Acute nonbacterial gastroenteritis is one of the most important infectious diseases in China and severely affects infants and young children.18–21,26
However, studies are seldom performed in adult populations for elucidating the importance of the norovirus and other enteric viruses. In this study, norovirus was identified in 26.4% of the studied adult subjects with acute nonbacterial gastroenteritis in Beijing between July 2007 and June 2008, though the rate was higher than that of the previous study in children in China;23
this was in agreement with the published studies reporting that the prevalence rate varied between 3.5% and 47.3%.27,28
The difference in the prevalence of norovirus infection may be due partly to the different sensitive methods used at distinct locations of the genome in different studies. In this study, the norovirus infection rate (26.4%) was higher than that of rotavirus A (6.1%), astrovirus (1.8%), and sapovirus (0.5%) in the adult populations; the data differed from the results of the previous studies performed with children in China and other countries,3–6,23
in which the highest rate was detected for rotavirus A infection. The discrepancy of the highest prevalent rates between adults and children implied that norovirus affected adults more frequently than children, but not rotavirus A. Alternatively, although the sensitive method was only used on the detection of norovirus, the Ct value was defined as less than 30 cycles; this may counteract the query of the higher prevalence of norovirus than other enteric viruses. Moreover, the prevalence of other enteric viruses only showed an individual percent far lower than the prevalence of norovirus, and above all, we could deduce that norovirus was the top enteric virus than any other virus in adults.
Norovirus was detected almost continuously throughout the study period, and centralized in the winter and spring for outbreaks. In this study, norovirus mixed with other enteric viruses, comprised of rotavirus A, astrovirus, and sapovirus, respectively, were acquired both in outbreaks and sporadic cases, indicating that co-infection occurred in adults regularly, but no more than three virus infections were detected simultaneously. Significant associations of co-infection between sporadic and outbreak cases were found, and no significant distribution of age and male gender was reported between solo norovirus infection and co-infection patients. To date, the real importance of the mixed infections was not well elucidated, but it had been demonstrated that no significant differences were found in the clinical symptoms of the patients with multiple viral infections.28,29
The results of molecular epidemiology disclosed that GII.4/2006b accounted for the majority of sporadic cases and outbreaks during the surveillance period, no significant difference in the proportion and genotypes was discovered between outbreaks and sporadic cases, the data of this study was inconsistent with the results derived from which GII.4 predominated in outbreaks and GII.3 in sporadic cases,30
the main reason for the disagreement results may be explained partly by the geographic and the temporal and seasonal variations, and probably by the emergence of the new circulating GII.4 variants. Previous studies showed that although multiple genotypes of norovirus were reported, a trail of distinct GII.4 variants were identified as the main agents involved in the global epidemics. After almost a 2 to 4 years interval, a new variant GII.4 emerged and displaced the previous one, denoting GII.4 norovirus evolution is epochal, with periods of stasis followed by the emergence of novel epidemic strains that involve in a linear manner over time.15
Variant GII.4/2006b was first obtained in Spain in December 2005 and became the overwhelming predominant one since then, it was first isolated in China in July 2006 and then prevailed circulating in China.23,31
To date, the reasons for the variants GII.4/2006 circulating globally are still not fully understood, but may involve herd immunity and the ability of the virus to evade immune surveillance, and other factors such as difference in stability, infectious dose, and/or host-related factors that may also play an important role.32,33
Limitations in the study should be considered. First, long-time surveillance on norovirus and other enteric viruses in adults need to continue for further elucidating the dynamic changes of norovirus and other enteric viruses, and the proportion and genotypes in the outbreaks and sporadic cases; second, recombination of norovirus between genogroups and genotypes occurred frequently as another important feature of the norovirus evolution,31,33
therefore, recombination of the pathogen of norovirus should be considered in the study; third, detection on bacterial gastroenteritis should be considered, because bacterial infection might be a possible cause of acute gastro enteritis in our subjects.
In conclusion, the results of the study show that, in comparison with other enteric viruses, norovirus played a major role in acute nonbacterial gastroenteritis; no significant genetic relatedness of the dominant strains was found between sporadic cases and outbreaks. GII.4/2006b was identified as the predominant strain spreading throughout the year and predominated in the winter for outbreaks. Significant associations of co-infection between sporadic and outbreak cases were found, however, no significant associations of age and male gender were found with co-infection.