NoV epidemics resulted from the global spread of GII.4 strains that evolved under the influence of population immunity. Lineages show notable (and currently unexplained) differences in geographic prevalence. Establishing a global NoV network by which data on strains with the potential to cause pandemics can be rapidly exchanged may lead to improved prevention and intervention strategies.

Noroviruses are the causative agents of the majority of viral gastroenteritis outbreaks in humans. During the past 15 years, noroviruses of genotype GGII.4 have caused four epidemic seasons of viral gastroenteritis, during which four novel variants (termed epidemic variants) emerged and displaced the resident viruses. In order to understand the mechanisms and biological advantages of these epidemic variants, we studied the genetic changes in the capsid proteins of GGII.4 strains over this period. A representative sample was drawn from 574 GGII.4 outbreak strains collected over 15 years of systematic surveillance in The Netherlands, and capsid genes were sequenced for a total of 26 strains. The three-dimensional structure was predicted by homology modeling, using the Norwalk virus (Hu/NoV/GGI.1/Norwalk/1968/US) capsid as a reference. The highly significant preferential accumulation and fixation of mutations (nucleotide and amino acid) in the protruding part of the capsid protein provided strong evidence for the occurrence of genetic drift and selection. Although subsequent new epidemic variants differed by up to 25 amino acid mutations, consistent changes were observed in only five positions. Phylogenetic analyses showed that each variant descended from its chronologic predecessor, with the exception of the 2006b variant, which is more closely related to the 2002 variant than to the 2004 variant. The consistent association between the observed genetic findings and changes in epidemiology leads to the conclusion that population immunity plays a role in the epochal evolution of GGII.4 norovirus strains.Since the beginning of viral gastroenteritis outbreak surveillance in the early 1990s, noroviruses have become recognized as the major cause of reported outbreaks of acute viral gastroenteritis worldwide. Noroviruses form a genus within the family Caliciviridae and are genetically and antigenically highly variable. Currently, five distinct genogroups (GGs) are recognized. Strains belonging to GGI, GGII, and GGIV are known to cause infections in humans. The GGs have been subdivided further into genotypes, defined by a minimum amino acid sequence identity over the complete capsid sequence of 80% (1).The strains most commonly identified as the cause of outbreaks belong to genotype GGII.4. In The Netherlands, this was the case for 68% of all norovirus outbreaks that were characterized during 12 years of surveillance and for up to 81% of all health care-related outbreaks. Since their first detection in The Netherlands in January 1995, the GGII.4 strains have consistently been present in the Dutch population (46). These observations are in agreement with those of other surveillance studies worldwide (3,4,15,17,29,36,55).During the past 15 years, four epidemic norovirus seasons have occurred, in the winters of 1995-1996, 2002-2003, 2004-2005, and 2006-2007. These worldwide epidemics were invariantly caused by the predominant genotype, GGII.4, and were attributed to the emergence of new variant lineages of this genotype (4,3...

White spot syndrome virus (WSSV), the sole member of the virus family Nimaviridae, is a large double-stranded DNA virus that infects shrimp and other crustaceans. By alignment of three completely sequenced isolates originating from Taiwan (WSSV-TW), China (WSSV-CN) and Thailand (WSSV-TH), the variable loci in the genome were mapped. The variation suggests the spread of WSSV from a common ancestor originating from either side of the Taiwan Strait to Thailand, but support for this hypothesis through analysis of geographical intermediates is sought. RFLP analysis of eight Vietnamese WSSV isolates, of which six were collected along the central coast (VN-central) and two along the south coast (VN-south), showed apparent sequence variation in the variable loci identified previously. These loci were characterized in detail by PCR amplification, cloning and sequencing. Relative to WSSV-TW, all VN-central isolates showed a~8?5 kb deletion in the major variable region ORF23/24, whereas the VN-south isolates contain a deletion of~11?5 or~12?2 kb, compared to a~1?2 or~13?2 kb deletion in WSSV-CN and WSSV-TH, respectively. The minor variable region ORF14/15 showed deletions of various sizes compared with WSSV-TH for all eight VN isolates. The data suggest that the VN isolates and WSSV-TH have a common lineage, which branched off from WSSV-TW and WSSV-CN early on, and that WSSV entered Vietnam by multiple introductions. A model is presented for the spread of WSSV from either side of the Taiwan Strait into Vietnam based on the gradually increasing deletions of both 'variable regions'. The number and order of repeat units within ORF75 and ORF125 appeared to be suitable markers to study regional spread of WSSV.

