Application of PCR to detect Norwalk virus in fecal specimens from outbreaks of gastroenteritis

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.

Norovirus

Norovirus, an RNA virus of the family Caliciviridae, is a human enteric pathogen that causes substantial morbidity across both health care and community settings. Several factors enhance the transmissibility of norovirus, including the small inoculum required to produce infection (<100 viral particles), prolonged viral shedding, and its ability to survive in the environment. In this review, we describe the basic virology and immunology of noroviruses, the clinical disease resulting from infection and its diagnosis and management, as well as host and pathogen factors that complicate vaccine development. Additionally, we discuss overall epidemiology, infection control strategies, and global reporting efforts aimed at controlling this worldwide cause of acute gastroenteritis. Prompt implementation of infection control measures remains the mainstay of norovirus outbreak management.

Genotyping ofNorovirusStrains Detected in Outbreaks between April 2002 and March 2003 in Osaka City, Japan

Noroviruses (NVs) are the major cause of food- and waterborne nonbacterial gastroenteritis in Japan. Between April 2002 and March 2003, a total of 111 fecal specimens from 40 outbreaks of acute nonbacterial gastroenteritis in Osaka City, Japan were subject to NV detection. Seventy-two samples (64.9%) from 31 outbreaks (77.5%) were NV positive by a real time reverse transcription (RT)-PCR assay. To further determine the genotype of individual NV strains, we sequenced the capsid N-terminal/shell (N/S) domain of some representative strains from each outbreak. The 51 NV strains detected in this study were segregated into 15 genotypes (6 in genogroup I and 9 in genogroup II), and GII/5 genotype NV was a dominant outbreak genotype.

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

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

Norovirus infectivity in humans and persistence in water

To examine the long-term infectivity of human norovirus in water, 13 study subjects were challenged at different time points with groundwater spiked with the prototype human norovirus, Norwalk virus. Norwalk virus spiked in groundwater remained infectious after storage at room temperature in the dark for 61 days (the last time point tested). The Norwalk virus-seeded groundwater was stored for 1,266 days and analyzed, after RNase treatment, by reverse transcription-quantitative PCR (RT-qPCR) to detect Norwalk virus RNA contained within intact capsids. Norwalk virus RNA within intact capsids was detected in groundwater for 1,266 days, with no significant log(10) reduction throughout 427 days and a significant 1.10-log(10) reduction by day 1266. Purified Norwalk virus RNA (extracted from Norwalk virus virions) persisted for 14 days in groundwater, tap water, and reagent-grade water. This study demonstrates that Norwalk virus in groundwater can remain detectable for over 3 years and can remain infectious for at least 61 days. (ClinicalTrials.gov identifier NCT00313404.).

Randomized, double-blinded clinical trial for human norovirus inactivation in oysters by high hydrostatic pressure processing

Contamination of oysters with human noroviruses (HuNoV) constitutes a human health risk and may lead to severe economic losses in the shellfish industry. There is a need to identify a technology that can inactivate HuNoV in oysters. In this study, we conducted a randomized, double-blinded clinical trial to assess the effect of high hydrostatic pressure processing (HPP) on Norwalk virus (HuNoV genogroup I.1) inactivation in virus-seeded oysters ingested by subjects. Forty-four healthy, positive-secretor adults were divided into three study phases. Subjects in each phase were randomized into control and intervention groups. Subjects received Norwalk virus (8FIIb, 1.0 × 10(4) genomic equivalent copies) in artificially seeded oysters with or without HPP treatment (400 MPa at 25°C, 600 MPa at 6°C, or 400 MPa at 6°C for 5 min). HPP at 600 MPa, but not 400 MPa (at 6° or 25°C), completely inactivated HuNoV in seeded oysters and resulted in no HuNoV infection among these subjects, as determined by reverse transcription-PCR detection of HuNoV RNA in subjects' stool or vomitus samples. Interestingly, a white blood cell (granulocyte) shift was identified in 92% of the infected subjects and was significantly associated with infection (P = 0.0014). In summary, these data suggest that HPP is effective at inactivating HuNoV in contaminated whole oysters and suggest a potential intervention to inactivate infectious HuNoV in oysters for the commercial shellfish industry.

Quantification of Norwalk virus inocula: Comparison of endpoint titration and real-time reverse transcription-PCR methods

Human noroviruses (NoV) are the leading cause of epidemic acute gastroenteritis. In order to fully characterize features such as persistence and infectious dose, precise quantification of virus concentration is necessary. The purpose of this study was to compare two methods [endpoint titration RT-PCR and quantitative RT-PCR (RT-qPCR)] with respect to quantification of Norwalk virus (NV) in inocula made from purified stock suspensions of human fecal specimens. A full-length NV RNA transcript was developed to facilitate quantification using RT-qPCR and provided log linear detection in the range of 49-4.9 x 10(4) genome equivalent copies (GEC) per reaction. Endpoint titration RT-PCR was used to estimate PCR detection units, and RT-qPCR was used to estimate genome copies in two NV inocula (8fIIa and 8fIIb) used in previous human challenge studies. Overall, RT-qPCR was 1.1-1.6 log(10) more sensitive (lower detection limit) than endpoint titration RT-PCR when the same RNA release method, PCR primers and thermocycle program were used. These findings have important implications for many experimental interpretations, not the least of which is estimating the median infectious dose in human challenge studies.

Molecular epidemiology of norovirus gastroenteritis outbreaks in North Carolina, United States: 1995–2000

Noroviruses (NoVs) are the most common cause of acute non-bacterial gastroenteritis outbreaks in the US. We investigated 16 gastroenteritis outbreaks in North Carolina (NC), from 1995 to 2000, to further characterize the epidemiology of NoV using RT-PCR on stool and ELISA on sera. NoV were identified in 14 outbreaks by RT-PCR. Sequence analyses of the amplicons indicated the outbreak strains belonged to the following clusters: five GII/4, three GI/3, one GI/4, one GII/2, one GII/5, one GII/7, and one GII/13 (prototype strain). We detected NoV in stool samples from one outbreak but could not determine its specific cluster within the GII genogroup based on polymerase sequence analysis. The five GII/4 strains were classified as the "95/96 US common strain" and occurred throughout the 5-year period. In contrast to national trends, the majority (86%) of NoV outbreaks identified in North Carolina were foodborne. Of the 12 food-related NoV outbreaks, we were able to document transmission by food handlers in two outbreaks. Person-to-person transmission from primary cases was suggested in three outbreaks. Our results indicate that NoVs are important agents of viral gastroenteritis outbreaks in NC.

