Prolonged norovirus infection after pancreas transplantation: a case report and review of chronic norovirus

A non-human primate model for human norovirus infection

Norovirus infection can cause gastrointestinal disease in humans. Development of therapies and vaccines against norovirus have been limited by the lack of a suitable and reliable animal model. Here we established rhesus macaques as an animal model for human norovirus infection. We show that rhesus macaques are susceptible to oral infection with human noroviruses from two different genogroups. Variation in duration of virus shedding (days to weeks) between animals, evolution of the virus over the time of infection, induction of virus-specific adaptive immune responses, susceptibility to reinfection and preferential replication of norovirus in the jejunum of rhesus macaques was similar to infection reported in humans. We found minor pathological signs and changes in epithelial cell surface glycosylation patterns in the small intestine during infection. Detection of viral protein and RNA in intestinal biopsies confirmed the presence of the virus in chromogranin A-expressing epithelial cells, as it does in humans. Thus, rhesus macaques are a promising non-human primate model to evaluate vaccines and therapeutics against norovirus disease.

Standardization of an antiviral pipeline for human norovirus in human intestinal enteroids demonstrates nitazoxanide has no to weak antiviral activity

Human noroviruses (HuNoVs) are the leading cause of acute gastroenteritis. In immunocompetent hosts, symptoms usually resolve within 3 days; however, in immunocompromised persons, HuNoV infection can become persistent, debilitating, and sometimes life-threatening. There are no licensed therapeutics for HuNoV due to a near half-century delay in its cultivation. Treatment for chronic HuNoV infection in immunosuppressed patients anecdotally includes nitazoxanide, a broad-spectrum antimicrobial licensed for treatment of parasite-induced gastroenteritis. Despite its off-label use for chronic HuNoV infection, nitazoxanide has not been clearly demonstrated to be an effective treatment. In this study, we standardized a pipeline for antiviral testing using multiple human small intestinal enteroid lines representing different intestinal segments and evaluated whether nitazoxanide inhibits replication of five HuNoV strains in vitro. Nitazoxanide did not exhibit high selective antiviral activity against any HuNoV strain tested, indicating it is not an effective antiviral for HuNoV infection. Human intestinal enteroids are further demonstrated as a model to serve as a preclinical platform to test antivirals against HuNoVs to treat gastrointestinal disease. Abstr.

A Standardized Antiviral Pipeline for Human Norovirus in Human Intestinal Enteroids Demonstrates No Antiviral Activity of Nitazoxanide

Human noroviruses (HuNoVs) are the leading cause of acute gastroenteritis. In immunocompetent hosts, symptoms usually resolve within three days; however, in immunocompromised persons, HuNoV infection can become persistent, debilitating, and sometimes life-threatening. There are no licensed therapeutics for HuNoV due to a near half-century delay in its cultivation. Treatment for chronic HuNoV infection in immunosuppressed patients anecdotally includes nitazoxanide, a broad-spectrum antimicrobial licensed for treatment of parasite-induced gastroenteritis. Despite its off-label use for chronic HuNoV infection, nitazoxanide has not been clearly demonstrated to be an effective treatment. In this study, we established a standardized pipeline for antiviral testing using multiple human small intestinal enteroid (HIE) lines representing different intestinal segments and evaluated whether nitazoxanide inhibits replication of 5 HuNoV strains in vitro . Nitazoxanide did not exhibit high selective antiviral activity against any HuNoV strains tested, indicating it is not an effective antiviral for norovirus infection. HIEs are further demonstrated as a model to serve as a pre-clinical platform to test antivirals against human noroviruses to treat gastrointestinal disease.

Long Term Norovirus Infection in a Patient with Severe Common Variable Immunodeficiency

