovirus-expressed Norwalk virus capsid antigen

A total of 3,250 serum specimens collected in England in 1991 and 1992 were tested by an indirect enzyme-linked immunosorbent assay for antibody to Norwalk virus using baculovirus-expressed capsid antigen, and 2,382 (73.3%) were positive. The prevalence of Norwalk virus antibody differed regionally. It was lowest (24.6%) in 6- to 11-month-old infants and increased to 89.7% in persons over 60 years old.

[Acute diarrhea associated viruses : An update]

Viruses recovered in stools are either cultivable viruses (enteroviruses, adenoviruses excepted type 40 and 41), or "fastidious" non cultivable viruses (rotaviruses adenoviruses 40 and 41, Norwalk, calcivirus, astrovirus, SRSV and SRV). Non cultivable viruses have been associated with many cases of diarrhea. Norwalk, two strains of calicivirus and SRV/SRSV, appear to be capable of causing outbreak. Rotavirus, astrovirus and most fastidious adenoviruses are associated with endemic spread. Specific or catch-all methods are used for diagnosis. Among the latter, electron microscopy is the most commonly used when the virus is recognizable and present in sufficient quantities. Small spherical viruses in the range 20-35 nm present greater difficulties. Polyacrylamide gel electrophoresis gives interesting epidemiological results for rotavirus. Specific methods are latex agglutination and enzyme immunoassays essentially for rotavirus and adenoviruses (all types or only 40 and 41). False positive results are few with well-designed kits. False negative results are seen in atypical strains and antigenic variants. In an outbreak, it is essential to make electron microscopic examinations. In individual cases, if no electron microscope is available, it is possible to make the diagnosis of rotavirus - and perhaps adenovirus 40 and 41 with a commercial kit. However a small number of stools contain more than one virus and they may act in synergy. In contrast many asymptomatic children may carry viruses.