Detection of serum antibodies to bovine norovirus in veterinarians and the general population in the Netherlands

The close genetic relationship of human and animal strains of norovirus has raised the possibility of transmission of noroviruses from animals to humans and may explain the emergence of certain norovirus strains. To assess if exposure to bovine noroviruses (NoV) might result in infection in humans, an enzyme immunoassay (EIA) was designed and validated in order to detect antibodies against bovine norovirus. This and two other EIAs were used to test sera from 210 veterinarians and 630 matched population controls for IgG and IgA antibodies to recombinant capsid protein of bovine NoV (rBoV), Norwalk virus (rNV), and Lordsdale virus (rLDV). Of 840 participants, IgG reactivity to rBoV was found in 185 (22%), to rNV in 638 (76%) and to rLDV in 760 (90%). IgG reactivity to rBoV was more common in veterinarians (58/210: 28%) than in controls (127/630: 20% [P = 0.03]). IgA reactivity to rBoV was similar in both veterinarians and controls. Cross-reactivity of IgA and IgG antibodies to rBoV and rNV was seen, but 26% of all specimens positive rBoV antibodies showed high IgG reactivity to rBoV but low reactivity to rNV, suggesting a specific response to bovine antigen. No evidence of overall cross-reactivity of antibodies to rBoV and rLDV was seen. Among veterinarians, youth spent on farm (Odds Ratio [OR] = 1.8) and membership of the bovine practitioners' society (OR = 2.7) were significantly associated with IgG seroreactivity to rBoV. These data indicate that bovine strains of NoV may infect humans though less frequently than human strains.

Norovirus and foodborne disease, United States, 1991-2000

Analysis of foodborne outbreaks shows how advances in viral diagnostics are clarifying the causes of foodborne outbreaks and determining the high impact of norovirus infections.

Efforts to prevent foodborne illness target bacterial pathogens, yet noroviruses (NoV) are suspected to be the most common cause of gastroenteritis. New molecular assays allow for better estimation of the role of NoV in foodborne illness. We analyzed 8,271 foodborne outbreaks reported to the Centers for Disease Control and Prevention from 1991 to 2000 and additional data from 6 states. The proportion of NoV-confirmed outbreaks increased from 1% in 1991 to 12% in 2000. However, from 1998 to 2000, 76% of NoV outbreaks were reported by only 11 states. In 2000, an estimated 50% of foodborne outbreaks in 6 states were attributable to NoV. NoV outbreaks were larger than bacterial outbreaks (median persons affected: 25 versus 15), and 10% of affected persons sought medical care; 1% were hospitalized. More widespread use of molecular assays will permit better estimates of the role of NoV illness and help direct efforts to control foodborne illness.

Cellular and humoral immunity following Snow Mountain virus challenge

Little is known about the immune response to noroviruses. To elucidate the immunobiology of norovirus infection in humans, 15 volunteers were challenged with Snow Mountain virus (SMV), a genogroup 2 norovirus. We assessed the cellular and humoral immune response and infection by analyzing stool, serum, saliva, and peripheral blood mononuclear cell (PBMC) responses pre- and postchallenge. In contrast to Norwalk virus (NV), SMV infection was not dependent upon blood group secretor status. Nine of 15 volunteers were infected and showed a >/=4-fold increase over the prechallenge anti-SMV serum immunoglobulin G (IgG) titer, mostly subclass IgG1. Although serum IgG elicited by SMV infection was cross-reactive with Hawaii virus (HV), another genogroup 2 norovirus, salivary IgA was less cross-reactive. Neither SMV-elicited serum IgG nor salivary IgA cross-reacted with NV, a genogroup 1 norovirus. Significant increases in serum gamma interferon (IFN-gamma) and IL-2, but not IL-6 or IL-10, were noted on day 2 postchallenge. For the majority of volunteers, both infected and uninfected, PBMCs stimulated with norovirus virus-like particles secreted IFN-gamma and other Th1 cytokines, suggesting previous norovirus exposure in most volunteers. Like the IgG antibodies, the SMV-activated T cells were cross-reactive with HV but not NV. IFN-gamma production was dependent upon CD4(+) cells, consistent with a predominant, but not exclusive, Th1 response. To our knowledge, this is the first report characterizing T-cell and cytokine responses following live norovirus challenge.

Experimental norovirus infections in non-human primates

Noroviruses, with Norwalk virus as the prototype strain, are the most common cause of viral gastroenteritis in people of all ages. Limited information on the immunology of Norovirus infections has been obtained by studies both in the natural setting and in experimentally infected volunteers. Interpretation of these studies is difficult due to the lack of information on the history of Norovirus exposure and the cross-reactivity of antibodies. An animal model for Norovirus infections would be important to study the immune response, e.g., for vaccine assessment. In the present study the susceptibility of common marmosets, cotton top tamarins, cynomolgus, and rhesus macaques to Norovirus infection was tested. Following oral inoculation, low level replication may have occurred in common marmosets and cotton top tamarins but not in cynomolgus macaques, based on short-term viral shedding; neither clinical symptoms nor antibody responses were observed in these species. In contrast, rhesus macaques were found susceptible to Norwalk virus infection as one animal shed virus for a longer period of time and developed Norwalk virus specific IgM and IgG responses. Further research on Norovirus susceptibility in rhesus macaques may yield an animal model to study the immune response and pathogenesis after Norovirus infection.

