This is the first nationwide survey on AGI in the general population in China. Our results show that AGI represent a substantial burden in China. Based on the estimates of 0.56 AGI episodes per person-year and 56.1% of cases seeking medical care for AGI, 748 million cases of AGI and 420 million medical consultations occur each year throughout the country. The mean number of days taken off work was 2.4 days. If extrapolated to the population, this equates to about 167 million working days lost. Based on an incidence of 0.56 AGI episodes per person-year, by extrapolating the average of the proportion of AGI that is foodborne published from the USA (25%), England and Wales (26%) and Australia (32%) [15–18], it is estimated that there are 209 million cases of foodborne disease in China in one year. As the actual fraction of AGI due to foodborne disease is unknown, the extrapolation is merely to bring in context from other studies.
It is difficult to compare AGI rates between studies due to the use of different case definitions and study designs. Using the standard symptom-based case definition for AGI proposed by Majowicz et al. , we obtained similar results as compared with those using the chosen definition in our primary analysis. We thus reported the suggested minimum set of results in this article, in order to facilitate international comparisons. Overall the estimated rate of AGI in China falls within the range of incidence reported in similar retrospective studies performed in other countries [4, 5, 19–23]. The AGI incidence estimated in the Chinese population was lower when compared with Australia (0.8), Poland (0.9), Italy, USA and Canada (1.0), New Zealand (1.1), Denmark (1.4), and higher compared with France (0.3), Ireland and Japan (0.4). The observed differences between countries could be related to different exposures based on lifestyle, such as food consumption habits. The estimates of disease burden in the community differ substantially between retrospective and prospective study designs even when using identical case definitions. Prospective cohort studies tend to give lower rates with a rate of 0.28 episodes per person-year in The Netherlands  and a rate of 0.27 in the United Kingdom . This discrepancy was once attributed to telescoping, with respondents remembering disease episodes as having occurred more recently than they actually did. However in other studies [26–28], it is reported that the shorter 7-day recall period yields significantly greater annual estimates compared to the longer 30-day recall period, which is contrary to telescoping resulting in the true burden of disease being underestimated. The number of episodes per year based on the point prevalence estimate (0.4%) in the present study was 0.81, much higher than the estimate (0.56) based on the monthly prevalence. These differences also highlight the need for further standardization of the survey methods used.
This study found a higher rate of AGI in females and in children < 5 years, which has also been the case in other studies [12, 29]. Handling food and caring for children may underlie this pattern, bringing females more frequently in contact with enteric pathogens than males. It is likely that the behaviour of young children may increase their exposure to pathogens via person-to-person and environmental transmission. Identification of groups of people vulnerable to gastroenteritis is useful to guide resource allocation. Seasonality of AGI was observed in this study, with a peak of incidence during the summer months as in Australia . These months are traditionally associated with increased rates of laboratory proved Salmonella and Vibrio gastroenteritis [31, 32]. It should be noted that although Australia has a summertime seasonality, because it is in the southern hemisphere, this is really a wintertime seasonality typical of norovirus transmission. Further research is needed to understand why this may be different in China.
In our study, people who were less wealthy and who had a lower level of education were more likely to report AGI, a finding different from the United States and Australia, but similar to Malaysia and Malta [30, 33–35]. This may be due to the different lifestyle behaviours in this group such as health habits, eating-out patterns and awareness of hygienic conditions. However, the multivariate analysis showed that the variation across income groups was not significant.
The proportion of cases reporting that consuming contaminated food (36.5%) was a cause of their illness was lower than those reported in Hong Kong (45.0%) but much higher than the proportion in Ireland (18.5%) [29, 36]: an indication of some uncertainty. In practice, people seldom actually know the cause of their AGI, although they often attribute it to food. However, if there is no better way to get the foodborne proportion of AGI, this subjective proportion may be used to estimate the incidence of foodborne AGI, similar to Evans et al. .
The proportion of cases who visited a doctor (56.1%) and the level of medicine use (89.2%) found in this study were higher than the proportions observed in other countries cited above. These differences may reflect important differences in healthcare systems in the studied countries. However, the fact that a relatively high proportion of cases visited a doctor may also suggest that this study is primarily capturing the severe cases. Antibiotics are very rarely for the treatment of AGI. However, their use was reported by 49.8% of those with AGI, much higher when compared with other countries [4, 5, 19, 22, 28, 38]. Inappropriate usage of antimicrobial agents to treat humans is a major problem in China, as many Chinese receive antibiotics before they are assessed by a doctor or before diagnostic tests are performed [32, 39, 40]. If there are 748 million episodes of AGI each year in China, the number of AGI cases who are treated with antibiotics may be 372 million. This is of major concern, particularly in view of the increase in antibiotic resistant pathogens and the potential complications arising from taking antibiotics. It is important to understand the factors influencing the prescription and use of antibiotics, and to promote appropriate usage of antibiotics by people in China.
Of the respondents with AGI who visited a doctor, about one in third provided a stool sample, which was higher than in other countries [4, 5, 20, 22, 23]. If there are 420 million medical consultations each year in China, the number with stool exam is 137 million. This provides a rich opportunity to document pathogen-specific illness rates. However, in China, if a stool specimen is received, it is routinely tested for white blood cells but not cultured for pathogen . Although most of the stool exams are for white blood cells rather than culture, the fact that the practice of seeking health care and submitting a stool sample is important. As cheap and fast non-culture diagnostic tests are disseminated, their use in China could revolutionize our approach to foodborne disease surveillance. The potential for this should be further studied, particularly since there is a summertime (bacterial pathogen) seasonality.
The main limitation in this study was recall bias, which may arise because individuals with a particular condition are likely to remember their experiences differently from those who are not similarly affected. Attempts were made to reduce this bias by asking the actual date of onset, which gives more accurate results. Another limitation was caused by the potential co-existence of AGI symptoms with respiratory symptoms, both of which can be caused by either enteric disease or respiratory infection. In the present study, respondents were not asked about respiratory symptoms or illnesses; hence, AGI associated with respiratory illness was not excluded from our analysis, which means that the study may have in part also measured symptoms caused by non-intestinal infections . When estimating the burden of foodborne disease based on data from population studies, it is necessary to consider this issue. The advantage of this study is that we used a face-to-face interview, which may reach the potential participants who do not have access to a telephone; also the respondents were selected by the "next birthday" method, which was easy to operate and successfully in randomly selecting respondents. The response rate was quite high in this study compared to surveys from other countries cited above.
Estimating the frequency of AGI cases in a population is an important issue in evaluating the burden of foodborne disease. Although different sources of limitation could have restricted the efficacy of our study in providing the burden of AGI in the Chinese population, our study provides a contribution to a comprehensive estimate of the global burden of foodborne disease. The data regarding healthcare-seeking behaviour and the proportion of patients who have a stool sample submitted for analysis in particular, will help fill a gap in the efforts to estimate the size of each layer in the surveillance pyramid for AGI in China. Although the data obtained are not pathogen specific, the dataset can assist in the efforts to calculate the cost and disease burden of AGI in China. Due to the methodological challenges inherent in the cross-sectional study design, the results are not conclusive; nevertheless, they suggest that a substantial amount of AGI occurs which leaves room for preventive measures, e.g. in terms of efforts to prevent foodborne disease outbreaks, improve food safety and improve hygiene.