Efficacy of a recombinant Norwalk virus protein enzyme immunoassay for the diagnosis of infections with Norwalk virus and other human "candidate" caliciviruses

Norovirus infections and seroprevalence of genotype GII.4-specific antibodies in a Spanish population

Genotype II.4 noroviruses (NoVs) are a leading cause of epidemic acute gastroenteritis in children and adults worldwide. The prevalence of different NoV genotypes causing outbreaks and sporadic cases of acute gastroenteritis in the region of Valencia, Spain, during a 4-year period (2008-11) was investigated. NoVs were detected in 42 out of 55 (76.3%) outbreaks and in 26 out of 332 (7.8%) sporadic cases of acute gastroenteritis. Genogroup GII strains were predominant in outbreaks and sporadic cases. Different genotype GII.4 variants were found (Yerseke_2006a, Den Haag_2006b, Apeldoorn_2007, and New Orleans_2009), with the latter variant detected most frequently (35.3%). A recombinant P domain of the NoV GII.4 Apeldoorn_2007 variant was produced in Escherichia coli and used as the coating antigen in an enzyme immunoassay to survey the IgG antibody seroprevalence against NoV GII.4 in a Spanish population. Baculovirus-expressed virus-like particles (VLPs) of NoV GII.4 Den Haag_2006b variant were also produced and used as antigen to compare seroreactivity. Of the 434 serum specimens analyzed, 429 (98.6%) had antibodies against the P domain. The comparison of reactivities of 30 serum samples to the NoV GII.4 P polypeptide and VLP showed reproducible results with a correlation coefficient of r = 0.794. Titers of antibodies to the P domain increased gradually and significantly with age, reaching the highest levels at the age group of 41-50 years. These results confirm the high prevalence of NoV GII.4 infections in Valencia from early childhood.

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.

Genetic and phenotypic characterization of GII-4 noroviruses that circulated during 1987 to 2008

The predominance and continual emergence of new variants in GII-4 noroviruses (NVs) in recent years have raised questions about the role of host immunity and histo-blood group antigens (HBGAs) in NV evolution. To address these questions, we performed a genetic and phenotypic characterization of GII-4 variants circulating in the past decade (1998 to 2008). Ninety-three GII-4 sequences were analyzed, and of them, 16 strains representing 6 genetic clusters were selected for further characterization. The HBGA binding properties were determined by both saliva- and oligosaccharide-binding assays using P particles as a model of NV capsid. The antigenic properties were also examined by enzyme immunoassay (EIA), Western blot analysis, and receptor blocking assay, using P-particle-specific antibodies from immunized mice and GII-4 virus-infected patients. Our results showed that 15 of the 16 GII-4 viruses bound to saliva of all A, B, and O secretors. Oligosaccharide binding assays yielded largely consistent results, although the binding affinities to some oligosaccharides varied among some strains. The only nonbinder had a mutation in the binding site. While antigenic variations were detected among the 16 strains, significant cross-blocking on the HBGA binding was also noted. Sequence alignment revealed high conservation of HBGA binding interfaces with some variations in adjacent regions. Taken together, our data suggested that the ability of GII-4 to recognize different secretor HBGAs persisted over the past decade, which may explain the predominance of GII-4 over other genotypes. Our data also indicated that both the host immunity and HBGAs play a role in NV evolution. While host immunity may continue driving NV for antigenic change, the functional selection by the HBGAs tends to lock the architecture of the capsid/HBGA interfaces and allows only limited variations outside the HBGA binding sites. A potential outcome of such counterselection between theses two factors in NV evolution is discussed.

Surveillance and risk factors of norovirus gastroenteritis among children in a southern city of China in the fall-winter seasons of 2003-2006

AIM

Noroviruses (NoVs) are an important cause of acute gastroenteritis but knowledge on the disease burden and epidemiology in children in the developing countries remains limited. In this study, we performed a surveillance of NoV gastroenteritis in children of China to address some of the questions.

