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Norovirus (NoV) is one of the leading causes of acute
gastroenteritis and responsible for more than 212,000 deaths annually worldwide.Currently,
there is no vaccine to prevent NoV diarrhea. Development of a suitable vaccine
is challenging since the viruses are difficult to culture and undergo rapid
genetic diversification through antigenic shift. Bangladesh is a densely
populated low-income country with high burden of diarrheal diseases. Despite the
recognition of NoV as a leading cause of gastroenteritis, the virus is least
studied and poorly understood in low-income countries. This study aimed to
elucidate genomic features of NoVs identified in hospitalized patients.

International Centre for Diarrhoeal Disease Research,
Bangladesh (icddr,b) has been running its hospital based diarrhea etiology
surveillance for last 40 years but NoV has never been included to this
surveillance. During 2015-16, we tested NoV RNA in a sub-sample of stools from
hospitalized patients by using multiplex one step real-time RT-PCR. Genotyping
and recombination of NoV capsid and polymerase genes were determined by sanger

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Overall, NoV RNA was identified in 83 (18%) out of 450
stool specimens. Majority of the NoV positives were genogroup GII (89%) and
remaining were GI (7%) and mixed GI and GII (4%). Among 56 NoV strains which
were successfully genotyped, more than half were recombinants consisting of
different genotypes of GII which include GII.P16/GII.4 (16%); GII.P16 /GII.3 (16%);
GII.P16/GII.2 (5%); GII.Pe/GII.17 (4%); GII.P7/GII.6 (7%); GII.Pm/GII.1 (5%);
and GII.Pg/GII.12 (2%). Other major genotypes were GII.P4/GII.4 (27%) and
GII.P7/GII.6 (7%). No recombinant GI was identified. Remarkably, higher
proportion of recombinant strains (90%, 19/21) were detected in 2016 compared
to 2015 (34%, 12/35). Our phylogenetic analysis reveals that most of these
recombination events might be due to the incorporation of the polymerase
GII.P16 component into the mainstream of NoV strains i.e. GII.2, GII.3, and
GII.4 resulting in generation of the emerging strains.

Our study reveals high incidence of recombinant NoV genotypes
which were facilitated by recombination between capsid and polymerase genes
from distinct GII genotypes. Although their pandemic potential is yet to be
investigated, the identification of the emerging variants as well as
contemporary global strains in this study is an illustration of the enormous
diversity of NoV strains in Bangladesh.

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