Single base substitutions in the capsid region of the norovirus genome during viral shedding in cases of infection in areas where norovirus infection is endemic

Epidemiological Features of Human Norovirus Genotypes before and after COVID-19 Countermeasures in Osaka, Japan

We investigated the molecular epidemiology of human norovirus (HuNoV) in all age groups using samples from April 2019 to March 2023, before and after the COVID-19 countermeasures were implemented. GII.2[P16] and GII.4[P31], the prevalent strains in Japan before COVID-19 countermeasures, remained prevalent during the COVID-19 pandemic, except from April to November 2020; in 2021, the prevalence of GII.2[P16] increased among children. Furthermore, there was an increase in the prevalence of GII.4[P16] after December 2022. Phylogenetic analysis of GII.P31 RdRp showed that some strains detected in 2022 belonged to a different cluster of other strains obtained during the present study period, suggesting that HuNoV strains will evolve differently even if they have the same type of RdRp. An analysis of the amino acid sequence of VP1 showed that some antigenic sites of GII.4[P16] were different from those of GII.4[P31]. The present study showed high infectivity of HuNoV despite the COVID-19 countermeasures and revealed changes in the prevalent genotypes and mutations of each genotype. In the future, we will investigate whether GII.4[P16] becomes more prevalent, providing new insights by comparing the new data with those analyzed in the present study.

Clinical and virological characteristics of viral shedding in children with norovirus gastroenteritis

BACKGROUND

The correlation between the clinical manifestations and fecal viral load of norovirus (NoV) infection remains unknown.

METHODS

We established a SYBR® Green-based real-time quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) method to quantify NoV and then sequenced its genomes from the feces of patients admitted at the Chang Gung Memorial Hospital from 2017 to 2018.

RESULTS

NoV GII.4 Sydney (n = 21, 36.2%) and GII.P16-GII.2 (n = 19, 32.8%), the two predominant genotypes found among 58 isolates, were closely related to the Taiwan variant 2012a cluster in the VP1 region and genotypes of China strain. An increase in viral load could be observed on Day 3 following the onset of NoV infection. The viral load then declined rapidly from days 10-15 but remained high for >1 month in a severe combined immunodeficiency patient. Significantly longer shedding was found in patients with fever (p = 0.03) or infected by the GII.4 Sydney strain (p < 0.01).

CONCLUSION

The qRT-PCR-mediated method proposed in this work could quantify the viral load in patients with NoV infection. Significant viral shedding over a period of 2 weeks in children with acute gastroenteritis and >1 month in an immunodeficient patient was observed. Significantly longer shedding could be correlated with infection by the GII.4 Sydney strain and febrile patients.

Back-estimation of norovirus infections through wastewater-based epidemiology: A systematic review and parameter sensitivity

The amount of norovirus RNA (Ribonucleic Acid) in raw wastewater, collected from a wastewater treatment plant (WWTP), can provide an indication of disease prevalence within the sampled catchment. However, an accurate back-estimation might be impeded by the uncertainties from in-sewer/in-sample degradation of viral RNA, variable shedding magnitude, and difficulties in measurement within raw wastewater. The current study reviewed the published literature regarding the factors of norovirus shedding, viral RNA decay in wastewater, and the occurrence of norovirus RNA in raw wastewater based on molecular detection. Sensitivity analysis for WBE back-estimation was conducted using the reported data of the factors mentioned above considering different viral loads in wastewater samples. It was found that the back-estimation is more sensitive to analytical detection uncertainty than shedding variability for norovirus. Although seasonal temperature change can lead to variation of decay rates and may influence the sensitivity of this pathogen-specific parameter, decay rates of norovirus RNA contribute negligibly to the variance in estimating disease prevalence, based on the available data from decay experiments in bulk wastewater under different temperatures. However, the effects of in-sewer transportation on viral RNA decay and retardation by sewer biofilms on pipe surfaces are largely unknown. Given the highest uncertainty from analytical measurement by molecular methods and complexity of in-sewer processes that norovirus experienced during the transportation to WWTP, future investigations are encouraged to improve the accuracy of viral RNA detection in wastewater and delineate viral retardation/interactions with wastewater biofilms in real sewers.