METHODS

Faecal specimens from children (<5 years of age) at outpatient clinics of the Nan Fang Hospital in Guangzhou, China during the fall-winter seasons in 2003-2006 were tested for rotaviruses (RVs) and NoVs. A questionnaire on clinical records and hygiene habits was collected from each patient.

RESULTS

Among 957 stool specimens tested, 488 (51%) specimens were positive for RVs. NoVs were detected in 112 (24%) of the 469 RV negative specimens. The Genogroup II (GII), particularly GII-4, viruses were predominant. No significant difference of clinical symptoms, hospitalisation and patient care expenses were found between children infected with NoVs and RVs. Consumption of uncooked food is a risk for NoV infection. Contact with diarrhoea patients is a suspected risk factor. Cutting nails frequently is a protective factor against NoV infection.

CONCLUSIONS

NoVs are an important cause of acute gastroenteritis in children which need special attention of patient care at the clinics in addition to RVs. The awareness of those risk factors may help future disease control and prevention.

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.

E. coli-expressed recombinant norovirus capsid proteins maintain authentic antigenicity and receptor binding capability

The baculovirus expression system has been widely used to produce the capsid proteins of Norovirus (NV) and the proteins form virus-like particles (VLPs) that are useful in many studies, such as immunology, diagnosis, and host-receptor interaction. We report here the application of the E. coli expression system in the production of recombinant NV capsid proteins. In a direct comparison of a previous well-characterized NV strain (VA387), we have demonstrated that the E. coli-expressed capsid proteins maintain the same antigenicity and receptor binding specificity as that of the baculovirus-expressed capsid, although the E. coli-expressed VA387 proteins did not form VLPs. Using the E. coli-expression system, we characterized the receptor-binding patterns of three additional NV strains (OIF1998, Parris Island and VA115), in which OIF1998 binds to HBGA of nonsecretors but did not bind or binds weakly to the HBGA of secretors, as seen in strain VA207. Parris Island binds to HBGA of types A and B but not type O secretors and nonsecretors. VA115 did not show specific binding to any A, B, O secretor nor nonsecretor, which is also observed when the capsid protein of this strain was expressed in baculovirus. Our data indicate that VLP formation is not required for receptor binding, and that the bacteria expression system offers a simple alternative for large production of NV capsid protein for various research purposes, particularly for strains generating low yields in the insect cells.

Expression of recombinant capsid proteins of chitta virus, a genogroup II Norwalk virus, and development of an ELISA to detect the viral antigen

The second open reading frame (ORF2) gene of the Chitta virus (CHV) was cloned to construct a recombinant baculovirus. The CHV ORF2 is predicted to encode a capsid protein of 535 amino acids (aa). CHV showed a high aa identity in the capsid region with genogroup II Norwalk virus (NV) (65-85%), but a low aa identity with genogroup I NV (44-46%). Phylogenetic analysis of the ORF2 gene demonstrated that CHV is genetically closely related to the Hawaii virus included in genogroup II NV. The recombinant capsid protein of CHV (rCHV) self-assembled to form empty virus-like particles (VLPs) when expressed in insect cells with the recombinant baculovirus. An enzyme-linked immunosorbent assay (ELISA) based on antisera to rCHV was developed to detect CHV antigen in stools. The antigen ELISA appeared to be highly specific to both rCHV and CHV-like strains. In addition, combined use of antigen ELISAs using antibodies against two antigenically distinct recombinant VLPs, the recombinant Chiba virus (rCV) and recombinant Seto virus (rSEV), enabled us to determine the genetic as well as antigenic relationship among these three viruses.

Diagnosis of foodborne viral infections in patients

A significant global problem is the microbiological contamination of foods and water. The microorganisms associated with about half of the foodborne disease outbreaks still go unrecognized, primarily as a result of inadequate diagnostic methods and sampling. A significant amount of food- and waterborne diseases are associated with viruses, information that has been obtained only in recent years. Improved diagnostic methods have established that caliciviruses are the most important non-bacterial pathogens associated with food- and waterborne outbreaks, and are the major cause of seafood-associated gastroenteritis.