Calicivirus infection in pediatric small intestine transplant recipients: pathological considerations

Human calicivirus (HuCV), a common cause of mild gastroenteritis in the general population, produces a prolonged diarrheal illness in pediatric recipients of small intestinal transplant (IT). By use of reverse-transcription polymerase chain reaction to detect the viral RNA polymerase gene in stool and tissue from gastrointestinal biopsies, 5 pediatric IT recipients with high-volume diarrhea were diagnosed with HuCV enteritis. Histopathologic findings of biopsies obtained at different gastrointestinal sites were studied retrospectively to identify characteristic features of HuCV enteritis and to distinguish these changes from rejection. Controls were 8 pediatric IT recipients with high-volume diarrhea but negative HuCV reverse-transcription polymerase chain reaction assays during the same time period. All HuCV biopsies showed increased mononuclear infiltrates in the lamina propria and villous blunting. Reactive disarray of surface epithelial cells and increased apoptosis in the surface epithelium and superficial lamina propria were characteristic features (in 4/5 patients). Increased glandular apoptosis was also present in 3/5 patients. Findings were more pronounced in jejunal allograft than ileal allograft, and were present in both graft and native bowel. In comparison with the control group, the architectural changes, surface epithelial reactive changes, and superficial apoptosis were characteristic of HuCV enteritis, while the presence of glandular apoptosis was a feature shared with cases of mild acute cellular rejection HuCV may cause severe allograft dysfunction after pediatric IT. Calicivirus infection has clinical and histological features that overlap with allograft rejection. Knowledge of the characteristic histologic features of HuCV enteritis aids in differential diagnosis.

Coexistence of multiple genotypes, including newly identified genotypes, in outbreaks of gastroenteritis due to Norovirus in Japan

Norovirus (NV) (formerly called Norwalk-like virus) is the most common cause of acute nonbacterial gastroenteritis in humans. Recently, we reported an NV genotyping scheme based on variability in the capsid N-terminal/shell (N/S) domain gene (Katayama et al., Virology 299:225-239, 2002). We found 19 genotypes, including nine of genogroup I and 10 of genogroup II. In the present study, we investigated the molecular epidemiology of NV from 66 outbreaks that occurred in Saitama Prefecture, Japan, from 1997 to 2002. We screened 416 stool specimens by a real-time reverse transcription (RT)-PCR method (Kageyama et al., J. Clin. Microbiol. 41:1548-1557, 2003) and detected 156 NV-positive specimens, from which we amplified the capsid N/S domain gene by RT-PCR and then cloned the PCR products. After sequencing these clones, we obtained 368 sequence variants (strains). By applying our classification scheme to the strains from Saitama and other published strains, we identified a total of 31 genotypes, including an additional five genotypes for genogroup I and seven for genogroup II. Of the 31 genotypes, 26 were present in the Saitama area during that time period. These results provide additional evidence for the great diversity of human NV genotypes. Specimens from all shellfish-related infections contained multiple genotypes, including several new genotypes. On the other hand, single genotypes were observed mostly in outbreaks that originated in semiclosed communities. Thus, the number of NV genotypes in each outbreak depended on the route of transmission.

Fast detection of Noroviruses using a real-time PCR assay and automated sample preparation

BACKGROUND

Noroviruses (NoV) have become one of the most commonly reported causative agents of large outbreaks of non-bacterial acute gastroenteritis worldwide as well as sporadic gastroenteritis in the community. Currently, reverse transcriptase polymerase chain reaction (RT-PCR) assays have been implemented in NoV diagnosis, but improvements that simplify and standardize sample preparation, amplification, and detection will be further needed. The combination of automated sample preparation and real-time PCR offers such refinements.

METHODS

We have designed a new real-time RT-PCR assay on the LightCycler (LC) with SYBR Green detection and melting curve analysis (Tm) to detect NoV RNA in patient stool samples. The performance of the real-time PCR assay was compared with that obtained in parallel with a commercially available enzyme immunoassay (ELISA) for antigen detection by testing a panel of 52 stool samples. Additionally, in a collaborative study with the Baden-Wuerttemberg State Health office, Stuttgart (Germany) the real-time PCR results were blindly assessed using a previously well-established nested PCR (nPCR) as the reference method, since PCR-based techniques are now considered as the "gold standard" for NoV detection in stool specimens.

RESULTS

Analysis of 52 clinical stool samples by real-time PCR yielded results that were consistent with reference nPCR results, while marked differences between the two PCR-based methods and antigen ELISA were observed. Our results indicate that PCR-based procedures are more sensitive and specific than antigen ELISA for detecting NoV in stool specimens.

CONCLUSIONS

The combination of automated sample preparation and real-time PCR provided reliable diagnostic results in less time than conventional RT-PCR assays. These benefits make it a valuable tool for routine laboratory practice especially in terms of rapid and appropriate outbreak-control measures in health-care facilities and other settings.

Development and application of a capsid VP1 (region D) based reverse transcription PCR assay for genotyping of genogroup I and II noroviruses

Noroviruses (NoV), previously called "Norwalk-like viruses", have emerged as the single most important cause of acute gastroenteritis worldwide. Most diagnostic reverse transcription-polymerase chain reaction (RT-PCR) assays target the viral RNA-dependent RNA polymerase; however, the major capsid protein (VP1) is the reference genomic region for establishing genotypes. In this study, we analyzed complete NoV VP1 sequences (n=100) and determined a region (region D) that was most suitable to differentiate between genotypes. Within region D, we designed two genogroup specific, broadly reactive, degenerate primer sets (GI and GII). The region D primers were evaluated in a single-tube one-step RT-PCR assay using a panel of 81 (31 GI, 50 GII) NoV strains from both outbreaks and sporadic cases. In total, 95% of the samples tested positive using the new region D primer sets. Phylogenetic analysis of region D sequences (36 deduced amino acids for GI, 56 deduced amino acids for GII), revealed 19 clusters (7 within GI and 12 within GII) including three new genetically distinct clusters, two of which were unresolved using region A sequences. Phylogenetic analysis of the complete VP1 sequences revealed identical grouping of strains and confirmed the newly identified clusters using region D. In summary, we successfully developed and evaluated a broadly reactive RT-PCR assay for reliable genotyping of GI and GII noroviruses.

