Molecular detection and sequence analysis of human caliciviruses from acute gastroenteritis outbreaks in Hungary

Scientific Opinion on an update on the present knowledge on the occurrence and control of foodborne viruses

A review of the biology, epidemiology, diagnosis and public health importance of foodborne viruses was performed. Data needs to support a risk assessment were also identified. In addition possible control options and their anticipated impact to prevent or reduce the number of foodborne viral human infections were identified, including the scientific reasons for and against the establishment of food safety criteria and process hygiene criteria for viruses for certain food categories. Food may be contaminated by virus during all stages of the food supply chain, and transmission can occur by consumption of food contaminated during the production process (primary production, or during further processing), or contaminated by infected food handlers. Transmission of zoonotic viruses (e.g. HEV) can also occur by consumption of products of animal origin. Viruses do not multiply in foods, but may persist for extended periods of time as infectious particles in the environment, or in foods. At the EU-level it is unknown how much viral disease can be attributed to foodborne spread. The relative contribution of different sources (shellfish, fresh produce, food handler including asymptomatic shedders, food handling environment) to foodborne illness has not been determined. The Panel recommends focusing controls on preventive measures to avoid viral contamination rather than trying to remove/inactivate these viruses from food. Also, it is recommended to introduce a microbiological criteria for viruses in bivalve molluscs, unless they are labelled "to be cooked before consumption". The criteria could be used by food business operators to validate their control options. Furthermore, it is recommended to refine the regulatory standards and monitoring approaches in order to improve public health protection. Introduction of virus microbiological criteria for classification of bivalve molluscs production areas should be considered. A virus monitoring programme for compliance with these criteria should be risk based according to the findings of a sanitary survey.

Outbreak studies of a GII-3 and a GII-4 norovirus revealed an association between HBGA phenotypes and viral infection

Noroviruses are the major viral pathogen of epidemic acute gastroenteritis. Two outbreaks of norovirus gastroenteritis that occurred in China in 2003 and 2006, caused by a GII-4 and a GII-3 strain, respectively, were studied to investigate potential association between viral infection and histo-blood types of hosts. The histo-blood group antigen (HBGA) phenotypes of 146 subjects (16 from the GII-3 and 130 from the GII-4 outbreaks) were determined in a saliva-based EIA. Our results showed that the secretor status of individuals was strongly associated with infection in the two outbreaks (P=0.0007, OR=0.044, 95% CI, 0.003-0.765); none of the nonsecretor in either outbreak developed symptomatic infection. The infection rate of individuals with the ABH and Lewis blood types varied between the two outbreaks. In the GII-4 outbreak, association of ABH blood types with noroviral infection (P=0.001, Chi-square=16.13) has been observed, in which the type A individuals had an increased risk of infection [61% in the symptomatic group (n=41) vs. 30% in the asymptomatic group (n=89), P=0.0001], while the type O individuals showed a decreased infection rate (17% vs. 48% in the two groups, P=0.0048). In the GII-3 outbreak, however, individuals with the H antigen only appeared to have a higher rate of infection (33% vs. 14%, P=0.059). Our study provided further evidence in the association between noroviral infection and the HBGA types of hosts. While the nonsecretor phenotype appears naturally resistant to these two strains, additional determinants on the HBGAs also may play roles in host range of the two strains.

Epidemiology of acute gastroenteritis outbreaks caused by Noroviruses in Okayama, Japan

Noroviruses (called formerly "Norwalk-like viruses") cause food-borne gastroenteritis outbreaks. These outbreaks were thought to be related to shellfish consumption, although non-shellfish related outbreaks also occurred frequently in Japan. To clarify the epidemiology of Norovirus outbreaks, 435 stool samples were collected from 60 acute non-bacterial gastroenteritis outbreaks occurring over 8 years in Okayama, Japan. Using reverse transcription-PCR (RT-PCR), Noroviruses were detected in 257 cases (59.1% of all samples) from 46 outbreaks (77% of all outbreaks). The majority of the 46 Norovirus outbreaks (89%) occurred during November to March; notably one-third occurred in December. Restaurants, schools, and welfare institutions accounted for the major settings in 50%, 20%, and 15% of the Norovirus outbreaks, respectively. This was similar to other reports from Japan, but differed from those from the United Kingdom. The transmission routes were assigned in 27 of the Norovirus outbreaks. In 18 outbreaks the routes were related to human contact (7 from food handlers and 11 from person-to-person contact), whereas those related directly to shellfish occurred only in 9 outbreaks. These results suggest that transmission routes related to human contact are more important than recognized previously in the context of preventive medicine. Furthermore, all outbreaks in which some of the samples contained dual genogroups of Noroviruses were related to shellfish, suggesting that consumption of contaminated shellfish frequently results in mixed Norovirus infections in contrast to other transmission routes and that coexistence of genogroups is a useful marker for shellfish-related outbreaks.