The epidemiology of enteric caliciviruses from humans: a reassessment using new diagnostics

In the United States, acute gastroenteritis is one of the most commonly noted illnesses on hospital discharge records and death certificates, yet few of these cases have an etiologic diagnosis. The application of new molecular diagnostic methods has shown caliciviruses (previously referred to as the Norwalk family of viruses or small round structured viruses) to be the most common cause of acute gastroenteritis (AGE) outbreaks in the United States, and they may emerge as a common cause of sporadic cases of AGE among both children and adults. Novel molecular methods have permitted outbreak strains to be traced back to their common source and have led to the first identification of virus in implicated vehicles of infection-water, shellfish, and foods contaminated both at their source and by food handlers. The broad application of these methods to routine diagnosis in hospitals and public health laboratories is advancing our appreciation of the full burden of calicivirus-associated diarrhea, and it is opening new avenues for its prevention and control.

Seroprevalence against Norwalk-like human caliciviruses in beijing, China

Serum specimens from 1,109 individuals at different ages in Beijing, China, were collected between April 1996 and March 1997 and tested for IgG antibodies against human caliciviruses using enzyme immune assays (EIAs). Baculovirus-expressed recombinant Norwalk virus (rNV) and Mexican virus (rMxV) capsid proteins were used as antigens. The seroprevalence was 89% for rNV and 91% for rMxV. Similar seroprevalence between the two antigens was observed in individual age groups and both genders. Infants had a high seroprevalence (99% for NV and 94% for MxV) at birth. The lowest seroprevalence (41% for rNV and 36% for rMxV) was at 7-11 months of age. A sharp increase in seroprevalence occurred in early childhood, with 65% and 70% at one, 85% and 90% at three, and 100% and 98% at 8-9 years of age for rNV and rMxV, respectively. Forty-three individuals had antibody against rNV but not rMxV and 63 individuals had antibody against rMxV but not rNV, indicating different levels of exposure to the two strains in these individuals. This is the first report of surveillance of antibodies against NV-like viruses in China. The observed high prevalence and early age antibody acquisition suggest that infection by these two human calicivirus strains is common in this population.

Seroepidemiological study of genogroup I and II calicivirus infections in South and southern Africa

Diarrhoea is associated with the daily death of between 180 and 200 children under the age of 5 years in South Africa. Until recently, many cases and outbreaks of diarrhoea were not associated with a known aetiologic agent. Previous studies using baculovirus-expressed Norwalk virus (NV) and Mexico virus (MxV) capsid antigens have shown that human calicivirus infection is common in South Africa. In this study, our surveillance was extended to different populations, as well as to four other southern African countries: Namibia, Angola, Zimbabwe, and Mozambique. More than 1,700 specimens, some involved in previous cohort studies of infectious diseases, were enrolled in the surveillance. The overall seroprevalence of antibody against NV was >90% for all cohorts except for Mozambican refugees that had 83. 8% sero-positivity. The MxV antibody prevalence was higher than NV, with >95% positivity for all cohorts, except for one in Namibia that had 81% exposure. This study is one of only a few reporting on the concurrent incidence of NV and MxV infections in a cohort study, and has determined that small round structured viruses are prevalent in the local populations of South and Southern Africa. These agents may account for a number of previously unknown or unidentified causes of diarrhoeal illness, in both adults and children, in southern Africa.