Molecular Epidemiology of Outbreaks of Viral Gastroenteritis in New York State, 1998–1999

This investigation evaluated the role of Norwalk-like virus (NLV) and other viruses (rotavirus, enteric adenovirus, and enterovirus) in 11 outbreaks of acute nonbacterial gastroenteritis that occurred in multiple settings in a span of 18 months in New York State. To determine the etiology of illness, patients' stool specimens were analyzed with a combination of reverse-transcription polymerase chain reaction (RT-PCR) and nucleotide sequencing, cell culture, and ELISA diagnostic techniques. NLV was detected from all of these outbreaks, with an overall detection rate of 64% (51 of 79) for all specimens tested. Repeated attempts to isolate other viral pathogens were unsuccessful. Phylogenetic analysis of a subset of 27 specimens from these outbreaks showed the presence of both genogroup I and genogroup II NLVs. A spectrum of different nucleotide sequences were detected, demonstrating interoutbreak sequence variation and unrelated infections. NLV is a significant causative agent of diarrhea outbreaks in New York State.

Waterborne outbreak of gastroenteritis associated with a norovirus

The Wyoming Department of Health investigated an outbreak of acute gastroenteritis among persons who dined at a tourist saloon in central Wyoming during October 2001. Human caliciviruses (HuCVs) were suspected as the etiological agent of the outbreak based on the incubation period, duration of illness, and symptoms observed in ill patrons. A retrospective cohort study demonstrated that ill patrons were 4.5 times more likely to have exposure to drinking water and/or ice than nonill patrons. No food items were associated with illness. An environmental investigation gave evidence that the saloon's groundwater was contaminated with sewage. Water from the saloon's only well was processed for viruses. The processed water sample and stool samples collected from three ill patrons were analyzed by reverse transcription-PCR (RT-PCR) for the presence of HuCV. All positive RT-PCR results were confirmed by sequence and phylogenetic analyses of cloned RT-PCR products. A genogroup I, subtype 3, HuCV stain was found to be present in the well water sample and two stool samples. In addition, a genogroup II, subtype 6, strain was detected in one stool sample. The identification of the same HuCV strain in both the well water and stool samples strongly suggests a link between exposure to well water and the outbreak of gastroenteritis. The presence of a genogroup II, subtype 6, strain in one of the stool samples suggests that multiple HuCV strains may have been involved in this outbreak. The laboratory isolation of HuCV strains from outbreak-associated drinking water is relatively novel in the United States. This investigation outlines the procedure for virus isolation and illustrates the utility of RT-PCR for the identification of HuCV in large volumes of water and stool samples obtained during outbreaks of acute nonbacterial gastroenteritis.

Calicivirus enteritis in an intestinal transplant recipient

Protracted diarrhea of uncertain etiology is a significant problem following intestinal transplantation. We report an infant who developed severe secretory diarrhea 178 days after intestinal transplantation that persisted for more than 120 days. Repeated allograft biopsies demonstrated only nonspecific inflammation. Enzyme immunoassay (for rotavirus), culture, and reverse transcription polymerase chain reaction [calicivirus (Norwalk-like virus)] were used to identify the allograft viral infection. A heavy density of calicivirus RNA nucleotide sequences (genogroup II, strain Miami Beach) was isolated from the jejunal and ileal allograft. Following a reduction in immunosuppressive therapy, diarrhea and enteritis remitted in association with the disappearance of all calicivirus RNA sequences. Calicivirus may cause severe allograft dysfunction in intestinal transplant recipients.

Human susceptibility and resistance to Norwalk virus infection

Infectious diseases have influenced population genetics and the evolution of the structure of the human genome in part by selecting for host susceptibility alleles that modify pathogenesis. Norovirus infection is associated with approximately 90% of epidemic non-bacterial acute gastroenteritis worldwide. Here, we show that resistance to Norwalk virus infection is multifactorial. Using a human challenge model, we showed that 29% of our study population was homozygous recessive for the alpha(1,2)fucosyltransferase gene (FUT2) in the ABH histo-blood group family and did not express the H type-1 oligosaccharide ligand required for Norwalk virus binding. The FUT2 susceptibility allele was fully penetrant against Norwalk virus infection as none of these individuals developed an infection after challenge, regardless of dose. Of the susceptible population that encoded a functional FUT2 gene, a portion was resistant to infection, suggesting that a memory immune response or some other unidentified factor also affords protection from Norwalk virus infection.

International collaborative study to compare reverse transcriptase PCR assays for detection and genotyping of noroviruses

To allow more rapid and internationally standardized assessment of the spread of noroviruses (previously called Norwalk-like viruses [NLVs]) as important food-borne pathogens, harmonization of methods for their detection is needed. Diagnosis of NLVs in clinical diagnostic laboratories is usually performed by reverse transciptase PCR (RT-PCR) assays. In the present study, the performance of five different RT-PCR assays for the detection of NLVs was evaluated in an international collaborative study by five laboratories in five countries with a coded panel of 91 fecal specimens. The assays were tested for their sensitivity, detection limit, and ease of standardization. In total, NLVs could be detected by at least one RT-PCR assay in 69 (84%) of the samples that originally tested positive. Sensitivity ranged from 52 to 73% overall and from 54 to 100% and 58 to 85% for genogroup I and II viruses, respectively. In all, 64% of the false-negative results were obtained with a set of diluted stools (n = 20) that may have lost quality upon storage. Sensitivity was improved when these samples were excluded from analysis. No one single assay stood out as the best, although the p1 assay demonstrated the most satisfactory overall performance. To promote comparability of data, this assay will be recommended for newly starting groups in future collaborative studies.

Broadly reactive and highly sensitive assay for Norwalk-like viruses based on real-time quantitative reverse transcription-PCR

We have developed an assay for the detection of Norwalk-like viruses (NLVs) based on reverse transcription-PCR (RT-PCR) that is highly sensitive to a broad range of NLVs. We isolated virus from 71 NLV-positive stool specimens from 37 outbreaks of nonbacterial acute gastroenteritis and sequenced the open reading frame 1 (ORF1)-ORF2 junction region, the most conserved region of the NLV genome. The data were subjected to multiple-sequence alignment analysis and similarity plot analysis. We used the most conserved sequences that react with diverse NLVs to design primers and TaqMan probes for the respective genogroups of NLV, GI and GII, for use in a real-time quantitative RT-PCR assay. Our method detected NLV in 99% (80 of 81) of the stool specimens that were positive by electron microscopy, a better detection rate than with the two available RT-PCR methods. Furthermore, our new method also detected NLV in 20 of 28 stool specimens from the same NLV-related outbreaks that were negative for virus by electron microscopy. Our new assay is free from carryover DNA contamination and detects low copy numbers of NLV RNA. It can be used as a routine assay for diagnosis as well as for elucidation of the epidemiology of NLV infections.