Viral shedding in gastroenteritis in children caused by variants and novel recombinant norovirus infections

Human norovirus (NoV) is the leading cause of acute gastroenteritis and the rapid transmission of NoV renders infection control problematic. Our study aimed to investigate viral shedding in gastroenteritis in children caused by variants of emerging norovirus strains infections.We used RNA-dependent RNA polymerase (RdRp) sequencing to measure NoV genome copies in stool to understand the relationship between the clinical manifestations and viral shedding in hospitalized patients. The near full-length NoV genome sequence was amplified via reverse transcription-polymerase chain reaction (RT-PCR) and NoV recombination was analyzed using the Recombination Analysis Tool (RAT).From January 2015 to March 2018, 77 fecal specimens were collected from hospitalized pediatric patients with confirmed NoV gastroenteritis. The NoV genotypes were GII.4 (n = 22), non-GII.4 (n = 14), GII.4 Sydney (n = 21), and GII.P16-GII.2 (n = 20). Viral load increased from days 2 to 9 from the illness onset, resulting in an irregular plateau without peaks. After day 9, the viral load declined gradually and most viral shedding in feces ceased by day 15. The average viral load was highest in GII.4 Sydney followed by GII.P16-GII.2 infections and lowest in non-GII.4 infections. GII.4 unclassified infections showed the longest viral shedding time, followed by GII.4 Sydney infections, GII.P16-GII.2 recombinant infection resulted in the shortest duration. NoVs evolved to form a group of GII.P16-GII.2 variants during the 2017 to 2018 period.The viral load and shedding period and was different in variants of NoV infections in children. High mutation rate of emerging and re-emerging variants was observed to an enhanced epidemic risk rendering continuous surveillance.

Detection of feline norovirus using commercial real-time RT-PCR kit for the diagnosis of human norovirus infection

Feline noroviruses (FNoVs) are potential clinical pathogens in cats. To perform an epidemiological study of FNoV infection, it is necessary to develop a simple and effective method for virus detection. We investigated whether a commercial human NoV quantitative RT-PCR kit for the detection of human NoVs used in medical practice can be applied for FNoV detection. This kit was capable of detecting the FNoV gene regardless of the genogroup (GIV and GVI) in experimental and field samples. Based on the above findings, it is possible to detect FNoVs using human NoV tests. The relationship between FNoV infection and gastroenteritis in cats may be clarified by applying these methods to an epidemiological survey of FNoVs.

Identification of GII.14[P7] norovirus and its genomic mutations from a case of long-term infection in a post-symptomatic individual

Norovirus is a leading cause of acute gastroenteritis worldwide. Norovirus shedding typically lasts one week to one month after the onset of diarrhea in immunocompetent hosts. The occurrence of mutations in the genome during infection has contributed to the evolution of norovirus. It has been suggested that genomic mutations in the P2-domain of capsid protein VP1, the major antigenic site for virus clearance, are involved in the evasion of host immunity and prolonged shedding of norovirus. In our previous study, we found a case of long-term shedding of GII.14 norovirus in a post-symptomatic immunocompetent individual that lasted about three months. In this study, we characterized the genomic sequence of the GII.14 strain to gain insight into the context of long-term shedding. By sequencing a 4.8 kb region of the genome corresponding to half of ORF1 and the entire ORF2 and ORF3, which encode several non-structural proteins and the structural proteins VP1 and VP2, the GII.14 strain was found to be classified as recombinant GII.14[P7]. Six point-mutations occurred during the three-month period of infection in a time-dependent manner in the genomic regions encoding RNA-dependent RNA polymerase, VP1, and VP2. Three of the six mutations were sense mutations, but no amino acid substitution was identified in the P2-domain of VP1. These results suggest that there is a mechanism by which long-term shedding of norovirus occurs in immunocompetent individuals independent of P2-domain mutations.