Norovirus is the most common cause of acute non-bacterial gastroenteritis. Immunocompromised patients can become chronically infected, with or without symptoms. In Europe, common variable immunodeficiency (CVID) is one of the most common inborn errors of immunity. A potentially severe complication is CVID-associated enteropathy, a disorder with similar histopathology to celiac disease. Studies suggest that chronic norovirus infection may be a contributor to CVID enteropathy, and that the antiviral drug ribavirin can be effective against norovirus. Here, a patient with CVID-like disease with combined B- and T-cell deficiency, had chronic norovirus infection and enteropathy. The patient was routinely administered subcutaneous and intravenous immunoglobulin replacement therapy (SCIg and IVIg). The patient was also administered ribavirin for ~7.5 months to clear the infection. Stool samples (collected 2013–2016) and archived paraffin embedded duodenal biopsies were screened for norovirus by qPCR, confirming a chronic infection. Norovirus genotyping was done in 25 stool samples. For evolutionary analysis, the capsid (VP1) and polymerase (RdRp) genes were sequenced in 10 and 12 stool samples, respectively, collected before, during, and after ribavirin treatment. Secretor phenotyping was done in saliva, and serum was analyzed for histo-blood group antigen (HBGA) blocking titers. The chronic norovirus strain formed a unique variant subcluster, with GII.4 Den Haag [P4] variant, circulating around 2009, as the most recent common ancestor. This corresponded to the documented debut of symptoms. The patient was a secretor and had HBGA blocking titers associated with protection in immunocompetent individuals. Several unique amino acid substitutions were detected in immunodominant epitopes of VP1. However, HBGA binding sites were conserved. Ribavirin failed in treating the infection and no clear association between ribavirin-levels and quantity of norovirus shedding was observed. In conclusion, long term infection with norovirus in a patient with severe CVID led to the evolution of a unique norovirus strain with amino acid substitutions in immunodominant epitopes, but conservation within HBGA binding pockets. Regularly administered SCIg, IVIg, and ~7.5-month ribavirin treatment failed to clear the infection.

Norovirus disease among older adults

Norovirus, a leading cause of gastroenteritis outbreaks worldwide, results in substantial direct and indirect healthcare costs. Adults older than 65 years of age bear a significant proportion of the disease burden, and the disease course in this population is often more severe and protracted. In this narrative review, we discuss the epidemiology of norovirus infection, mechanisms of pathogenesis, and transmission pertinent to outbreaks along with infection prevention and control efforts. We also describe the clinical manifestations of norovirus disease with a focus on individuals older than 65 years of age, diagnosis and available treatment options, and the challenges and progress within vaccine development.

Prolonged norovirus infections correlate to quasispecies evolution resulting in structural changes of surface-exposed epitopes

In this study, we analyzed norovirus (NoV) evolution in sequential samples of six chronically infected patients. The capsid gene was amplified from stool samples, and deep sequencing was performed. The role of amino acid flexibility in structural changes and ligand binding was studied with molecular dynamics (MD) simulations. Concentrations of capsid-specific antibodies increased in sequential sera. Capsid sequences accumulated mutations during chronic infection, particularly in the surface-exposed antigenic epitopes A, D, and E. The number of quasispecies increased in infections lasting for >1 month. Interestingly, high genetic complexity and distances were followed by ongoing NoV replication, whereas lower genetic complexity and distances preceded cure. MD simulation revealed that surface-exposed amino acid substitutions of the P2 domain caused fluctuation of blockade epitopes. In conclusion, the capsid protein accumulates numerous mutations during chronic infection; however, only those on the protein surface change the protein structure substantially and may lead to immune escape.

Histo-blood group antigens of glycosphingolipids predict susceptibility of human intestinal enteroids to norovirus infection

The molecular mechanisms behind infection and propagation of human restricted pathogens such as human norovirus (HuNoV) have defied interrogation because they were previously unculturable. However, human intestinal enteroids (HIEs) have emerged to offer unique ex vivo models for targeted studies of intestinal biology, including inflammatory and infectious diseases. Carbohydrate-dependent histo-blood group antigens (HBGAs) are known to be critical for clinical infection. To explore whether HBGAs of glycosphingolipids contribute to HuNoV infection, we obtained HIE cultures established from stem cells isolated from jejunal biopsies of six individuals with different ABO, Lewis, and secretor genotypes. We analyzed their glycerolipid and sphingolipid compositions and quantified interaction kinetics and the affinity of HuNoV virus-like particles (VLPs) to lipid vesicles produced from the individual HIE-lipid extracts. All HIEs had a similar lipid and glycerolipid composition. Sphingolipids included HBGA-related type 1 chain glycosphingolipids (GSLs), with HBGA epitopes corresponding to the geno- and phenotypes of the different HIEs. As revealed by single-particle interaction studies of Sydney GII.4 VLPs with glycosphingolipid-containing HIE membranes, both binding kinetics and affinities explain the patterns of susceptibility toward GII.4 infection for individual HIEs. This is the first time norovirus VLPs have been shown to interact specifically with secretor gene-dependent GSLs embedded in lipid membranes of HIEs that propagate GII.4 HuNoV ex vivo, highlighting the potential of HIEs for advanced future studies of intestinal glycobiology and host-pathogen interactions.