Broadly reactive and highly sensitive assay for Norwalk-like viruses based on real-time quantitative reverse transcription-PCR

We have developed an assay for the detection of Norwalk-like viruses (NLVs) based on reverse transcription-PCR (RT-PCR) that is highly sensitive to a broad range of NLVs. We isolated virus from 71 NLV-positive stool specimens from 37 outbreaks of nonbacterial acute gastroenteritis and sequenced the open reading frame 1 (ORF1)-ORF2 junction region, the most conserved region of the NLV genome. The data were subjected to multiple-sequence alignment analysis and similarity plot analysis. We used the most conserved sequences that react with diverse NLVs to design primers and TaqMan probes for the respective genogroups of NLV, GI and GII, for use in a real-time quantitative RT-PCR assay. Our method detected NLV in 99% (80 of 81) of the stool specimens that were positive by electron microscopy, a better detection rate than with the two available RT-PCR methods. Furthermore, our new method also detected NLV in 20 of 28 stool specimens from the same NLV-related outbreaks that were negative for virus by electron microscopy. Our new assay is free from carryover DNA contamination and detects low copy numbers of NLV RNA. It can be used as a routine assay for diagnosis as well as for elucidation of the epidemiology of NLV infections.

Broadly reactive and highly sensitive assay for Norwalk-like viruses based on real-time quantitative reverse transcription-PCR

We have developed an assay for the detection of Norwalk-like viruses (NLVs) based on reverse transcription-PCR (RT-PCR) that is highly sensitive to a broad range of NLVs. We isolated virus from 71 NLV-positive stool specimens from 37 outbreaks of nonbacterial acute gastroenteritis and sequenced the open reading frame 1 (ORF1)-ORF2 junction region, the most conserved region of the NLV genome. The data were subjected to multiple-sequence alignment analysis and similarity plot analysis. We used the most conserved sequences that react with diverse NLVs to design primers and TaqMan probes for the respective genogroups of NLV, GI and GII, for use in a real-time quantitative RT-PCR assay. Our method detected NLV in 99% (80 of 81) of the stool specimens that were positive by electron microscopy, a better detection rate than with the two available RT-PCR methods. Furthermore, our new method also detected NLV in 20 of 28 stool specimens from the same NLV-related outbreaks that were negative for virus by electron microscopy. Our new assay is free from carryover DNA contamination and detects low copy numbers of NLV RNA. It can be used as a routine assay for diagnosis as well as for elucidation of the epidemiology of NLV infections.

Evaluation of the NucliSens Basic Kit assay for detection of Norwalk virus RNA in stool specimens

Norwalk-like viruses (NLVs) are a genetically diverse group of human caliciviruses that are the most common cause of epidemic gastroenteritis and are detected typically in stool by reverse transcription (RT)-PCR or electron microscopy (EM). The application of a rapid nucleic acid sequence-based amplification (NASBA) assay for the detection of NLV RNA in stool is described using the NucliSens Basic Kit. Primers and probes for the NLV Basic Kit assay were based on the RNA polymerase region of the prototype NLV, Norwalk virus (NV) genome and could consistently detect 10(4) RT-PCR detectable units of NV RNA in a stool filtrate. When compared directly with RT-PCR on a dilution series of NV stool filtrate, the NucliSens Basic Kit assay was equally sensitive. Cross-reactivity studies with a representative panel of other enteric pathogens were negative. When applied to 15 stool specimens from NV-challenged volunteers, the NASBA Basic Kit application for NV detection yielded 100% sensitivity, 50% specificity, and 67% concordance, using RT-PCR as the 'gold standard'. Despite the specificity of the NASBA primer/probe sequences for NV, other representatives from both NLV genogroups I and II could be detected by the Basic Kit assay in outbreak stool specimens, although the results were inconsistent. Our results suggest that the NucliSens Basic Kit assay provides a rapid and sensitive alternative to RT-PCR for detecting NV RNA in stool specimens. However, improvements in test specificity and primer design will be needed before the assay can be used routinely in the clinical setting.

Zirconium hydroxide effectively immobilizes and concentrates human enteric viruses

BACKGROUND

Detection of human enteric viruses in foods and environmental samples requires concentration of viruses from complex matrices before application of molecular or cultural methods. Previous studies have described the use of zirconium hydroxide to concentrate bacteria from clinical, environmental, and food samples.

AIMS

Our study describes the application of zirconium hydroxide to concentrate human enteric viruses.

METHODS

Poliovirus type 1, hepatitis A virus (HAV) strain HM-175, and Norwalk virus (NV) were used as models. Virus recovery was evaluated both as loss to discarded supernatants and as recovery in the precipitated pellets.

RESULTS

Poliovirus type 1, based on the plaque assay recoveries, ranged from 16 to 59% with minimal loss to the supernatant (1-5%). For both HAV and NV, RT-PCR amplicons of appropriate sizes were detected and confirmed in the pellet fraction with no visible amplicons from the supernatant.

SIGNIFICANCE AND IMPACT OF THE STUDY

This rapid and inexpensive method shows promise as an alternative means to concentrate enteric viruses.

Applied technique for increasing calicivirus detection in shellfish extracts

AIMS

Optimal detection of enteric RNA viruses in clinical, environmental, and food products using reverse transcription-PCR (RT-PCR) when inhibitory substances in extracted sample materials are present.

METHODS AND RESULTS

We adapted a device for detection of RNA viruses in plant tissues and insects to detect a calicivirus strain (San Miguel sea lion virus, serotype 17) in water and oyster tissue extracts. This single, compartmentalized tube-within-a-tube (TWT) device for RT-PCR-nested PCR was compared to a conventional protocol of RT-PCR-nested PCR. In the presence of 100 mg of shellfish tissue extract equivalent, this TWT device decreases the calicivirus assay detection limit 10-fold over that of conventional RT-PCR-nested PCR while maintaining an identical detection limit of viral nucleic acid suspended in water. Both the conventional and TWT methods estimated the total particle-to-infectious particle ratio for this strain of calicivirus at approximately 40 : 1.