Seroprevalence of noroviruses in swine

Noroviruses (NVs) are important human pathogens that cause acute gastroenteritis. Genetically related animal enteric NVs have also been described, but there is no evidence of interspecies transmission of NVs. In this study we characterized antibody prevalence among domestic pigs by using recombinant capsid antigens of two human NVs (Norwalk and Hawaii) and one swine NV (SW918) that is genetically related to GII human NVs. Recombinant SW918 capsid protein expressed in baculovirus self-assembled into virus-like particles (VLPs) that were detected by antibodies against GII (Hawaii and Mexico), but not GI (Norwalk and VA115), human NVs. NVs recognize human histo-blood group antigens as receptors, but SW918 VLPs did not bind to human saliva samples with major histo-blood group types. Seventy-eight of 110 (71%) pig serum samples from the United States and 95 of 266 (36%) pig serum samples from Japan possessed antibodies against SW918. Serum samples from pigs in the United States were also tested for antibodies against human NVs; 63% were positive for Norwalk virus (GI) and 52% for Hawaii virus (GII). These results indicate that NV infections are common among domestic pigs; the finding of antigenic relationships between SW918 and human NVs and the detection of antibodies against both GI and GII human NVs in domestic animals highlights the importance of further studies on NV gastroenteritis as a possible zoonotic disease.

Norovirus and histo-blood group antigens: demonstration of a wide spectrum of strain specificities and classification of two major binding groups among multiple binding patterns

Noroviruses, an important cause of acute gastroenteritis, have been found to recognize human histo-blood group antigens (HBGAs) as receptors. Four strain-specific binding patterns to HBGAs have been described in our previous report. In this study, we have extended the binding patterns to seven based on 14 noroviruses examined. The oligosaccharide-based assays revealed additional epitopes that were not detected by the saliva-based assays. The seven patterns have been classified into two groups according to their interactions with three major epitopes (A/B, H, and Lewis) of human HBGAs: the A/B-binding group and the Lewis-binding group. Strains in the A/B binding group recognize the A and/or B and H antigens, but not the Lewis antigens, while strains in the Lewis-binding group react only to the Lewis and/or H antigens. This classification also resulted in a model of the norovirus/HBGA interaction. Phylogenetic analyses showed that strains with identical or closely related binding patterns tend to be clustered, but strains in both binding group can be found in both genogroups I and II. Our results suggest that noroviruses have a wide spectrum of host range and that human HBGAs play an important role in norovirus evolution. The high polymorphism of the human HBGA system, the involvement of multiple epitopes, and the typical protein/carbohydrate interaction between norovirus VLPs and HBGAs provide an explanation for the virus-ligand binding diversities.

The P domain of norovirus capsid protein forms dimer and binds to histo-blood group antigen receptors

Noroviruses (NVs) are the most important pathogen of epidemic nonbacterial gastroenteritis. The recent finding that NVs recognize human histo-blood group antigens (HBGAs) as receptors provided a new approach to study the pathogenesis of NVs. Using computational and site-directed mutagenesis approaches, our investigators previously identified a plausible binding pocket in the P domain of the NV capsids. In this study, we further characterize the role of the P domain in the interaction with human HBGA receptors using three NV strains representing three binding patterns. Our results show that the isolated P domain, although it did not form virus-like particles (VLPs), formed dimers, and the dimers bound HBGAs with the same patterns as those of the intact viral capsids. In contrast, the S domain, which formed small, thin-layer VLPs, did not bind A, B, or H HBGAs. A chimera containing the S domain of VA387 and the P domain of MOH revealed a binding pattern of the P donor strain (MOH). Deletion experiments revealed that an intact P domain is necessary for receptor binding. The P domain dimers are stable over a broad range of pH (2 to 11) or under strong denaturing conditions. Taken together, our results suggest that the P domain of NV contains essential elements for strain-specific binding to receptors. Further study of the P domain will provide useful information about the virus-receptor interaction. The high yield and easy production of the recombinant P protein in the Escherichia coli expression system will provide a simple approach to this goal.