Newer approaches in diagnosis of diarrhea in immunocompromised patients

PURPOSE OF REVIEW

This review will focus on the epidemiology and cause of diarrheal illness in solid organ transplant and stem-cell transplant population recipients with a specific focus on the role of advanced multiplex technology in the diagnosis of diarrhea within this patient population.

RECENT FINDINGS

A wide range of infectious and noninfectious causes of diarrhea have been described in immunocompromised patients. The most common infections noted are Clostridioides difficile, norovirus, and cytomegalovirus, whereas immunosuppressive drugs and mucositis are the most common noninfectious causes of diarrhea. Historically, diagnostic evaluation has been limited to an array of single pathogen assays. Newer multiplex assays have become available that allow rapid, sensitive detection of a wide range of pathogens in a single assay. These assays have improved the number of patients with a diagnosed pathogen but may identify colonizing pathogens that are not pathogenic. Studies are needed to inform the discrimination and optimal use of these newer assays.

SUMMARY

Diarrhea is a common complication in immunocompromised patients and is associated with greater morbidity and rare mortality. New diagnostics facilitate detection of recognized pathogens and may allow for improved outcomes through the use of pathogen-targeted therapy.

Infectious complications after kidney transplantation

Infectious complications are one of the leading causes of hospitalization and mortality in kidney transplant recipients. They are more frequent during the year following transplantation, and in the elderly. Community infections, such as pyelonephritis and pneumonia, are from far the most common infections. However, the field of opportunistic infections has been particularly moving as routine prophylaxis for cytomegalovirus and pneumocystosis have altered their patterns. Emergence of new infections, as BK nephritis, followed by chronic infections by Norovirus and E hepatitis, and increasing incidence of invasive fungal infections and mycobacterial infections have raised concerns. An increasing number of infections may be prevented by prophylaxis, but also by vaccines who should be encouraged, especially for influenza, pneumococcal diseases and zoster. Access to transplantation is now possible for human immunodeficiency virus infected patients, with good results. The field of infectious diseases is thus changing in kidney transplant recipients, due to high-risk recipients, new immunosuppressive drugs, and development of new diagnostic, therapeutic and preventive methods.

Genetic Susceptibility to Human Norovirus Infection: An Update

Noroviruses are the most common etiological agent of acute gastroenteritis worldwide. Despite their high infectivity, a subpopulation of individuals is resistant to infection and disease. This susceptibility is norovirus genotype-dependent and is largely mediated by the presence or absence of human histo-blood group antigens (HBGAs) on gut epithelial surfaces. The synthesis of these HBGAs is mediated by fucosyl- and glycosyltransferases under the genetic control of the FUT2 (secretor), FUT3 (Lewis) and ABO(H) genes. The so-called non-secretors, having an inactivated FUT2 enzyme, do not express blood group antigens and are resistant to several norovirus genotypes, including the predominant GII.4. Significant genotypic and phenotypic diversity of HBGA expression exists between different human populations. Here, we review previous in vivo studies on genetic susceptibility to norovirus infection. These are discussed in relation to population susceptibility, vaccines, norovirus epidemiology and the impact on public health.

Norovirus antivirals: Where are we now?

Human noroviruses inflict a significant health burden on society and are responsible for approximately 699 million infections and over 200 000 estimated deaths worldwide each year. Yet despite significant research efforts, approved vaccines or antivirals to combat this pathogen are still lacking. Safe and effective antivirals are not available, particularly for chronically infected immunocompromised individuals, and for prophylactic applications to protect high‐risk and vulnerable populations in outbreak settings. Since the discovery of human norovirus in 1972, the lack of a cell culture system has hindered biological research and antiviral studies for many years. Recent breakthroughs in culturing human norovirus have been encouraging, however, further development and optimization of these novel methodologies are required to facilitate more robust replication levels, that will enable reliable serological and replication studies, as well as advances in antiviral development. In the last few years, considerable progress has been made toward the development of norovirus antivirals, inviting an updated review. This review focuses on potential therapeutics that have been reported since 2010, which were examined across at least two model systems used for studying human norovirus or its enzymes. In addition, we have placed emphasis on antiviral compounds with a defined chemical structure. We include a comprehensive outline of direct‐acting antivirals and offer a discussion of host‐modulating compounds, a rapidly expanding and promising area of antiviral research.

