Epidemiological dynamics of norovirus GII.4 variant New Orleans 2009

Norovirus (NoV) is one of the major causes of diarrheal disease with epidemic, outbreak or sporadic patterns in humans of all ages worldwide. NoVs of genotype GII.4 cause nearly 80-90% of all NoV infections in humans. Periodically, some GII.4 strains become predominant generating major pandemic variants. Retrospective analysis of the GII.4 NoV strains detected in Italy between 2007-2013 indicate that the pandemic variant New Orleans 2009 emerged in Italy in the late 2009, became predominant in 2010-2011 and continued to circulate in a sporadic fashion until April 2013. Upon phylogenetic analysis based on the small diagnostic regions A and C, the late New Orleans 2009 NoVs circulating during 2011-2013 appeared genetically different from the early New Orleans 2009 strains that circulated in 2010. For a selection of strains a 3.2-kb-long genome portion at the 3' end was sequenced. In the partial ORF1 and in the full-length ORF2 and ORF3, the 2011-2013 New Orleans NoVs comprised at least three distinct genetic sub-clusters. By comparison with sequences retrieved from the databases, these sub-clusters were also found to circulate globally, suggesting that the local circulation reflected repeated introductions of different strains, rather than local selection of novel viruses. Phylogenetic sub-clustering did not correlate with changes in residues located in predicted putative capsid epitopes, although several changes affected the P2 domain in epitopes A, C, D and E.