Genetic diversity of norovirus and sapovirus in hospitalized infants with sporadic cases of acute gastroenteritis in Chiang Mai, Thailand

Stool specimens from hospitalized infants with sporadic gastroenteritis in Chiang Mai, Thailand, between July 2000 and July 2001 were examined for norovirus and sapovirus by reverse transcription-PCR and sequence analysis. These viruses were identified in 13 of 105 (12%) specimens. One strain was found to be a recombinant norovirus.

Transcriptional enhancement of RT-PCR for rapid and sensitive detection of Noroviruses

Previously reported nucleic acid sequence-based amplification (NASBA) primers specific for the GII Noroviruses were adapted for reverse transcriptase-polymerase chain reaction (RT-PCR), and detection sensitivity was then enhanced by a subsequent in vitro transcription of the RT-PCR amplicons. The NASBA-derived primers performed comparably to other broadly reactive GII Norovirus primers with respect to detection limits (i.e. 1 RT-PCR amplifiable unit (RT-PCRU) per reaction). Detection limits improved by approximately 1 log(10) to 0.3 RT-PCRU per reaction when transcriptional enhancement and electrochemiluminescence (ECL) hybridization followed RT-PCR. The method shows promise for improved detection sensitivity in instances where very low levels of virus contamination might be anticipated.

Mutations within the P2 domain of norovirus capsid affect binding to human histo-blood group antigens: evidence for a binding pocket

Noroviruses (NORs) are an important cause of acute gastroenteritis. Recent studies of NOR receptors showed that different NORs bind to different histo-blood group antigens (HBGAs), and at least four distinct binding patterns were observed. To determine the structure-function relationship for NORs and their receptors, two strains representing two of the four binding patterns were studied. Strain VA387 binds to HBGAs of A, B, and O secretors, whereas strain MOH binds to HBGAs of A and B secretors only. Using multiple sequence alignments, homology modeling, and structural analysis of NOR capsids, we identified a plausible "pocket" in the P2 domain that may be responsible for binding to HBGA receptors. This pocket consists of a conserved RGD/K motif surrounded by three strain-specific hot spots (N(302), T(337), and Q(375) for VA387 and N(302), N(338), and E(378) for MOH). Subsequent mutagenesis experiments demonstrated that all four sites played important roles in binding. A single amino acid mutation at T(337) (to A) in VA387 or a double amino acid mutation at RN(338) (to TT) in MOH abolished binding completely. Change of the entire RGD motif to SAS abolished binding in case of VA387, whereas single amino acid mutations in that motif did not have an apparent effect on binding to A and B antigens but decreased binding to H antigen. Multiple mutations at the RGK motif of MOH (SIRGK to TFRGD) completely knocked out the binding. Mutation of N(302) or Q(375) in VA387 affected binding to type O HBGA only, while switch mutants with three amino acid changes at either site from MOH to VA387 resulted in a weak binding to type O HBGAs. A further switch mutant with three amino acid changes at E(378) from MOH to VA387 diminished the binding to type A HBGA only. Taken together, our data indicate that the binding pocket likely exists on NOR capsids. Direct evidence of this hypothesis requires crystallography studies.

Molecular epidemiology of human calicivirus gastroenteritis outbreaks in Hungary, 1998 to 2000

Between November 1998 and November 2000, 196 stool specimens from 21 outbreaks of acute nonbacterial gastroenteritis occurring in 11 of the 19 counties of Hungary were collected and tested for human caliciviruses. Human caliciviruses were detected and characterized by a type-common enzyme-linked immunosorbent assay (EIA) and reverse transcription-polymerase chain reaction (RT-PCR) followed by cloning and sequencing. Twenty (95%) and 14 (67%) outbreaks were positive by EIA and RT-PCR, respectively, and 12 RT-PCR-positive outbreaks were also confirmed by sequencing. Comparative sequence analysis revealed 13 Norwalk-like virus sequences in the 12 outbreaks, including 11 Norwalk-like virus genogroup II (seven in Hawaii-like, two Lordsdale-like, one Melksham-like, and one Hillingdon-like) and two Norwalk-like virus genogroup I (related to Southampton-like and Desert Shield-like clusters) viruses. Multiple Norwalk-like virus clusters, with a predominance of Hawaii-like viruses, played an important role in nonbacterial gastroenteritis outbreaks during the study period. This is the first country-wide molecular epidemiological investigation of human calicivirus-associated, gastroenteritis outbreaks in Hungary and Central-Eastern Europe.