Chronic norovirus infection in primary immune deficiency disorders: an international case series

OBJECTIVE

Predictive factors associated with clinical outcomes of chronic norovirus infection (CNI) in primary immunodeficiency diseases (PIDD) are lacking.

METHOD

We sought to characterize CNI using a multi-institutional cohort of patients with PIDD and CNI using the Clinical Immunology Society's CIS-PIDD Listserv e-mail group.

RESULTS

Thirty-four subjects (21 males and 13 females) were reported from centers across North America, Europe, and Asia. All subjects were receiving high doses (median IgG dose: 1200 mg/kg/month) of supplemental immunoglobulin therapy. Fifty-three percent had a complete absence of B cells (median B-cell count 0; range 0-139 cells/μL). Common Variable Immune Deficiency (CVID) subjects manifested a unique phenotype with B-cell lymphopenia, non O+ blood type, and villous atrophy (logistic regression model, P = 0.01). Five subjects died, all of whom had no evidence of villous atrophy.

CONCLUSION

While Norovirus (NoV) is thought to replicate in B cells, in this PIDD cohort of CNI, B-cell lymphopenia was common, indicating that the presence of B lymphocytes is not essential for CNI.

Successful treatment of chronic norovirus gastroenteritis with nitazoxanide in a pediatric kidney transplant recipient

Uncontrolled studies suggest a benefit of nitazoxanide for the treatment of norovirus gastroenteritis in immunocompromised individuals. Here, we report the use of nitazoxanide in a 13-year-old male kidney transplant recipient who developed intractable norovirus gastroenteritis. Reduction of immunosuppression was not possible due to refractory TCMR. Administration of oral immunoglobulin and switching from tacrolimus to sirolimus failed to produce a meaningful clinical response. Treatment with a 14-day course of nitazoxanide resulted in prompt resolution of diarrhea as well as clearance of norovirus from the stool despite intense immunosuppression. Nitazoxanide may be considered as an option for the treatment of intractable norovirus gastroenteritis in pediatric transplant patients when reduced immunosuppression is not feasible or other treatment options have failed. Further studies to evaluate the safety and effectiveness of nitazoxanide in immunocompromised children are needed.

Intrahost Norovirus Evolution in Chronic Infection Over 5 Years of Shedding in a Kidney Transplant Recipient

Noroviruses are the leading cause of acute gastroenteritis, and they can affect humans of all age groups. In immunocompromised patients, norovirus infections can develop into chronic diarrhea or show prolonged asymptomatic virus shedding. Chronic norovirus infections are frequently reported for solid organ transplant recipients, with rapid intrahost norovirus evolution seen. In this report, we describe a case of chronic norovirus infection in an immunocompromised patient who was followed up for over 5 years. The purpose of the study was to specify the norovirus evolution in a chronically infected immunocompromised host and identify possible selection sites in norovirus capsid protein. During the follow-up period, 25 sequential stool samples were collected and nine of them were selected to generate amplicons covering viral RNA-dependent RNA polymerase (RdRp) and viral capsid protein (VP1) genes. Amplicons were sequenced using next-generation sequencing. Single nucleotide polymorphisms were defined, which demonstrated a nearly 3-fold greater mutation rate in the VP1 genome region compared to the RdRp genome region (7.9 vs. 2.8 variable sites/100 nucleotides, respectively). This indicates that mutations in the virus genome were not accumulated randomly, but are rather the result of mutant selection during the infection cycle. Using ShoRAH software we were able to reconstruct haplotypes occurring in each of the nine selected samples. The deduced amino-acid haplotype sequences were aligned and the positions were analyzed for selective pressure using the Datamonkey program. Only 12 out of 25 positive selection sites were within the commonly described epitopes A, B, C, and D of the VP1 protein. New positive selection sites were determined that have not been described before and might reflect adaptation of the norovirus toward optimal histo-blood-group antigen binding, or modification of the norovirus antigenic properties. These data provide new insights into norovirus evolutionary dynamics and indicate new putative epitope “hot-spots” of modified and optimized norovirus–host interactions.