Expression and characterization of Sapporo-like human calicivirus capsid proteins in baculovirus

Sapporo-like caliciviruses reveal typical calicivirus morphology and cause acute gastroenteritis. This study describes the expression in baculovirus of capsid proteins of two Sapporo-like calicivirus strains (Hou/86 and Hou/90). Eight different constructs of the capsid genes were compared for production of the proteins. Constructs containing short (9 or 19 nt) upstream sequences failed to produce capsid proteins but extension of the upstream sequence to 73 nt resulted in production of capsid proteins. Expressed capsid protein with the MEG tri-peptide as the N-terminus self-formed virus-like particles (VLPs). Expressed protein with an upstream AUG failed to form VLPs. Addition of His-tag to the N-terminus of capsid protein also blocked VLP formation. Of three Norwalk-Hou/90 chimeric capsid gene constructs, one resulted in production of chimeric capsid and the protein did not form VLPs. Recombinant capsid proteins for each of Hou/86 and Hou/90 were further characterized. The expressed capsid antigens of the two strains were antigenically distinct but shared a common epitope(s). Further study of these proteins should allow development of immunologic assays for diagnosis and should help to clarify the epidemiology of Sapporo-like caliciviruses in humans.

Prevalence of antibodies to astrovirus types 1 and 3 in children and adolescents in Norfolk, Virginia

OBJECTIVE

To determine the prevalence of antibody to human astrovirus types 1 (HAstV-1) and 3 (HAstV-3) in children.

METHODS

Sera from children hospitalized in Norfolk, VA, for noninfectious conditions were collected for a 1-month period every 6 months from 1993 to 1996 and tested by enzyme immunoassay for antibody to HAstV-1 and HAstV-3 with the use of baculovirus-expressed recombinant capsid proteins as antigens.

RESULTS

The seroprevalence of 393 infants and children to HAstV-1 decreased from 67% in infants <3 months of age to 7% by 6 to 8 months of age, consistent with loss of transplacental antibodies. Children acquired HAstV-1 antibody with a peak prevalence of 94% at 6 to 9 years of age (P < 0.001). Antibodies to HAstV-3 exhibited a lower prevalence, with 26% positive at <3 months, 0% at 6 to 11 months and 42% by 6 to 9 years of age. HAstV-1 seroprevalence in children O to 2 months of age decreased from 89% in November, 1993, to 40% in November, 1996 (P = 0.009).

CONCLUSIONS

Astrovirus type-specific antibody prevalence can be measured by baculovirus-expressed capsid antigens in an enzyme immunoassay. Children developed antibody to HAstV-1 (94%) and to HAstV-3 (42%) by 6 to 9 years of age indicating frequent exposure to these enteric viruses in infancy and early childhood.

The role of human caliciviruses in epidemic gastroenteritis

Members of the Caliciviridae family of small, positive-sense RNA viruses exhibit a broad host range. The Norwalk and Norwalk-like caliciviruses in this family are major etiologic agents of epidemic gastroenteritis in humans. This illness characteristically lasts 24-48 h and often occurs in group settings such as families, schools, institutions, or communities. The spread of the human caliciviruses is considered to be predominantly by person-to-person contact via the fecal-oral-route. However, the ingestion of calicivirus-contaminated food or water can result in large-scale common-source outbreaks. Many basic features concerning the biology and replication of the human caliciviruses are not known because they have not yet been grown in cell culture and the virus does not appear to replicate in animal models other than the chimpanzee. Sequence analysis of RT-PCR-generated DNA fragments derived from serotypically distinct reference strains (such as the Norwalk, Hawaii, and Snow Mountain viruses) and other circulating strains associated with gastroenteritis has provided evidence for marked genetic diversity among these viruses. Moreover, analysis of the antigenic relationships among these viruses using paired sera from individuals infected with well-characterized reference strains or from animals immunized with recombinant "virus-like particles" (VLPs) suggests that several serotypes of these viruses are circulating worldwide. The availability of molecular techniques for the detection of these fastidious viruses has enabled epidemiologic studies that have strengthened the association of human caliciviruses with acute gastroenteritis and has demonstrated a potential role for antigenic diversity in the natural history of these pathogens.

