Collection authors Miren Iturriza-Gómara and John Harris on the importance of detecting sporadic norovirus cases in the community
Norovirus infects people of all ages across the world, and in high income countries it has been identified as the principal cause of vomiting and diarrhoea. Norovirus has also been identified as the leading cause of foodborne gastroenteritis. Norovirus disease is relatively mild, and most people who get norovirus will not seek medical care, and even if they do, a laboratory diagnosis is unlikely unless the person is thought to be part of an outbreak.
Data from targeted studies and modelling have demonstrated that most norovirus disease occurs in infants and young children, but norovirus is not routinely investigated as a cause of paediatric gastroenteritis. With regards to norovirus strain diversity, most of the available data comes from outbreak causing strains. However, there is an increasing number of studies that have characterised norovirus strains associated with sporadic disease in children.
In the relatively small number of countries in which norovirus surveillance is in place, it is limited to foodborne and/or healthcare associated outbreaks. It has become apparent, despite the high diversity of noroviruses, strains of the GII-4 genotype are the most common worldwide. The reasons for their persistence in the population are not fully understood, but their ability to mutate generating strains with new antigenic profiles is believed to be one of the main drivers of their success. Existing surveillance data provide clear evidence that most outbreaks, and in particular those associated with healthcare settings, are caused by GII-4 strains. These strains evolve continuously through drift, generating a “cloud” of GII-4 variant strains from which a new pandemic strain emerges every two to four years. However, information on the diversity and co-circulation of different GII-4 strains associated with sporadic gastroenteritis is scarce due to a lack of any systematic surveillance.
A recent analysis of norovirus using archived samples from large studies of sporadic gastroenteritis conducted in the UK and Malawi, identified that the pandemic GII-4 strains Sydney/2012 and New Orleans/2009 were circulating in the community up to 18 years prior to their recognition as pandemic strains. The data suggest that although the majority of strains circulating among sporadic cases belonged to the strains identified as epidemic in their corresponding years or period, the range of GII-4 strains found in sporadic cases was more diverse than that associated with outbreaks. These data also suggest the need for a more systematic and representative approach to norovirus diagnosis and strain surveillance that includes capturing more cases of sporadic gastroenteritis across the entire population. This may allow for the early detection of novel strains and the increase the ability to assess a strains potential to become epidemic or pandemic.
The development of new surveillance strategies, perhaps focused around children and schools, coupled with the use of home testing kits, might be one approach to filling surveillance gaps, particularly in gaining understanding of circulating strains in the community. Furthermore, as the prospect of norovirus vaccination gets closer, better understanding the relationship between strain diversity and degree of cross-protection is required in order to design the most effective strategies for vaccination and vaccine development.
Miren Iturriza-Gómara is Professor of Virology at the University of Liverpool.
Her research interest include enteric virus infections and vaccines and has worked
on the molecular epidemiology and evolution of rotavirus and norovirus for over 15 years.
John Harris is a lecturer at the University of Liverpool. He has worked on the surveillance of gastrointestinal pathogens, with a particular interest in norovirus for over ten years. In 2014 he was awarded a PhD from the University of Liverpool researching the epidemiology of norovirus outbreaks in hospitals.