The role of multiplex molecular panels for the diagnosis of gastrointestinal infections in immunocompromised patients

PURPOSE OF REVIEW

An increasing number of laboratories have implemented multiplex molecular panels for the diagnosis of gastrointestinal infections. This review focuses on recent data addressing the performance of US Food and Drug Administration-cleared multiplex gastrointestinal panels and discusses the advantages and limitations of these tests in the immunocompromised population.

RECENT FINDINGS

Testing for gastrointestinal pathogens using multiplex molecular panels increases sensitivity and detection of coinfections compared with routine testing methods. Furthermore, multiplex panels reduce turnaround time and may allow for more informed decisions regarding treatment and infection control measures. However, the routine use of multiplex gastrointestinal panels has led to an increase in the detection of certain organisms, such as enteroaggregative Escherichia coli and sapovirus, which many clinical laboratories did not specifically test for in the past. This has created a degree of confusion on how to best interpret the results of multiplex panels, especially in the immunocompromised host.

SUMMARY

Multiplex molecular panels provide a rapid and sensitive tool for the diagnosis of infectious diarrhea, and may allow for more timely decisions regarding the management of immunosuppressed patients. However, there are limitations associated with multiplex panels, including the interpretation of results and the cost associated with testing. Clinical microbiologists should work closely with clinicians to develop evidence-based algorithms to guide test utilization in this area.

Inhibition of Calcineurin or IMP Dehydrogenase Exerts Moderate to Potent Antiviral Activity against Norovirus Replication

Norovirus is a major cause of acute gastroenteritis worldwide and has emerged as an important issue of chronic infection in transplantation patients. Since no approved antiviral is available, we evaluated the effects of different immunosuppressants and ribavirin on norovirus and explored their mechanisms of action by using a human norovirus (HuNV) replicon-harboring model and a surrogate murine norovirus (MNV) infectious model. The roles of the corresponding drug targets were investigated by gain- or loss-of-function approaches. We found that the calcineurin inhibitors cyclosporine (CsA) and tacrolimus (FK506) moderately inhibited HuNV replication. Gene silencing of their cellular targets, cyclophilin A, FKBP12, and calcineurin, significantly inhibited HuNV replication. A low concentration, therapeutically speaking, of mycophenolic acid (MPA), an uncompetitive IMP dehydrogenase (IMPDH) inhibitor, potently and rapidly inhibited norovirus replication and ultimately cleared HuNV replicons without inducible resistance following long-term drug exposure. Knockdown of the MPA cellular targets IMPDH1 and IMPDH2 suppressed HuNV replication. Consistent with the nucleotide-synthesizing function of IMPDH, exogenous guanosine counteracted the antinorovirus effects of MPA. Furthermore, the competitive IMPDH inhibitor ribavirin efficiently inhibited norovirus and resulted in an additive effect when combined with immunosuppressants. The results from this study demonstrate that calcineurin phosphatase activity and IMPDH guanine synthase activity are crucial in sustaining norovirus infection; thus, they can be therapeutically targeted. Our results suggest that MPA shall be preferentially considered immunosuppressive medication for transplantation patients at risk of norovirus infection, whereas ribavirin represents as a potential antiviral for both immunocompromised and immunocompetent patients with norovirus gastroenteritis.

Chronic norovirus infection and common variable immunodeficiency

Chronic infection with norovirus is emerging as a significant risk for patients with immunodeficiency - either primary or secondary to therapeutic immunosuppression. Patients with primary immunodeficiency present a range of pathological responses to norovirus infection. Asymptomatic infections occur and differentiating viral carriage or prolonged viral shedding after self-limiting infection from infection causing protracted diarrhoea can be challenging, due to relatively mild pathological changes that may mimic other causes of diarrhoea in such patients (for instance pathogenic bacteria or parasites or graft-versus-host disease). However, a subset of patients with common variable immunodeficiency (CVID) experience a severe norovirus-associated enteropathy leading to intestinal villous atrophy and malabsorption. Symptomatic infection of up to 8 years has been demonstrated with clinical and histological recovery on viral clearance. Although oral immunoglobulins and nitazoxanide have been used to treat noroviral infections associated with immunosuppression, ribavirin is the only agent to date that has been linked to viral clearance in the Noroviral enteropathy associated with CVID.

Chronic norovirus infection among solid organ recipients in a tertiary care hospital, the Netherlands, 2006-2014

OBJECTIVES

Immunocompromised patients can suffer prolonged norovirus symptoms and virus shedding for many years. Little is known about the prevalence of chronic norovirus infection among solid organ transplant (SOT) recipients. In this study, 2182 SOT recipients were retrospectively tested for chronic norovirus infection.

