Noroviruses (NoVs) cause over 90% of nonbacterial epidemic gastroenteritis and around 50% of all gastroenteritis cases worldwide. Progress made so far in the molecular biology of norovirus has revealed the presence of a particularly important genotype of the virus, known as NoV genogroup II, genotype 4 (GII.4), as the cause of global pandemics of gastroenteritis, accounting for ~80% of all NoV infections. Over the last decade NoV epidemiology and transmission has mirrored that of influenza A virus with new antigenic epidemic GII.4 variants of NoV arising approximately every two to three years. The pandemic GII.4 NoVs and their associated period of activity include; the US-95/96 variant in 1996-8, Farmington Hills 2002 in 2002, Hunter virus 2004 in 2004, Den Haag 2006b in 2007-9 and New Orleans 2009 during 2010-12.
To understand the processes responsible for the emergence of antigenically distinct GII.4 variants we examined molecular epidemiological trends of NoV-associated acute gastroenteritis in Australia and New Zealand between 2009 and 2014.
Overall, 1195 NoV-positive stools representing 925 outbreaks and 252 acute infections were genotyped from cases of acute gastroenteritis. We performed an evolutionary analysis of complete GII.4 capsid sequences to determine the origins of recently circulating GII.4 variants. The GII.4 variant, New Orleans 2009, was predominant throughout 2010-12 causing 75.2% of all infections and was associated with three consecutive winter epidemics between 2010 and 2012. However, during 2012, New Orleans 2009 was replaced by an emergent GII.4 variant, Sydney 2012, which continues to be the predominant norovirus in 2014. Both viruses were preceded by pre-epidemic variants that circulated around two years before their pandemic emergence. In summary, this study revealed NoV GII.4 variants as the predominant cause of acute gastroenteritis between 2009 and 2014 and documented the replacement of New Orleans 2009 by a novel variant, Sydney 2012, as the current most prevalent NoV in circulation.