Correlation of patient immune responses with genetically characterized small round-structured viruses involved in outbreaks of nonbacterial acute gastroenteritis in the United States, 1990 to 1995

Small round-structured viruses (SRSVs) are a genetically and antigenically diverse group of caliciviruses that are the most common cause of outbreaks of acute nonbacterial gastroenteritis. We have applied both molecular techniques to characterize SRSVs in fecal specimens and serologic assays using four different expressed SRSV antigens to examine the distribution of outbreak strains in the United States and determine if the immune responses of patients were strain specific. Strains from 23 outbreaks of SRSV gastroenteritis were characterized by reverse transcription-PCR and nucleotide sequencing of a 277-base region of the capsid gene. These strains segregated into two distinct genogroups, I and II, comprising four and six clusters of strains respectively, each representing a distinct phylogenetic lineage. Serum IgG responses in patients were measured by enzyme immunoassay using expressed capsid antigens of Norwalk virus (NV), Toronto virus (TV), Hawaii virus (HV), and Lordsdale virus (LV), representing four of the 10 clusters. While strains in genogroups I and II were antigenically distinct, within genogroups, the specificity of the immune response varied greatly. Patients infected with genogroup I strains which had as much as 38.5% aa divergence from NV demonstrated relatively homologous seroresponses to the single NV antigen. In contrast, in genogroup II, homologous seroresponses to TV and HV were only present when the infecting strains showed less than 6.5% aa divergence from these antigens. These results suggest that TV and HV represent not only separate genetic clusters in genogroup II but also separate antigenic groups, each of which is related but distinguishable. In addition, two genetically distinct SRSV strains were identified for which we have no homologous antigen. This study suggests that while current molecular diagnostics are capable of detecting the full range of SRSVs, additional expressed antigens will be required to detect an immune response to SRSV infection caused by all the antigenically diverse strains.

Study of Norwalk virus and Mexico virus infections at Ga-Rankuwa Hospital, Ga-Rankuwa, South Africa

Small round-structured viruses (SRSVs) or Norwalk-like viruses have been implicated as an important causative agent of gastroenteritis outbreaks. We used the relatively newly developed recombinant enzyme immunoassays (EIAs) to determine the seroprevalence of Norwalk virus (NV) and Mexico virus (MxV) in a family-based cohort and an antenatal clinic cohort at Ga-Rankuwa Hospital, Ga-Rankuwa, South Africa. High prevalences (96 to 99%) of anti-NV and anti-MxV antibodies were detected in both cohorts. We also investigated the pattern of antibody acquisition in a cohort of infants and young children without gastroenteritis and found that by 48 months of age all children had acquired adult antibody levels to both these viruses. Lastly, we tested 276 stool specimens collected from infants and young children with gastroenteritis for the presence of NV or MxV antigen by recombinant EIAs to each virus, by electron microscopy (EM), and by reverse transcription (RT)-PCR. NV and MxV antigens were present in 1.8 and 4.3% of the stool specimens, respectively, by the recombinant EIAs; 9.2% were positive for SRSVs by EM, and 25% of these SRSVs gave a positive result by RT-PCR for primer pair 35-36 directed to a region of the RNA-dependent RNA polymerase gene. The seroprevalence studies indicate a high level of exposure to these viruses in both children and adults. Although the viral antigens are not highly prevalent in diarrheal stools, it was determined by the two assays for NV and MxV that children are, nevertheless, infected early in life.

Outbreaks of gastroenteritis in elderly nursing homes and retirement facilities associated with human caliciviruses

Eleven outbreaks of acute gastroenteritis, eight of which were in nursing homes or retirement facilities, were reported in virginia during the winter of 1993-1994. Serum samples (four outbreaks) and stool samples (two outbreaks) from involved people were tested for human calicivirus (HuCV) infection by enzyme immune assays (EIAs) using recombinant Norwalk virus (rNV) and Mexico virus (rMX) capsid antigens and reverse transcription-polymerase chain reaction (RT-PCR). Of the 31 pairs of acute and convalescent serum specimens tested, 24 had a fourfold or more titer increase to rMX and 4 responded to rNV. In all four outbreaks, the geometric mean titers (GMTs) against rMX were significantly higher than those against rNV in the convalescent, but not in the acute phase of illness. The antibody response to rMX among these patients was also higher than to rNV (summary mean 32-fold increase vs. 0.7-fold increase, respectively, P < .001). Antigen was detected in 5 of 21 stool specimens tested by the rMX EIA, RNA in 12 of 17 stool specimens tested by RT-PCR, and small round structured virus (SRSV) particles in 12 of 21 by electron microscopy (EM); none were positive by the rNV EIA. Sequence analysis of the RT-PCR-amplified products from the viral RNA polymerase region revealed 92-93% amino acid identity with Snow Mountain agent (SMA), 86% with MX, 58-59% with NV, and 31-32% with Sapporo HuCV, suggesting that these viruses belong to the SMA HuCV genogroup.