METHODS

The first and last norovirus positive faecal samples of SOT recipients were sequenced to distinguish between persisting infection and re-infection. Patient charts were reviewed to obtain data on health status and treatments.

RESULTS

In all, 101 of 2182 (4.6%) recipients were norovirus infected and 23 (22.8%) of these developed chronic norovirus infection. Chronic norovirus infection was found among allogeneic heart, kidney and lung transplant recipients. The median shedding period at the end of the study period was 218 days (range 32-1164 days).

CONCLUSIONS

This study shows that chronic norovirus infection is not a rare phenomenon among SOT recipients in a tertiary-care hospital. Further research is needed to study the risk of norovirus transmission to other immunocompromised patients in the hospital and to the general population.

Chronic Norovirus Infections in Cardiac Transplant Patients

Norovirus is a common self-limiting gastrointestinal infection, but in transplant recipients, symptoms can last for months and result in serious health complications. As there is currently no established treatment for chronic norovirus infection in transplant patients, management has been directed at symptom control, trial of various antivirals, and ultimately reductions in immunosuppression. We present 3 cases of chronic norovirus infection in cardiac transplant patients to illustrate various approaches to diagnosis, the prolonged nature of disease symptoms, and treatment options. When managing a transplant recipient with chronic diarrhea, considering a broad differential as well as maintaining a high suspicion for infectious etiologies is key. A stepwise approach to management includes termination of diarrhea-causing medications, trials of nitazoxanide and immunoglobulin, and reductions in immunosuppressive therapies. Although brief discontinuation of immunosuppression is often required to achieve symptom, graft rejection is often a complication.

Norovirus Regulation by Host and Microbe

Norovirus (NoV) infection is the leading cause of epidemic gastroenteritis globally, and can lead to detrimental chronic infection in immunocompromised hosts. Despite its prevalence as a cause of diarrheal illness, the study of human NoVs (HNoVs) has historically been limited by a paucity of models. The use of murine NoV (MNoV) to interrogate mechanisms of host control of viral infection has facilitated the exploration of different genetic mouse models, revealing roles for both innate and adaptive immunity in viral regulation. MNoV studies have also recently identified important interactions between the commensal microbiota and NoV with clear extensions to HNoVs. In this review, we discuss the most current understanding of how the host, the microbiome, and their interactions regulate NoV infections.

Predictors of persistent diarrhea in norovirus enteritis after solid organ transplantation

Solid organ transplant (SOT) recipients may develop protracted diarrheal illness from norovirus. We performed a retrospective chart review between January 2010 and April 2014 to identify predictors of persistent diarrhea in transplant recipients with norovirus enteritis. A total of 152 SOT recipients with mean age of 31.5 years (SD 23.1) were included: 43.4% male, 34.2% pediatric patients. Allograft types were abdominal 136 (89.5%) (kidney [39.5%], liver-small bowel [23%], other [27%]) and thoracic 16 (10.5%). The median time to diagnosis of first norovirus enteritis episode from date of transplantation was 1.7 (0.3-5.3) years. At time of presentation, diarrhea was present in 141 (93%). Thirty percent had persistent diarrhea at 2 weeks. Hospitalization was required for treatment in 121 (80%) of episodes with the mean length of stay of 10±15.2 days. Most (91%) infections were due to norovirus genogroup II, and gastrointestinal coinfections were seen in 23 (19%) norovirus enteritis episodes. Nausea at time of diagnosis (P=.002) and cytomegalovirus (CMV) infection in the preceding 90 days (P=.036) were identified as independent risk factors for persistent diarrhea using univariate and multivariable logistic regression. Our study shows that nausea on presentation and prior CMV infection were associated with persistent diarrhea in patients with norovirus enteritis.

Norovirus in Transplantation

Noroviruses are among the most common cause of diarrhea in transplant recipients. The clinical spectrum of norovirus infection after transplant is increasingly being recognized. As substantial morbidity is now associated with norovirus infections in this population; the quest for rapid diagnostic modalities and newer therapies has expanded. Transplant recipients with norovirus infection are at risk for several complications, including protracted illness with malnutrition, organ failure, and chronic viral shedding. This review summarizes the current knowledge on the epidemiology, complications, diagnosis, and treatment of norovirus infection in the transplant setting.