Static and Evolving Norovirus Genotypes: Implications for Epidemiology and Immunity.

Noroviruses are major pathogens associated with acute gastroenteritis worldwide. Their RNA genomes are diverse, with two major genogroups (GI and GII) comprised of at least 28 genotypes associated with human disease. To elucidate mechanisms underlying norovirus diversity and evolution, we used a large-scale genomics approach to analyze human norovirus sequences. Comparison of over 2000 nearly full-length ORF2 sequences representing most of the known GI and GII genotypes infecting humans showed a limited number (≤5) of distinct intra-genotypic variants within each genotype, with the exception of GII.4. The non-GII.4 genotypes were comprised of one or more intra-genotypic variants, with each variant containing strains that differed by only a few residues over several decades (remaining "static") and that have co-circulated with no clear epidemiologic pattern. In contrast, the GII.4 genotype presented the largest number of variants (>10) that have evolved over time with a clear pattern of periodic variant replacement. To expand our understanding of these two patterns of diversification ("static" versus "evolving"), we analyzed using NGS the nearly full-length norovirus genome in healthy individuals infected with GII.4, GII.6 or GII.17 viruses in different outbreak settings. The GII.4 viruses accumulated mutations rapidly within and between hosts, while the GII.6 and GII.17 viruses remained relatively stable, consistent with their diversification patterns. Further analysis of genetic relationships and natural history patterns identified groupings of certain genotypes into larger related clusters designated here as "immunotypes". We propose that "immunotypes" and their evolutionary patterns influence the prevalence of a particular norovirus genotype in the human population.

Sequential Gastroenteritis Outbreaks in a Single Year Caused by Norovirus Genotypes GII.2 and GII.6 in an Institutional Setting.

BACKGROUND: Norovirus is a leading cause of acute gastroenteritis worldwide. Improved diagnostic capability has been instrumental in the characterization of archival norovirus strains associated with gastroenteritis outbreaks that were investigated decades ago. One such investigation was that of 2 sequential gastroenteritis outbreaks that occurred in 1971 at the former Henryton State Hospital in Maryland. Approximately 40% of the resident population experienced clinical symptoms in both outbreaks, which occurred 11 months apart. METHODS: Stored stools and paired sera were re-analyzed to investigate the etiology of the 2 outbreaks. RESULTS: Different norovirus genotypes were identified as the etiological agents responsible for the illnesses, with GII.2 associated with the first outbreak and GII.6 with the second. The viruses were antigenically distinct as determined by analyses of hyperimmune sera raised against the corresponding virus-like particles in animals, as well as paired sera from infected individuals. CONCLUSIONS: The observed antigenic differences were consistent with the failure of the GII.2 strain to provide cross-protective immunity to the GII.6 strain a few months later. An understanding of antigenic diversity among norovirus genotypes will be important in the design of norovirus vaccines.