A semi-nested RT-PCR assay for detection of norovirus in rat fecal samples

Norovirus is a highly prevalent pathogen that can infect a wide range of host species. Thus far, there have only been two reports of norovirus infection in rats. Diagnostic assays for the detection of norovirus are well established, but a specific molecular assay for the diagnosis of norovirus infection in laboratory rats has not yet been reported. In this study, we describe the development of a sensitive, semi-nested RT-PCR assay for detection of norovirus in fecal samples from Rattus norvegicus, reared in animal facilities under different sanitary barrier conditions. Additionally, we describe the first report of the presence of norovirus in rat colonies from Brazilian animal facilities.

Human norovirus hyper-mutation revealed by ultra-deep sequencing

Human noroviruses (NoVs) are a major cause of gastroenteritis worldwide. It is thought that, similar to other RNA viruses, high mutation rates allow NoVs to evolve fast and to undergo rapid immune escape at the population level. However, the rate and spectrum of spontaneous mutations of human NoVs have not been quantified previously. Here, we analyzed the intra-patient diversity of the NoV capsid by carrying out RT-PCR and ultra-deep sequencing with 100,000-fold coverage of 16 stool samples from symptomatic patients. This revealed the presence of low-frequency sequences carrying large numbers of U-to-C or A-to-G base transitions, suggesting a role for hyper-mutation in NoV diversity. To more directly test for hyper-mutation, we performed transfection assays in which the production of mutations was restricted to a single cell infection cycle. This confirmed the presence of sequences with multiple U-to-C/A-to-G transitions, and suggested that hyper-mutation contributed a large fraction of the total NoV spontaneous mutation rate. The type of changes produced and their sequence context are compatible with ADAR-mediated editing of the viral RNA.

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Norovirus U-to-C hyper-mutants are present in patient samples.

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Analysis of hyper-mutants in cell culture suggests ADAR-mediated RNA edition.

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Hyper-mutation may contribute to norovirus diversity and evolution.

Long-term viral shedding and viral genome mutation in norovirus infection

The duration of viral shedding in the patients from two outbreaks and four sporadic cases of norovirus (NoV) infections was investigated. The longest period of viral shedding into feces was for 173 days in an inpatient from one case of outbreak. The VP1 sequence from two long-term viral shedding cases in the outbreak revealed four synonymous and one non-synonymous mutations in one inpatient at 26 days from the onset of illness, and nine synonymous and two non-synonymous mutations and a deletion, 10 synonymous mutations and a deletion in other inpatient at 29 days and 54 days from the onset of illness, respectively. Ten of the 11 amino acid positions detected in these two inpatients were in the outermost P2 domain of the viral capsid protein, and mutations at positions 295, 297, and 394 were shared in the inpatients. Mutations in the P2 domain were in epitopes A and D or near epitopes A, C, and E, suggesting that the long-term carrier state of norovirus infection contributes to the generation of escape mutants by host immunoselection.

Impact of genotype-specific herd immunity on the circulatory dynamism of norovirus: a 10-year longitudinal study of viral acute gastroenteritis

Human norovirus is a major cause of viral acute gastroenteritis worldwide. However, the transition of endemic norovirus genotypes remains poorly understood. The characteristics of natural immunity against norovirus are unclear because few studies have been performed in the natural infection setting. This prospective 10-year surveillance study of acute gastroenteritis in the province of Osaka, Japan, revealed that norovirus spread shows temporal, geographic, and age group-specific features in the humans. Genogroup II genotype 4 (GII.4) was detected in most sporadic pediatric cases, as well as in foodborne and nursing home outbreaks, respectively. The dominant genotypes in outbreaks at childcare facilities and schools shifted every season and involved GI, GII.2, GII.3, GII.4, and GII.6. Evidence at both the facility and individual levels indicated that genotype-specific herd immunity lasted long enough to influence the endemic norovirus genotype in the next season. Thus, norovirus circulates through human populations in a uniquely dynamic fashion.