Noroviruses are the most common cause of viral gastroenteritis. An increase in the number of globally reported norovirus outbreaks was seen the past decade, especially for outbreaks caused by successive genogroup II genotype 4 (GII.4) variants. Whether this observed increase was due to an upswing in the number of infections, or to a surveillance artifact caused by heightened awareness and concomitant improved reporting, remained unclear. Therefore, we set out to study the population structure and changes thereof of GII.4 strains detected through systematic outbreak surveillance since the early 1990s. We collected 1383 partial polymerase and 194 full capsid GII.4 sequences. A Bayesian MCMC coalescent analysis revealed an increase in the number of GII.4 infections during the last decade. The GII.4 strains included in our analyses evolved at a rate of 4.3–9.0×10−3 mutations per site per year, and share a most recent common ancestor in the early 1980s. Determinants of adaptation in the capsid protein were studied using different maximum likelihood approaches to identify sites subject to diversifying or directional selection and sites that co-evolved. While a number of the computationally determined adaptively evolving sites were on the surface of the capsid and possible subject to immune selection, we also detected sites that were subject to constrained or compensatory evolution due to secondary RNA structures, relevant in virus-replication. We highlight codons that may prove useful in identifying emerging novel variants, and, using these, indicate that the novel 2008 variant is more likely to cause a future epidemic than the 2007 variant. While norovirus infections are generally mild and self-limiting, more severe outcomes of infection frequently occur in elderly and immunocompromized people, and no treatment is available. The observed pattern of continually emerging novel variants of GII.4, causing elevated numbers of infections, is therefore a cause for concern.

During a 2-year survey in an academic hospital, 8 (8.4%) of all norovirus (NoV)-positive patients showed prolonged norovirus illness and shedding (duration, 21-182 days). All patients had underlying illnesses, resulting in some level of immunodeficiency in 5. Four patients were admitted to the hospital with gastroenteritis, 2 acquired norovirus while hospitalized, and 2 were outpatients. Genotypes GII.4 and GIIb-GII.3 were found. Reinfection occurred in 3 patients. Full capsid sequences were determined from strains detected in sequentially collected stool specimens to study evolution. The greatest number of amino acid mutations in a given patient was 11; they were detected in NoV isolates recovered over a 119-day period and were mapped to positions at or near putative antigenic sites. In the patient with most severe immune dysfunction, only 5 amino acids mutated over 182 days, suggesting immune-driven selection. The severe impact on patients and hospitals and the potential role of prolonged shedders as a reservoir for viral antigenic variants lead us to stress the importance of confinement of outbreaks of NoV infection that occur in hospitals.

Sapoviruses (SaVs) belong to the Caliciviridae family and can cause gastroenteritis in humans and swine. Despite extensive testing, human sapoviruses have been found only in sporadic cases and in one mixed outbreak in children between 1994 and 2007 in the Netherlands. Here we describe a change in sapovirus epidemiology in the Netherlands resulting in sapovirus outbreaks and infections in adults. From November 2007 to January 2009, 478 outbreaks of acute gastroenteritis were reported to the National Institute for Public Health and the Environment in the Netherlands as a part of ongoing surveillance. Sapoviruses were found to be the most likely cause of 19 outbreaks (4%). During the same 2-year period, sapovirus infections were reported in Sweden, Slovenia, and Hungary. In the Netherlands, further characterization of outbreak strains showed that 12 (63%) sapovirus outbreaks were caused by genotype I.2 viruses. Most patients were adults older than 60 years (range, 1 to 100 years). Phylogenetic analysis using all presently available SaV sequences showed high homology between genotype I.2 strains detected in different geographical regions (Sweden, Slovenia, Taiwan, Japan, and Russia) since 2007. These first reported outbreaks of sapovirus infections in adults in the Netherlands were remarkable. Detection of identical genotypes in many samples might suggest that these viruses have the same origin, and since the infection is spreading fast, the prevalence of sapovirus infection may be increasing. The incidence of sapovirus infections in these countries suggests that a substantial part of Europe is affected by this virus.

From 1994 through 2005, gastroenteritis outbreaks caused by norovirus generally increased in the Netherlands, with 3 epidemic seasons associated with new GGII.4 strains. Increased percentages of GGII.4 strains during these epidemics, followed by a sharp decrease in their absolute and relative numbers, suggest development of immunity.

A reporting system could provide early warning.