Prevalence of serum antibodies to Norwalk virus among Norwegian military recruits

The prevalence of serum antibodies against Norwalk virus among military recruits in Norway was investigated using an enzyme-linked immunosorbent assay (ELISA). A total of 1017 sera were assayed for anti-Norwalk virus total antibodies (ig), of which 300 (29.5%) were positive. Of 227 positive sera, 10.6 and 15.4% were positive for IgA and IgM, respectively, while 2.2% were positive for both. The prevalence of antibodies against Norwalk virus in south-east Norway was significantly higher than that in northern Norway.

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.

Study on occurrence of human calicivirus (Mexico strain) as cause of sporadic cases and outbreaks of calicivirus-associated diarrhoea in the United Kingdom, 1983-1995

The use of a recently developed EIA using antisera raised against purified baculovirus expressed recombinant Mexico virus (rMx) capsid protein is described for screening for human calicivirus in stools. The results show that MX-like viruses have been circulating in the UK periodically since 1983 and were an occasional cause of sporadic cases of diarrhoea in infants and outbreaks of infection among elderly patients in hospitals and old people's homes. Further evidence is presented that some strains of caliciviruses with characteristic surface morphology (HuCVs) and some with an indistinct appearance, small round structured viruses (SRSVs) are antigenically related to MxV. Tests on SRSVs from four unrelated outbreaks typed as UK3 failed to react in the Mx EIA or recombinant Norwalk virus (rNV) EIA. A 2-month survey of 206 children treated in two London hospitals for diarrhoea showed that only one was positive for MxV, a child known to be infected with HIV-1. None of the samples reacted in the rNV EIA.

Genetic and antigenic diversity of human caliciviruses (HuCVs) using RT-PCR and new EIAs

RT-PCR using primers from conserved regions of calicivirus genomes, followed by sequencing, permits characterization of genetic variation within the family. EIAs based on baculovirus-expressed viral capsid proteins and hyperimmune antisera against the capsid proteins were developed to detect HuCV antigens and antibodies. Serologic surveys using recombinant Norwalk virus (rNV) and recombinant Mexico virus (rMX, a SMA-like virus) EIAs showed that infections by HuCVs are common and that children acquire antibodies to HuCVs at an early age in both developed and developing countries. Three HuCV genogroups have been described that are represented by Norwalk virus (NV), Snow Mountain agent (SMA), and Sapporo virus, although recently accumulated sequences of HuCV strains indicate these genogroups can be further divided. These genogroups also correspond to distinct antigenic groups based on the results of the recombinant EIAs. The three genogroups co-circulate and have a worldwide distribution, although the SMA genogroup seems to be predominant currently. Application of these new assays for further characterization of the genetic and antigenic properties of HuCVs remains an important task for HuCV research.

Historical background and classification of caliciviruses and astroviruses

Infections caused by caliciviruses, i.e., vesicular exanthema virus of swine were recognised as a major cause of economic loss in the 1930s. However, it was not until the application of electronmicroscopy in the 1970s that caliciviruses and astroviruses were recognised and proven to be a cause of diarrhoea and vomiting. The following review briefly describes the steps which have led to the development of diagnostic tests and enabled the characterization of several members of the Caliciviridae and Astroviridae. In the past five years this has culminated in the sequencing of their genomes and the expression of viral proteins. This in turn has led to the development of improved diagnostic tests e.g., RT-PCR and enzyme immunoassays, and may pave the way towards producing effective vaccines in the future.