CONCLUSIONS

We believe that the TWT device with appropriate RT-PCR primers will decrease the detection limit for other calicivirus strains and RNA viruses in shellfish tissue extracts.

SIGNIFICANCE AND IMPACT OF THE STUDY

We believe that the TWT approach is applicable to other situations where RT and/or PCR inhibitory materials are present or nucleic acid targets of bacteria or viruses are at low levels in extracts of food products or clinical specimens.

Pretreatment to avoid positive RT-PCR results with inactivated viruses

Enteric viruses that are important causes of human disease must often be detected by reverse transcription-polymerase chain reaction (RT-PCR), a method that commonly yields positive results with samples that contain only inactivated virus. This study was intended to develop a pretreatment for samples, so that inactivated viruses would not be detected by the RT-PCR procedure. Model viruses were human hepatitis A virus, vaccine poliovirus 1 and feline calicivirus as a surrogate for the Norwalk-like viruses. Each virus was inactivated (from an initial titer of approximately 10(3) PFU/ml) by ultraviolet light, hypochlorite or heating at 72 degrees C. Inactivated viruses, that were treated with proteinase K and ribonuclease for 30 min at 37 degrees C before RT-PCR, gave a negative result, which is to say that no amplicon was detected after the reaction was completed. This antecedent to the RT-PCR method may be applicable to other types of viruses, to viruses inactivated in other ways and to other molecular methods of virus detection.

Sequence diversity of human caliciviruses recovered from children with diarrhea in Mendoza, Argentina, 1995-1998

Human caliciviruses were detected by EIA and/or RT-PCR in stool specimens from children with diarrhea treated at out- or in-patient facilities between 1995 and 1998 in Mendoza, Argentina. Mexico virus-like strains detected by primers NV36/51 were transiently prevalent in 1995/1996. Significantly more human caliciviruses were detected when primers were designed from contemporaneously circulating strains. Nucleotide sequences of a highly conserved region in the RNA polymerase gene of 10 selected human caliciviruses were determined. Eight strains were Norwalk-like viruses and two strains were Sapporo-like viruses. Seven of the eight Norwalk-like viruses also were positive by the recombinant Mexico virus antigen EIA. The seven Mexico virus EIA-positive strains revealed two patterns in the RNA polymerase sequences: two strains were closest to Mexico virus and the other five strains were closest to Lordsdale virus. One of the five "Lordsdale" viruses was found to be a naturally occurring recombinant between the Mexico virus and Lordsdale human calicivirus genetic clusters [Jiang et al., (1999b) Archives of Virology 144:2377-2387]. The Mexico virus EIA-negative strain had 73-77% nucleotide identity with the closest related Norwalk-like viruses, indicating it might belong to a new genetic cluster of the Norwalk-like virus genus. The two Sapporo-like viruses were distinct genetically; one belonged to the Houston/90 or Parkville cluster and the other to a new cluster. Some strains appeared to have short periods of prevalence and locally adapted primer pairs significantly increased detection rates. The finding of high diversity of circulating strains, including recombinant strains and strains with previously unrecognized genetic identities, highlights a need for studies of human caliciviruses in these children and other populations.

Improved detection of human enteric viruses in foods by RT-PCR

Human enteric viruses (including hepatitis A virus (HAV) and Norwalk-like viruses (NLVs)) are now recognized as common causes of foodborne disease. While methods to detect these agents in clinical specimens have improved significantly over the last 10 years, applications to food samples have progressed more slowly. In an effort to improve the sensitivity and speed of virus detection from non-shellfish food commodities by reverse transcription-polymerase chain reaction (RT-PCR), we (i) evaluated multiple RNA extraction methods; (ii) compared alternative NLV primer sets; and (iii) developed a one-step RT-PCR method. Hamburger and lettuce samples, processed for virus concentration using a previously reported filtration-extraction-precipitation procedure, were inoculated with HAV or NV. Several RNA extraction methods (guanidinium isothiocyanate, microspin column, QIAshredder Homogenizer, and TRIzol) and primer pairs were compared for overall RNA yield (microg/ml), purity (A(260)/A(280)), and RT-PCR limits of detection. The use of TRIzol with the QIAshredder Homogenizer (TRIzol/Shred) yielded the best RT-PCR detection limits (<1 RT-PCR amplifiable units/reaction for NV), and the NVp110/NVp36 primer set was the most efficient for detecting NV from seeded food samples. A one-step RT-PCR protocol using the TRIzol/Shred extraction method and the NVp110/NVp36 or HAV3/HAV5 primer sets demonstrated improved sensitivity (>10-fold) over the routinely used two-step method. HAV RNA was detected by RT-PCR at initial inoculum levels corresponding to <10 and <100 PFU per 300 microl sample concentrate (corresponding to 6 g food sample) for hamburger and lettuce, respectively. NV RNA was detected by RT-PCR at initial inoculum levels <5 and <50 RT-PCR amplifiable units per 300 microl concentrate (corresponding to 6 g food sample) for hamburger and lettuce, respectively. Residual RT-PCR inhibitors were effectively removed as evidenced by the ability to detect viral RNA in food concentrates without prior dilution. The methods reported here show promise for rapid, sensitive detection of human enteric viruses in foods.

A predominant role for Norwalk-like viruses as agents of epidemic gastroenteritis in Maryland nursing homes for the elderly

Stool specimens from 156 Maryland nursing home residents, who became ill during 20 outbreaks of gastroenteritis from November 1987 through February 1988, were analyzed. All tested negative for astroviruses, enteroviruses, Group A rotaviruses, Sapporo-like caliciviruses, and enteric bacteria (i.e., Salmonella, Clostridium, and Shigella species). Eighty-two (52%) were positive for Norwalk-like viruses (NLVs), members of the family Caliciviridae. Six distinct genetic clusters within genogroups I and II of the NLVs were detected; a genogroup II (GII) virus closely related to the Camberwell virus in the NLV GII/4 genetic cluster was the predominant strain. Serologic evidence of infection with > or = 1 NLV was detected in 61 (56%) of 109 patients tested against 3 NLV antigens (i.e., Norwalk, Hawaii, and Toronto viruses). Sixteen (80%) outbreaks met the definition for an NLV outbreak. Taken together with a retrospective analysis of bacterial gastroenteritis in this same setting, these data support a major role for NLVs as etiologic agents of gastroenteritis in elderly persons.