Within-host evolution results in antigenically distinct GII.4 noroviruses

Genogroup II, genotype 4 (GII.4) noroviruses are known to rapidly evolve, with the emergence of a new primary strain every 2 to 4 years as herd immunity to the previously circulating strain is overcome. Because viral genetic diversity is higher in chronic than in acute infection, chronically infected immunocompromised people have been hypothesized to be a potential source for new epidemic GII.4 strains. However, while some capsid protein residues are under positive selection and undergo patterned changes in sequence variation over time, the relationships between genetic variation and antigenic variation remains unknown. Based on previously published GII.4 strains from a chronically infected individual, we synthetically reconstructed virus-like particles (VLPs) representing early and late isolates from a small-bowel transplant patient chronically infected with norovirus, as well as the parental GII.4-2006b strain. We demonstrate that intrahost GII.4 evolution results in the emergence of antigenically distinct strains over time, comparable to the variation noted between the chronologically predominant GII.4 strains GII.4-2006b and GII.4-2009. Our data suggest that in some individuals the evolution that occurs during a chronic norovirus infection overlaps with changing antigenic epitopes that are associated with successive outbreak strains and may select for isolates that are potentially able to escape herd immunity from earlier isolates.

IMPORTANCE

Noroviruses are agents of gastrointestinal illness, infecting an estimated 21 million people per year in the United States alone. In healthy individuals, symptomatic infection typically resolves within 24 to 48 h. However, symptoms may persist for years in immunocompromised individuals, and development of successful treatments for these patients is a continuing challenge. This work is relevant to the design of successful norovirus therapeutics for chronically infected patients; provides support for previous assertions that chronically infected individuals may serve as reservoirs for new, antigenically unique emergent strains; and furthers our understanding of genogroup II, genotype 4 (GII.4) norovirus immune-driven molecular evolution.

Norovirus GII.4 and GII.7 capsid sequences undergo positive selection in chronically infected patients

Norovirus has become an important cause for infectious gastroenteritis. Particularly genotype II.4 (GII.4) has been shown to spread rapidly and causes worldwide pandemics. Emerging new strains evade population immunity and lead to high norovirus prevalence. Chronic infections have been described recently and will become more prevalent with increasing numbers of immunocompromized patients. Here, we studied norovirus evolution in three chronically infected patients, two genotypes II.4 and one II.7. A 719 and 757 nt region was analyzed for GII.4 and GII.7, respectively. This covers the entire hypervariable P2 domain of the VP1 capsid gene. Genetic variability at given and between different time points was assessed. Evolutionary adaptation was analyzed by Bayesian sampling of genealogies. This analysis clearly demonstrated positive selection rather than incidental drift for all three strains. The GII.7 and one GII.4 strain accumulated on average 5-9 mutations per 100 days, most of them non-synonymous. This is a much higher evolutionary rate than observed for noroviruses on a global level. Our data demonstrate that norovirus quasispecies are positively selected in chronically infected patients. The numbers of intraindividual amino acid mutations acquired in the capsid gene are similar to those separating consecutive GII.4 epidemic strains. Evolution in a given, chronically infected individual may thus generate novel genotypes at risk to expedite global evolution particularly for slowly evolving genotypes, as GII.7.

An integrated approach to identifying international foodborne norovirus outbreaks

Surveillance gaps can be bridged through analysis of combined molecular and epidemiologic data.

International foodborne norovirus outbreaks can be difficult to recognize when using standard outbreak investigation methods. In a novel approach, we provide step-wise selection criteria to identify clusters of outbreaks that may involve an internationally distributed common foodborne source. After computerized linking of epidemiologic data to aligned sequences, we retrospectively identified 100 individually reported outbreaks that potentially represented 14 international common source events in Europe during 1999–2008. Analysis of capsid sequences of outbreak strains (n = 1,456), showed that ≈7% of outbreaks reported to the Foodborne Viruses in Europe database were part of an international event (range 2%–9%), compared with 0.4% identified through standard epidemiologic investigations. Our findings point to a critical gap in surveillance and suggest that international collaboration could have increased the number of recognized international foodborne outbreaks. Real-time exchange of combined epidemiologic and molecular data is needed to validate our findings through timely trace-backs of clustered outbreaks.