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.

Noroviruses are major agents of viral gastroenteritis worldwide. The infectivity of Norwalk virus, the prototype norovirus, has been studied in susceptible human volunteers. A new variant of the hit theory model of microbial infection was developed to estimate the variation in Norwalk virus infectivity, as well as the degree of virus aggregation, consistent with independent (electron microscopic) observations. Explicit modeling of viral aggregation allows us to express virus infectivity per single infectious unit (particle). Comparison of a primary and a secondary inoculum showed that passage through a human host does not change Norwalk virus infectivity. We estimate the average probability of infection for a single Norwalk virus particle to be close to 0.5, exceeding that reported for any other virus studied to date. Infected subjects had a dose-dependent probability of becoming ill, ranging from 0.1 (at a dose of 10(3) NV genomes) to 0.7 (at 10(8) virus genomes). A norovirus dose response model is important for understanding its transmission and essential for development of a quantitative risk model. Norwalk virus is a valuable model system to study virulence because genetic factors are known for both complete and partial protection; the latter can be quantitatively described as heterogeneity in dose response models.

SummaryBackgroundThe Global Burden of Diseases, Injuries, and Risk Factors Study 2015 (GBD 2015) provides an up-to-date analysis of the burden of diarrhoeal diseases. This study assesses cases, deaths, and aetiologies spanning the past 25 years and informs the changing picture of diarrhoeal disease worldwide.MethodsWe estimated diarrhoeal mortality by age, sex, geography, and year using the Cause of Death Ensemble Model (CODEm), a modelling platform shared across most causes of death in the GBD 2015 study. We modelled diarrhoeal morbidity, including incidence and prevalence, using a meta-regression platform called DisMod-MR. We estimated aetiologies for diarrhoeal diseases using a counterfactual approach that incorporates the aetiology-specific risk of diarrhoeal disease and the prevalence of the aetiology in diarrhoea episodes. We used the Socio-demographic Index, a summary indicator derived from measures of income per capita, educational attainment, and fertility, to assess trends in diarrhoeal mortality. The two leading risk factors for diarrhoea—childhood malnutrition and unsafe water, sanitation, and hygiene—were used in a decomposition analysis to establish the relative contribution of changes in diarrhoea disability-adjusted life-years (DALYs).FindingsGlobally, in 2015, we estimate that diarrhoea was a leading cause of death among all ages (1·31 million deaths, 95% uncertainty interval [95% UI] 1·23 million to 1·39 million), as well as a leading cause of DALYs because of its disproportionate impact on young children (71·59 million DALYs, 66·44 million to 77·21 million). Diarrhoea was a common cause of death among children under 5 years old (499 000 deaths, 95% UI 447 000–558 000). The number of deaths due to diarrhoea decreased by an estimated 20·8% (95% UI 15·4–26·1) from 2005 to 2015. Rotavirus was the leading cause of diarrhoea deaths (199 000, 95% UI 165 000–241 000), followed by Shigella spp (164 300, 85 000–278 700) and Salmonella spp (90 300, 95% UI 34 100–183 100). Among children under 5 years old, the three aetiologies responsible for the most deaths were rotavirus, Cryptosporidium spp, and Shigella spp. Improvements in safe water and sanitation have decreased diarrhoeal DALYs by 13·4%, and reductions in childhood undernutrition have decreased diarrhoeal DALYs by 10·0% between 2005 and 2015.InterpretationAt the global level, deaths due to diarrhoeal diseases have decreased substantially in the past 25 years, although progress has been faster in some countries than others. Diarrhoea remains a largely preventable disease and cause of death, and continued efforts to improve access to safe water, sanitation, and childhood nutrition will be important in reducing the global burden of diarrhoea.FundingBill & Melinda Gates Foundation.

Attachment of Norwalk (NV), Snow Mountain (SMV), and Hawaii (HV) virus-like particles (VLPs) to specific ABH histo-blood group antigens was investigated by using human saliva and synthetic biotinylated carbohydrates. The three distinct Norwalk-like viruses (NLVs) have various capacities for binding ABH histo-blood group antigens, suggesting that different mechanisms for NLV attachment likely exist. Importantly, antisera from NV-infected human volunteers, as well as from mice inoculated with packaged Venezuelan equine encephalitis virus replicons expressing NV VLPs, blocked the ability of NV VLPs to bind synthetic H type 1, Le b , and H type 3, suggesting a potential mechanism for antibody-mediated neutralization of NV.Norwalk-like viruses (NLVs) are members of the family Caliciviridae and are a leading cause of acute gastroenteritis worldwide. Studies into the basic biology of NLVs have been limited to recombinant molecular approaches and human challenge studies, primarily because no tissue culture model is available and because an animal model for NLV infection has only recently been reported (21). The genome of Norwalk virus (NV), the prototype NLV, is a single-stranded, positive-polarity RNA of approximately 7.5 kb in length and is organized into three open reading frames (ORFs). The ORF2-encoded major capsid protein self-assembles into Norwalk virus-like particles (VLPs) when expressed from recombinant baculoviruses or Venezuelan equine encephalitis virus (VEE) replicons in insect or mammalian cells, respectively (3, 9, 11).Marionneau et al. have recently reported that baculovirusexpressed NV VLPs likely attach to either H types 1 or 3 on gastroduodenal epithelial cells of secretor-positive individuals (16). We describe a simple biochemical method to assess specific binding of three distinct VEE-expressed NLV VLPs to various ABH histo-blood group antigens from the type 1 and 3 biosynthesis pathways. We also evaluated and compared the receptor-blockade capacity of antisera from NV-infected human volunteers and from mice inoculated with two different candidate vaccines.Cloning, expression, and production of genogroup II NLV VLPs. To assess the ability of different NLVs to bind ABH histo-blood group antigens, Snow Mountain virus (SMV) and Hawaii virus (HV) ORF2 capsid genes were cloned from the stools of SMV-or HV-infected human volunteers (Fig. 1A). HV1 is identical in amino acid sequence to the published HV ORF2 (14), and the SMV1 consensus clone contains three amino acid alterations (13).HV1 and SMV1 capsid sequences were inserted into the pVR21 VEE replicon vector, and packaged HV1-and SMV1-carrying VEE replicon particles (VRPs) were produced as previously described for NV1 (3). To determine if the VRPs express NLV capsid proteins, baby hamster kidney (BHK) cell cultures were infected with VRP-NV1, VRP-SMV1, or VRP-HV1. Immunofluorescence analysis (IFA) with antiserum from human volunteers challenged with either NV, SMV, or HV demonstrated that all three capsid constructs were expressed from the VEE replico...

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