Prevalence of "Norwalk-like virus" infections in outbreaks of acute nonbacterial gastroenteritis observed during the 1999-2000 season in Osaka City, Japan

We have investigated the incidence of Norwalk-like viruses (NLVs) associated with outbreaks of acute nonbacterial gastroenteritis in Osaka City, Japan, since April 1996 using reverse transcription (RT)-PCR and electron microscopy methods. From the results of the first 3 years, between April 1996 and March 1999, we previously reported that multiple genetic types of NLVs were detected in 71.9% of outbreaks using RT-PCR with Ando's primers except for one outbreak [Iritani et al., 2000]. However, during the 1999-2000 season, NLV outbreak strains, which could not be detected by RT-PCR with Ando's primers, were increased. From probe typing and sequence analysis, 76.9% of these undetectable outbreak strains were classified into the P1-B type and the others were untypable. These untypable strains were closely related with Alphatron type strains detected in the Netherlands. The P2-B probe type of the NLV outbreak strains was predominant (88.2%) in the 1999-2000 season. The phylogram based on the 81 nucleotide sequences from these P2-B outbreak strains formed 2 clusters closely related with Lordsdale virus. The dominant genetic type of the P2-B outbreak strains, during the 1996-1997 season in Osaka City, belonged in one of these 2 clusters. These findings of the emergence of NLVs escaping the RT-PCR method strongly indicated the importance of probe typing and sequence analysis to survey NLV infections. Our surveillance of NLV infection in the outbreaks, for these 4 years, showed that the predominant probe type and dominant genetic type of NLV outbreak strains changed each season.

Selection of more appropriate PCR primer pairs for improved efficiency in detecting Norwalk-like virus (NLV) RNA

A pair of primers, NV35 and NV36, and another pair of primers, NV81 and NV82/SM82, are commonly used for polymerase chain reaction (PCR) detection of Norwalk-like virus (NLV) genome RNA sequences in authorized test laboratories in Japan. However, the efficiency of NLV genome RNA detection with these primer pairs has been less than satisfactory. In the present study, we attempted to establish more appropriately matched primer pairs for improved detection of NLV genome RNA sequences using a combination of primers including NV35, NV36, NV81, NV82/SM82, SR33, and SRs (a mixture of 4 primers SR46, SR48, SR50, and SR52). We also evaluated appropriate primers for improved reverse transcription of NLV genome RNA. Stool samples used for detection of NLV included 18 samples collected from NLV-infected patients who ingested oysters (group 1) and 13 samples collected from those who did not ingest oysters (group 2). Reverse transcription of RNA genome with primer NV35 was less efficient compared with that with primer SR33 or NV81. When PCR products obtained with NV35 and NV36 as a pair of primers were subjected to gel electrophoresis, a strong extra band was detected compared with those obtained with other primer pairs. Since this extra band may represent heterodimeric or homodimeric hybrids, or intramolecular hybrids derived from these primers, this template-independent hybridization could lower the efficiency of primer-dependent polymerase reaction. Of 18 primer pairs, a pair of NV81 and SRs provided the best detection of PCR products following reverse transcription of NLV RNA with SR33 or NV81. The detection rate was 61% for both reverse transcription with SR33 and that with NV81. After reverse transcription using SR33 as a primer, nested PCR using a pair of NV81 and SRs following primary PCR using a pair of NV81 and NV82/SM82 increased the detection rate to 89% in group 1 and 100% in group 2.

Outbreaks of acute gastroenteritis associated with Norwalk-like viruses in campus settings

Norwalk-like viruses (NLVs) are transmitted by fecally contaminated food, water, fomites, and person-to-person contact. They are a leading cause of acute gastroenteritis epidemics in industrialized countries. NLV outbreaks are characterized by a 12- to 48-hour incubation period; nausea, vomiting, and diarrhea for 24 to 72 hours; and high secondary attack rates. NLV infections spread rapidly on college and university campuses because of close living quarters, shared bathrooms and common rooms, many food handlers, popular self-service salad bars in dining halls, and person-to-person contact through sports and recreational activities. The illness is generally mild and self-limited but an outbreak can strain the resources of campus health services and cause high absenteeism among both students and staff. Treatment is primarily through antiemetic medication and oral rehydration. Prevention and control of NLV outbreaks rests on promoting hand washing; enforcement of strict hygiene in all food preparation areas; and prompt, rigorous cleaning of potentially contaminated areas where someone has been ill.

Nucleotide sequence analysis and development of consensus primers of RT-PCR for detection of Norwalk-like viruses prevailing in Japan

A total of 177 different nucleotide sequences of the RNA polymerase region of Norwalk-like viruses (NLVs) genomes, collected via a nation-wide survey project in Japan between 1989 and 1998, were examined by reverse transcription-polymerase chain reaction (RT-PCR) employing various primer pairs. The nucleotide sequences of different strains showed great diversity, with a range of 57 to 100% identities among strains. The strains could be classified into five clusters: Norwalk (NV), Snow Mountain agent/Bristol virus (SMA/BV), Toronto virus/Mexico virus (TV/MX), and Japan specific cluster 1 and 2 (JP-1 and JP-2). Within each cluster there is greater than 85% identity of amino acid sequence (more than 75% identity of nucleotide sequences), based on sequence homology analysis. We believe that two of the five clusters, JP-1and JP-2, define new specific clusters found in Japan according to phylogenetic and pair-wise comparison studies. An RT-PCR procedure was designed using new consensus primer pairs, P1/P2, P1/P3, and Y1/Y2 based on multiple alignment of collected nucleotide sequences, that are expected to detect nearly all NLVs prevailing in Japan. The usefulness of the primers was tested by ten different laboratories in Japan using a panel of ten fecal samples containing different virus strains. The identification of these primer pairs will facilitate routine diagnosis of NLV infection by RT-PCR and offers the potential for their direct detection in food and environmental samples.