Study of Norwalk-related viruses in Mexican children

Two-hundred Mexican children monitored from birth to 2 years of age in a cohort study of diarrhea were tested for Norwalk virus (NV) and Norwalk-related virus infection. Blood was collected quarterly and tested by an enzyme immunoassay (EIA) using the recombinant NV (rNV) particles as antigen. Stool was collected weekly and tested by an EIA using hyperimmune anti-sera from animals immunized with rNV and a reverse transcription-polymerase chain reaction (RT-PCR) with primers in the RNA polymerase region of NV. A high prevalence of serum antibody to NV (85% at age 2 years) was found by the antibody EIA. In 54 stool specimens selected from children who developed a high titer of serum antibody to rNV, none was positive for NV by the antigen EIA, but 6 yielded products by the RT-PCR. One stool specimen (MX virus) yielded a 3.3 kb RT-PCR product from the 3' end of the viral genome. The MX virus cDNA has a genomic organization like other caliciviruses. Sequence comparison showed that MX virus shares 80% nucleic acid and 91% amino acid sequence identity with Snow Mountain agent (SMA), but only 62% and 60% identity, respectively, with NV in the RNA polymerase region, suggesting that MX virus is a SMA-like virus.

A serosurvey of water-borne pathogens amongst canoeists in South Africa

Certain health risks have been associated with recreational exposure to faecally polluted water. Canoeing in certain South African waters is considered to be a high risk activity with regard to schistosomiasis, gastroenteritis and possibly hepatitis. In a cross-sectional study, a serosurvey was conducted amongst canoeists to ascertain whether or not they had a higher seroprevalence to hepatitis A virus, Norwalk virus and Schistosoma spp. than non-canoeists. In comparisons between the two groups, a significant association could not be demonstrated between canoeing and antibody response to hepatitis A and Norwalk viruses (P-values for age-adjusted chi 2 were 0.083 and 0.219 respectively), but a significant association could be demonstrated between canoeing and the antibody response to Schistosoma spp. (P < 0.001; age-adjusted).

Antibody prevalence and immunoglobulin IgG subclass pattern to Norwalk virus in Sweden

The antibody prevalence in Sweden to Norwalk virus (NV) was investigated using a baculovirus expressed capsid antigen. One hundred thirty-two serum samples were examined for IgA, total IgG and IgG subclass antibodies to Norwalk virus. In young children, NV IgG antibody prevalence was higher than the IgA prevalence, whereas no difference was found in individuals older than 21 years. The IgG antibody prevalence was 50% in children below 5 years of age and increased to > 80% in individuals older than 10 years of age. To examine the IgG antibody response in more detail, IgG subclass patterns were characterized. IgG 1 predominated in all age groups. IgG 4, usually detected after repeated exposure to antigen, was the second most prevalent subclass, but was only found in individuals older than 21 years of age. IgG 3 subclass antibodies were found in 13% and IgG 2 in 3% of the sera examined. IgG 3 subclass antibodies have been recognized as a marker for recent or ongoing viral infections.

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.

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.

Seroprevalence studies using a recombinant Norwalk virus protein enzyme immunoassay

A recombinant Norwalk virus (NV) protein enzyme immunoassay was used to study the age of acquisition of NV IgG in various populations. In London, England, there was little evidence of infection during the first 2 years of life. However, the prevalence of NV IgG rose steadily throughout the period that children attend school, reaching a peak of 70% in the group aged 11-16 years. High levels of maternal antibody were detected in infants aged < 3 months. Comparison of the acquisition of antibodies to three strains of human calicivirus in Japanese children in northern Japan indicated that although the majority had experienced infection with strains Japan and UK1 by the age of 12 years, only 22% possessed antibodies to NV. In Australian aborigines NV infection occurs early in life; by the age of 6 years over 90% of children were seropositive.