Descriptive study of foodborne disease using disease monitoring data in Zhejiang Province, China, 2016–2020 - BMC Public Health - BMC Public Health

Foodborne diseases impede socioeconomic development by straining health care systems, and harming national economies, tourism and trade. This study described the epidemiology of foodborne diseases caused by different pathogens in Zhejiang Province during the period 2016–2020. Over the 5 years, 75,124 cases with 4826 (6.42%) hospitalizations caused by Norovirus, Salmonella, V. parahaemolyticus, Diarrheagenic E. coli and Shigella from 31 hospitals were reported. Among 11 cities, 2028 cases in Huzhou city (14.33%), 1933 cases in Wenzhou city (13.66%), 1636 cases in Taizhou city (11.56%). The results were quite different from Sun Liang's report, in which Wenzhou city accounts for the largest percentage of illnesses [17].

The number of illnesses caused by Norovirus ranks first among all etiologies, which is consistent with Shanghai, in which Norovirus was the most common pathogen (43.10%) [18], but quite different from the studies in China's coastal provinces such as Hainan [19]. Wang [20] et al. reviewed 2447 papers in China that reported 1082 foodborne disease cases occurring between 1994 and 2005, in which V. parahaemolyticus caused the most events in littoral provinces, whereas in inland provinces, the largest percentage of cases were caused by Salmonella. Thus, there are regional differences in the distribution of pathogenic bacteria in China. These studies suggests that region-specific policies on foodborne disease control should be established.

Seasonality of foodborne illnesses was observed in this study. A seasonal trend was found for the V. parahaemolyticus, Salmonella and Diarrheagenic E. coli with the highest rates during summer period, peaking in August, this was similar in Enserink's [21] and Gong's [18] reports. However, the seasonal peak of infection attributed to some foodborne pathogens isn't in the summer. For instance, Norovirus infections showed the highest rate in November and March and the lowest in summer, which was in line with previous studies [18, 22, 23]. Seasonality related to the temperature, humidity and rainfall, all of which may affect exposure frequency and host immune status. These findings indicated that temperature is an important factor in foodborne illnesses, and investigation of the reasons for the seasonal dominance on foodborne diseases should be the focus of surveillance.

This study showed the distinctive differences between four main pathogens with age groups. In general, the positive detection rate was higher in people aged 19 ~ 30 and 31 ~ 40 years than that in those aged < 18 and 40 + years, which were infected by Norovirus, V. parahaemolyticus and Diarrheagenic E. coli. This was partly consistent with a study in China which found incidence of foodborne diseases in youth group was higher than that in elderly group [14]. Also, a study in France which found incidence of foodborne diseases in young was higher than that in elders, in which, elders (≥ 60 years) were at least likely to get infected with V. parahaemolyticus, whereas people aged 30 ~ 44 years were the most likely get infected [24]. Similar results were observed in a Shanghai study [25]. In contrast to previous studies which found children (< 5 years) and elder people more likely to get infected with Norovirus [26, 27], our study found that the highest proportion in Norovirus infections was people aged 19–30 years old. Among Salmonella infections, cases among children aged under 1 year old accounted for 26.30%, significantly higher than other age groups. Similar findings reported in Guangdong Province that children aged < 5 years were the group most affected by Salmonella (73%), of whom the infants under 1 year old were 81.5% [28]. As for gender distribution, though significantly different among four pathogens, all showed higher proportion in male. The Norovirus, V. parahaemolyticus and Diarrheagenic E. coli infection with the highest positive detection rates in the workers were observed. Foodborne illnesses among workers are liable to occur frequently because poor hygienic conditions at workers' camps and work situations, in the meantime, high summer temperatures impacting the transportation, distribution and storing of foods [29]. The related knowledge on what is safe should be handed down through families, work sites and credible institutions.

Analysis of exposed foods of foodborne illnesses in this study, the cases caused by Norovirus, V. parahaemolyticus and Diarrheagenic E. coli, the largest number of food categories involved were aquatic product infection (17.73%, 39.34% and 15.84%, respectively). On the contrary, a study showed the analysis of exposed foods of reported cases in Shandong Province, multiple foods (meaning more than two kinds of food) were the most commonly reported classification [30]. The reason for the different findings may be that Zhejiang is a coastal province with a vast sea area and various aquatic products. Therefore, consumers would be advised to separate raw and cooked foods, cook thoroughly as much as possible and keep food at safe temperatures to reduce the risk of foodborne diseases. However, avoiding all raw seafood should be difficult for those who are in the habit of eating seafood. As for cases infected by Salmonella, fruits, aquatic products and cooked meat products were identified as the most frequent food vehicles in the present study. Conversely, eggs have been reported as the most common classification for Salmonella infection in the US [31]. The main reason for this difference was cultural differences in eating habits. Yet it's worth noting that, the reported classification of multiple foods relatively high as well. That means people eat more and more diverse foods, on the other hand, the category of exposed foods in national foodborne disease surveillance system is not specified in enough detail.

Analysis of the settings, according to our analysis, private home was the most common exposure setting, followed by restaurant. However, the average annual case ratios in the Republic of Korea were the highest at restaurant (57%) [32]. Among cases reported in US, restaurants also the most common settings of preparation [31]. On the contrary, Wu et al. [33] from CDC of China found that, foodborne illnesses most frequently occurred in household (32%). Similar results were observed in a EU study [34]. These findings consistent with present results, this means a large proportion of foodborne diseases caused by foods improperly prepared or mishandled at home. The effective actions can include the following aspects: know the food they use, for example, read labels on food packages, make informed choices, become familiar with common food hazards; furthermore, government should focus on home settings to reduce infections.

In regard to clinical symptoms in general, results showed similar clinical symptoms, such as nausea, abdominal pain and watery diarrhea between patients caused by four pathogens, respectively. The proportion of fever was the highest in Salmonella while lowest in Diarrheagenic E. coli. The proportion of fever in Salmonella infections in our findings was close to that in another research [35]. As Most foodborne pathogens can cause acute gastroenteritis with gastrointestinal symptoms, it is difficult to distinguish the cases infected by different pathogens by symptoms.

The limitations of this study need to be explained. First, for many reported cases, information on certain aspects, such as food category, settings and etc. were missing or incomplete, so the conclusions might not be representative of unknown classifications. Second, information and detection data were collected from 31 hospitals and several laboratories. Though detection methods were unified and regular trainings were held, there was a chance of bias caused by the different conditions and levels of hospitals and laboratories. Third, inability to conduct an epidemiological investigation due to lack of patient cooperation, there were still some missing information.

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