"Norwalk-like viruses" as a cause of foodborne disease outbreaks

While outbreaks of foodborne disease remain an important public health concern, their aetiology is not identified in a majority of instances. In targeted studies, the application of newly developed molecular assays has demonstrated that a large proportion of these outbreaks may be caused by the "Norwalk-like viruses" (NLV), a genus of genetically related viruses belonging to the family Caliciviridae. NLV outbreaks associated with consumption of faecally contaminated oysters are frequently reported and can best be controlled by preventing contamination of oyster-harvesting waters. Infectious foodhandlers are another frequent source of contamination, and such transmission can be minimised by exclusion of ill foodhandlers and the maintenance of strict personal hygiene. Molecular assays have greatly refined the epidemiological investigation of foodborne NLV outbreaks, allowing the linking of outbreaks in different locations and permitting the identification of the virus in the implicated vehicle. The development of simpler and more sensitive assays and their use on a broader scale will assist in defining the true burden of foodborne NLV outbreaks and improve strategies for their prevention and control.

Detection of feline calicivirus, feline herpesvirus 1 and Chlamydia psittaci mucosal swabs by multiplex RT-PCR/PCR

A single tube, multiplex reverse transcription (RT)-polymerase chain reaction (PCR)/PCR assay was developed for detection of feline herpesvirus 1 (FHV1), Chlamydia psittaci and feline calicivirus (FCV) in cats with upper respiratory tract disease (URTD), incorporating a simple, rapid extraction procedure capable of extracting both DNA and RNA. The assay was found to be as sensitive in vitro as simplex assays that have previously been shown to be as sensitive as, or more sensitive than, culture for each pathogen in experimentally infected cats. Conjunctival alone or both conjunctival and oropharyngeal swabs were collected from cats in 104 households with URTD. FHV1 was detected in 18 (17.3%) and C. psittaci was detected in 12 (11.5%) households. The prevalence of C. psittaci was not significantly different to that determined using a duplex PCR assay for C. psittaci and FHV1. The prevalence of FCV was affected by sample storage temperature. Of samples stored at -70 degrees C, 0/31 were positive for FCV but FCV was detected in 10/73 (13.7%) samples stored at 4 degrees C (P=0.006). Of the samples stored at 4 degrees C, 3/19 (15.8%) conjunctival swabs were positive for FCV and 6/32 (18.8%) oropharyngeal/conjunctival swabs were positive for FCV (P=0.79). The potential utility of restriction endonuclease analysis of RT-PCR products resulting from amplification of the hypervariable region of the capsid protein gene of FCV in field samples, without prior cultivation, was also examined. The assay may have considerable importance for diagnosis and epidemiological surveys of feline upper respiratory tract pathogens.

Detection and identification of mammalian reoviruses in surface water by combined cell culture and reverse transcription-PCR

Reoviruses are a common class of enteric viruses capable of infecting a broad range of mammalian species, typically with low pathogenicity. Previous studies have shown that reoviruses are common in raw water sources and are often found along with other animal viruses. This suggests that in addition to the commonly monitored enteroviruses, reoviruses might serve as an informative target for monitoring fecal contamination of drinking water sources. Mammalian reoviruses were detected and identified by a combined cell culture-reverse transcription-PCR (RT-PCR) assay with novel primers targeting the L3 gene that encodes the lambda3 major core protein. Five of 26 (19.2%) cytopathic effect-positive cell culture lysates inoculated with surface water were positive for reoviruses by RT-PCR. DNA sequence analysis of RT-PCR products revealed significant sequence diversity among isolates, which is consistent with the sequence diversity among previously characterized mammalian reoviruses. Sequence analysis revealed persistence of a reovirus genotype at a single sampling site, while a sample from another site contained two different reovirus genotypes.

Multiplex polymerase chain reaction assay for simultaneous detection of Staphylococcus aureus and streptococcal causes of bovine mastitis

To improve diagnosis of mastitis in dairy cattle, a multiplex polymerase chain reaction (PCR) assay was developed for the simultaneous detection of the four major bacterial causes of bovine mastitis, Staphylococcus aureus, Streptococcus agalactiae, Streptococcus dysgalactiae, and Streptococcus uberis. The target sequence was the 16S to 23S rRNA spacer regions. The performance of the assay was examined with 117 milk samples collected from a subclinically infected herd, and the diagnostic specificities and sensitivities of the multiplex PCR were compared with conventional culture. PCR was significantly more sensitive than culture for detection of S. aureus and S. uberis, but there were no significant differences in sensitivities between PCR and culture for the detection of S. agalactiae and S. dysgalactiae. The results suggest that this multiplex PCR assay could be used as an alternative method in routine diagnosis for rapid, sensitive, and specific simultaneous detection of S. aureus, S. agalactiae, S. dysgalactiae, and S. uberis in milk samples.

Evaluation of nine sets of PCR primers in the RNA dependent RNA polymerase region for detection and differentiation of members of the family Caliciviridae, Norwalk virus and Sapporo virus

Norwalk virus and Sapporo virus were approved as type species of the genus "Norwalk-like viruses" and the genus "Sapporo-like viruses," respectively, in the family Caliciviridae. A total of 116 stool specimens containing Norwalk virus (NV) or Sapporo virus (SV) were tested by RT-PCR and Southern hybridization to evaluate nine sets of PCR primers and seven internal oligonucleotide probes in the RNA dependent RNA polymerase region of NV and SV for detection and differentiation of viruses in the NV and SV. Fifty-five stool samples were collected from 11 outbreaks of NV and/or SV gastroenteritis in an infant home, where residents were infants under 2 years of age, in Sapporo, Japan. Sixty specimens were obtained in Sapporo from sporadic cases in children, mainly under 6 years of age, of acute gastroenteritis due to small round structured viruses detected by EM. There is no single primer pair to detect all NV and SV, and at least three primer pairs, G1 set, G2 set and Sapp35/Sapp36, are required to detect viruses in the NV and SV clades. Many NV and SV strains were successfully classified into one of the NV/genogroup I, NV/genogroup II and SV by single-round RT-PCR and Southern hybridization. The new detection method for SV reported in this study combined with those for NV previously reported may elucidate the importance of Norwalk virus and Sapporo virus as a cause of viral gastroenteritis in all age groups in the world.